I. Introduction

II. Skepticism and Modernity

1. Origin, Nature, and Scope of Modern Skepticism

2. Skepticism and the Critical Problem

3. Realism and Skepticism

III. The Workings of Human Knowledge According to the Aristotelian-Thomistic Tradition

4. The Nature of Man as Key to Understanding Knowledge

5. How We Know Things

6. Knowledge and Existence

IV. Response to the Skeptical Challenge

7. Presuppositions and their Justification

8. On Truth

9. The Representative Theory of Perception

10. Of Dreams and Illusions

11. Of Brains and Vats

12. Possibility and Probability

13. Artificial Problem, Artificial Solutions

V. Conclusion: Summary and Evaluation




“The Thomist refuses to admit any gap between mind and things.”


Frederick Wilhelmsen[1]


When, in 1879, Pope Leo XIII published his encyclical letter Aeterni Patris on the restoration of true Christian philosophy, he did so because he realized that unless there rise up strong Catholic philosophers willing to take on the philosophical challenges put forth by modern philosophy, countless souls would be deceived by the menacing errors that had sprung up since the time of the late Renaissance and René Descartes, and which were still threatening—perhaps then more than ever before—to undermine the intellec­tual and spiritual truths known by reason and divine revelation. Pope Leo recognized that the evils afflicting and endangering society at his time were due to the “false conclusions con­cerning divine and human things, which originated in the schools of [false] philoso­phy” and which had already been “accepted by the common consent of the masses.”[2] Thus, the Pope set out to restore true Christian philosophy in the world by calling for a revival in the philosophy of St. Thomas Aquinas (1225-1274), for the common temporal good and the salvation of souls.

In keeping with this spirit, and “for the refutation of pre­vailing errors,”[3] we shall devote this study to exploring the philosophical wisdom of St. Thomas Aquinas and his legacy, and to applying it to the fundamental error in modern philoso­phy that has come to be known as the “critical problem”: the question of whether there exists anything outside of the knowing subject and, if so, whether there can be true knowledge of these things. Essentially originating with René Descartes (1596-1650), this critical problem has become so influential that any epistemologist, Thomistic or not, writing in the post-Cartesian era is now expected to analyze it, to comment on it, and perhaps even to use it as the starting point of his epistemological investigation.

It was about this critical problem, which puts into question the existence of a world outside the mind, that the existentialist Martin Heidegger remarked: “The ‘scandal of philosophy’ is not that this proof [that there is a “connection between the ‘in me’ and the ‘outside of me’”] has yet to be given, but that such proofs are expected and attempted again and again.”[4] Though Heidegger, of course, would disagree with the solution proposed in this paper (inasmuch as for him, man in his very being is already totally involved with the world he seeks to understand, and so Heidegger would take the “objective  knowing” of Thomistic realism as a deformation of that more basic “immersion in the world”) and would challenge the Thomistic realist on a different level, we remain undaunted in our endeavor to answer modern skepticism cogently and compellingly by means of the “golden wisdom of St. Thomas”[5] and the Aristotelian-Thomistic realist tradi­tion.

We thus pro­pose to argue in this paper that a sound investigation into the study of knowledge reveals that the so-called “critical problem” is merely contrived, based on false premises, and does not present a genuine challenge to the justification of objective knowledge, inasmuch as the problem can be reduced to absurdity and is based on gratui­tous assumptions about the nature of knowledge and the knowing subject. We will com­bat the critical problem by assuming at the outset—and later validate—the “pre-critical” attitude by explaining through the use of it why the critical paradigm is not warranted. Since the pre-critical paradigm, i.e., the realist Aristotelian-Thomistic paradigm, does not run into the same problems as the Cartesian critical paradigm does, and since it avoids the critical problem and can answer skeptical objections raised against it, we will conclude that the critical problem is an artificially-created “pseudo-problem” and must therefore be rejected.





“Men are most anxious to find truth, but very reluctant to accept it.”


Etienne Gilson[6]


Though it enjoyed a major revival after the Renaissance, epistemological skepticism was by no means “new” in the seventeenth century. It is difficult to pinpoint exactly who the “first skeptic” was, as skeptical thinkers can be found in Pre-Socratic times, but we can say that the first philosopher who took a serious skeptical stance in a systematized fashion was Pyrrho of Elis.[7] His skeptical position came to be referred to as “Pyrrhonism” and was taken up and defended centuries later by Sextus Empiricus (ca. 200 A.D.). Though Pyrrhonism had all but disappeared during the middle ages, it made its comeback in the thought of Michel de Montaigne (1533-1592), Pièrre Charron, and Francis Sánchez, after Sextus’s Outlines of Pyrrhonism had been translated into Latin and thus been made available to the scholars of the Renaissance.

Skepticism simpliciter is difficult to define, as the precise meaning of the notion is contested by different thinkers who all claim to be skeptics.[8] Etymologically speaking, a “skeptic” is “someone who examines,” deriving from the Greek word skeptesthai, “to examine.” In the ancient tradition, the Pyrrhonists held to a different—and more sophisti­cated, I venture to say—kind of skepticism than the Academics. For Pyrrho, skepti­cism consists in the suspension of judgment, i.e., the refusal to affirm or deny one claim over another, because “to every account an equal account is opposed.”[9] Thus it is that the Pyrrhonian skeptic holds no beliefs. The intended end for which he engages in the sus­pension of judgment is tranquility (ataraxia), the freedom from dis­turbance about what may or may not in fact be the case. Hence, Pyrrhonism actually in­tended to be a practical philosophy, a way of life.

The skeptics of the later Platonic Academy, on the other hand, viewed skepticism dif­ferently. Their skepticism was simply a denial of knowledge: nothing can be known, nothing can be affirmed with certainty. That is, the major distinction between the Aca­demic and the Pyrrhonist is that the Academic positively claims that nothing can be known, whereas the Pyrrhonist merely says that as far as he can see, there is not enough evidence for assenting to one position rather than its opposite, though such evi­dence may very well appear in the future.[10] Sextus saw the Academic stance so far removed from that of Pyrrhonism that he denied the Academics the label of “skeptics” and simply referred to them as “Academics.” In fact, we may classify the Academics as “nega­tive dogmatists,” inasmuch as they deny (hence “negative”) the possibility of knowledge, which, however, is in itself a positive knowledge claim (hence “dogmatic”).[11] It is readily obvious that this position carries with it an inconsistency that leads to its own refutation. When battling the Academics, St. Augustine was not hesitant to point out that if their denial of the possibility of knowledge “is true, then a man who knows merely it [the denial] itself, knows some truth.”[12]

During the time of the Renaissance, Sextus’s Outlines were translated into Latin, and thus Pyrrhonism was carried into the dawn of modernity, though the road to skepticism had already been paved by what Etienne Gilson (1884-1978) labels the “psychologism” of William of Ockham.[13] Among the Renaissance skeptics was Michel de Montaigne, whose thought, as it turned out, provided the bridge over which philosophy would travel—by means of the influence it had on René Descartes—from post-medieval phi­losophy into modernity.

In his Essays, Montaigne essentially revived the arguments of the Pyrrhonists, not those of the Academics. He concluded that only divine illumination and revelation, not philosophy or reason, give true certainty:

Since the senses cannot decide our dispute, being themselves full of uncertainty, it must be reason that does so. No reason can be established without another reason: there we go retreating back to infinity.[14]


[Man] will rise, if God by exception lends him a hand; he will rise by abandoning and renouncing his own means, and letting himself be raised and uplifted by purely celestial means.[15]


It is perhaps not surprising that Montaigne should have taken this position, or that skepti­cism in general should have resurfaced during the Renaissance, given the numerous quar­rels over philosophical and theological matters of the thinkers of the high and late middle ages. As Gilson relates:

There is never too much of a good thing, but there were too many varieties of the same thing, and the difficulty was that since Ockham was refuting Duns Scotus, the while Duns Scotus was correcting Bonaventura, or Thomas Aquinas straightening out Albertus Magnus, they could not all be right at the same time. But who was right?

By far the easiest way to solve the problem was to decide that every one was wrong.[16]


In his Discourse on the Method, Descartes struggled to free himself from the skepticism that Montaigne had revived, triggered by the endless disputations of the scholastics:

I will say nothing about philosophy but that, seeing that it has been cultivated for many ages by the most excellent minds that have lived, . . . nonetheless, there is still not to be found in it anything about which one does not dispute, and which, as a consequence, not be doubtful. . . .[17]


Greatly troubled by these uncertainties and by the corresponding doubt gushing forth from them, Descartes took the radical and revolutionary step to consider as “well nigh false all that which was merely probable.”[18] The moment he had made this seemingly innocuous decision, he had given birth, as it were, to modern philosophy.

Doubting a claim is one thing—declaring it “well nigh false” on the mere grounds that it is uncertain is quite another. In the face of uncertainty, the correct attitude that rea­son demands one adopt is that of epoché, as the ancient Greeks would say, i.e., that of suspension of judgment. If I do not know whether I should assent to or oppose some­thing, the reasonable course of action is to do neither, to simply refuse judgment until the issue at hand becomes clearer.

The real crux of the matter, however, is different and more far-reaching still. Regard­less of whether Descartes should have suspended judgment about, rather than considering false, that which was merely probable,[19] his procedure of discerning the true from the false—his litmus test, in other words—is what really constituted the break with all past philos­ophy. The first step he was convinced he needed to take was the radical expulsion of all opinions and views he had ever held, even those he thought he knew:

I was persuaded that . . . for all the opinions to which I had hitherto given credence, I could not do better than to undertake, once and for all, to get rid of them, in order to replace them afterwards either by other, better ones, or even by the same ones, when I would have adjusted them to the level of reason. And I firmly believed that I would, by this means, succeed in conducting my life much better than if I were to build only on old foundations and to rely only on the principles of which I had let myself be persuaded in my youth, without ever having examined whether they were true.[20]


In other words, he wanted a “clean slate,” starting all over again from the bottom up. All prior knowledge, belief, and opinion should be eradicated. This was the necessary pre­condition for his plan to “rebuild” the “house of knowledge” on a completely certain foundation, as he makes clear in his Meditations on First Philosophy:

For the purpose of rejecting all my opinions, it will be enough if I find in each of them at least some reason for doubt. And to do this I will not need to run through them all individually. . . . Once the foundations of a building are undermined, anything built on them collapses of its own accord; so I will go straight for the basic principles on which all my former beliefs rested.[21]


Thus was triggered the avalanche of radical skepticism in modern philosophy.

Descartes is, of course, the modern skeptic par excellence.[22] Never before in the his­tory of Western philosophy had anyone doubted so radically and systematically as he. Though the ancient skeptics and also the Renaissance Pyrrhonists all doubted as a matter of principle, none of them had actually called into question the existence of a world out­side the mind: “Greek philosophy does not know the problem of proving in a general way the existence of an external world. That problem is a modern invention. . . .”[23] For Descartes, doubting the existence of the external world was simply necessary in order to be consis­tent in his method of doubting everything it was possible to doubt:

I will suppose . . . that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me. I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgment. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things.[24]


As radical as this doubt is, Descartes was fully aware that he was merely “pretending for a time”[25] and not really calling this into question. His doubt was merely methodical, engaged in so that he could arrive at some point which is impossible to doubt—if such a point was to be found.

This, then, is another point on which Descartes differs from the ancient skeptics. The Pyrrhonists’ doubt was practical, aimed at a tranquil life, and therefore real; Descartes’ doubt was a matter of method to find an indubitable principle, and it was therefore not to be taken as genuine. This fact explains why the scope of Descartes’ skepticism was greater than that of the Pyrrhonists; it encompassed—or so he thought[26]—absolutely every­thing that was dubitable. Paradoxically, however, Descartes only doubted exces­sively in order ultimately to defeat skepticism[27]—beating the skeptic at his own game, so to speak. Once he had found his indubitable basis from which to start rebuilding the house of knowledge, he would make sure that all knowledge claims would be immune from skeptical attack. He would finally be able to have certitude regarding his knowl­edge. Thus it was necessary for him to make his doubt global.



It would be impossible to investigate, enumerate, and analyze all of the errors and in­consistencies in Descartes’ epistemology and method here. Such is not the intention of this paper. However, we must pay close attention to the fundamental change in the subse­quent philosophy it engendered, and to the radical skepticism it, even if unwittingly, pre­cipi­tated.

Closely bound up with Descartes’ radical doubt is what has come to be known as the “critique of knowledge” or simply the “critical problem.” The indubitable ground to which Descartes finally came, his “Archimedean point,” as it were, was, of course, the Cogito: “Cogito, ergo sum – I think, therefore, I am.”[28] But this basis led him to identify the ego, the “I,” merely with thought, not with the composite of body and soul as had been the traditional Aristotelian and Christian view of the self: “Thought; this alone is inseparable from me. . . . I am, then, in the strict sense only a thing that thinks; that is, I am a mind, . . . or intellect. . . .”[29] He was even more explicit about this conclusion in the Sixth Meditation: “I am really distinct from my body, and can exist without it.”[30]

The result of this radical bifurcation between the mind and the body, perhaps some­what reminiscent of Plato, but still essentially unprecedented in Western philosophy up until this point, led to an exaggerated view of the mind and gave way to episte­mology as a starting point from which all subsequent philosophical inquiry needed to proceed. From then on, all philosophy was supposed to start with the subject, with the I, and the exis­tence of everything that is not I, would first need to be proved. This is pre­cisely what makes up the so-called “critical problem”:

If I am philosophically—that is, critically—certain of only one thing at the outset of my philosophizing, and if this one thing is the existence of myself as a thinking principle, then how am I able (if I am able at all) to move from this primitive certitude to a knowledge of other things, and, most especially, to a knowledge of the existence of the extra-mental world, of things existing independently of my own understanding.[31]


Since I have arrived at the Cogito only through excessive doubt, how can I move from the thinking self to the world without abandoning the skeptical method which I put into place in order to keep myself from deception and false knowledge claims?

Though Descartes provided an answer to this question, most every thinker agrees that his answer was insufficient. The Frenchman relied on a few dubious a priori arguments[32] for the existence of God and tried to infer from them that God, being all-good, would never let anyone be deceived about anything he perceives “clearly and distinctly.”[33] One may express surprise at such a facile “solution” in the face of the otherwise rigorous Cartesian process of ensuring that every possibility of error is avoided. Descartes’ over­confidence in this answer can perhaps be explained by the fact that there was really no other way in which he could have solved the conundrum into which he had put himself.

With the Cogito, Descartes had accomplished the turn to the subject in philosophy—his very own “Copernican Revolution,” one without which Kant’s would have never seen the light of day. Prior to Descartes, philosophical inquiry had begun with things, with the world, and hence with natural philosophy. From then on, however, all philosophizing was to begin with thought and with doubt. The critical problem he had created—yes, created, not discovered, as will be shown later—would ensure that subsequent philosophers would always recognize the primacy of the thinking self, the subject, and try to work their way out into the world, the object: “It was Descartes who put subjective knowledge at the center of epistemology—and thereby made idealism a possible position for a modern philosopher to take.”[34] It is not surprising, therefore, that the thinkers following Descartes all accepted his revolu­tion and started with the subject, though with the most striking dif­ferences: Male­branche, Spinoza, Leibniz, Locke, Berkeley, Hume, Kant, and Hegel, to name just a few. In one way or another, all of them were idealists.

The critical problem is really the ultimate fruit of hyperbolic doubt, which is its only motivation. By unmasking it as a pseudo-problem, however, we can counter the skeptical attack in a way which is detrimental to all future attempts at irrational doubt. When we look closely at the origins of the critical problem in Descartes, we find that, far from doubting absolutely everything that is dubitable, Descartes actually presupposes as true the “representative theory of perception.” According to this position, the mind is a pas­sive container which receives little sense-images, or ideas, of the things in the outside world. What the mind is directly aware of is not the thing outside the mind, but only the representation of the thing inside the mind. This sense image is the “mediator” between the mind and the world. Thus the difficulty arises as to how we can know that the repre­sentations inside the mind are true representations of the things in the world and do not give us a false picture of extra-mental reality. Of course, this difficulty can never be re­solved, as all the access we have to the world outside the mind is through sense percep­tion, and if we take this view of how perception works, then it is impossible to “validate” our beliefs and knowledge claims about extra-mental reality.[35]

Without this theory of perception or one like it, however, the excessive Cartesian doubt is not possible, and the radical turn to the subject is therefore also thwarted. As a result we can say that the persuasiveness of the critique of knowledge and, thus, also the persuasiveness of extreme skepticism depend to a large extent on the tenability of the representative theory of perception, a position which Descartes ironically took for granted. Since, however, the representative theory of perception was endorsed by virtu­ally every modern philosopher,[36] it is clear why the critical problem was the funda­mental difficulty and starting point from which these thinkers proceeded. A few comments on perception, knowledge, and skepticism are therefore in order now.



As I already suggested, the critical problem as stated, i.e., in terms of the representa­tive theory of perception, cannot be solved. It cannot be solved because the very theory requires that direct access to the extra-mental world is not possible; but it is precisely this access which is needed to solve the critical problem. In other words, the theory necessi­tates that the only evidence which could refute it is inadmissible. Thus it has made itself non-falsifiable in principle. This is why Gilson can state that it is an illusion to think “that one can extract an ontology from an epistemology, and, by this or that method, discover in thought anything apart from thought.”[37] Since the critical problem begins with the mind and takes, as it must, the representative theory of perception as true, it necessarily follows that whatsoever we entertain in order to refute this theory, it will necessarily be in the order of thought, i.e., inside the mind; and so the critique of knowledge precludes its own solution by its premises. After all, we can never “step outside of ourselves” and “compare” the representational image with the object of which it is the image, since all of our access to this object is by means of the image, according to the theory.

Not surprisingly, therefore, many representationalists[38] claim that we can have no knowledge of the outside world. But here we must ask ourselves whether what they mean by “knowledge” is still the same as what the realist understands by the term. For the realist, there is no such radical bifurcation between the “inside” (i.e., mental) and “outside” (i.e., extra-mental) world, seeing that the subject and recipient of knowledge is always a man,[39] composed of body and soul, i.e., matter and form, and exists therefore as partly mental and partly extra-mental.[40] Man, then, in his very being, has already bridged the gap that the radical skeptic has created for himself. One may surely ask, therefore, why the realist should be troubled by the representationalist’s claim to the impossibility of knowledge—knowledge which, on the representationalist’s own admission, is not the knowledge of things anyway but only the knowledge of ideas. The notion of knowledge as the common man understands it is, by definition, realistic. That is to say, the only way we can be truly said to know that there is a rose is if we actually have direct access to the rose itself, not access to a mere representation of the rose.

It is expedient, therefore, to investigate the nature of empirical knowledge. We must closely investigate what empirical knowledge is and how it is acquired. To do so, we shall explore, quite “uncritically” for the moment, the epistemology of the Aristotelian-Thomistic tradition to see what insights it can give us into the nature of knowledge and the workings of the human mind. We shall later see how we can validate this tradition in the face of skeptical attacks and the representative theory of knowledge.




“It is the real things immediately and directly presented by sensation that are the soil in which our intellectual life takes root and grows.”


—John F. X. Knasas[41]


Aristotle (384-322 B.C.) opens his Metaphysics with the observation that “[a]ll men naturally desire to know.”[42] Such a desire can only be explained if knowledge is indeed possible, for the acquisition of knowledge is the final cause for which the desire exists. No one in the scholastic realist tradition ever doubted the possibility of knowledge. That knowledge is possible was self-evident to the scholastics, proven quite simply by the fact that knowing is something we do all the time. Someone who swims does not wonder about whether swimming is possible; he knows it is possible because he does it. It is, however, quite legitimate to inquire how knowledge—or swimming—is possible, and this is what we must now investigate.

What, however, is knowledge? Knowledge as such eludes precise definition.[43] In the De Veritate, St. Thomas Aquinas points out that three things are necessary for there to be knowledge: “an active power in the knower by which he judges about things, a thing known, and the union of both.”[44] Knowledge is a quality of the soul that is had when the knower becomes the thing known in an intentional way. To understand what this means and why this is important, we must first consider the nature of man, since the knowing subject in our discussion is always a human being.



The radical skeptics presuppose, without questioning it, a Platonic conception of man. This goes to show that it is really impossible to philosophize about knowing without first having a clear picture of the nature of man (and thus presupposing that at least some knowledge is possible). This is so because knowledge is never found in a vacuum but always as a relation between a knowing subject and a known object; it is necessarily relational. It is crucial, therefore, not to fall into error with regards to both who the knower is and what kinds of things are knowable.

According to Plato, man is a soul trapped in a body,[45] and there is no essential connec­tion between the two; in fact, the two are, as it were, at war with one another. The soul is the real self, and the body is only a prison from which the soul longs to free itself. Reality is mental in nature and grasped abstractly; everything physical is not really real, only part of a “shadow world,” and sensation cannot give us true knowledge. With this view in mind, the idealist struggles to explain the experience of knowledge and concludes either that knowledge is not possible but mere illusion (e.g., Hume, Unger), or he invents more or less bizarre arguments which must be accepted in order to have knowledge after all (e.g., Descartes, Leibniz, Kant).

The Platonic view of man is false, however, as Aristotle showed. Man is not two independent things, a soul and a body, but one unified whole, a body whose form, i.e., formal cause, is the soul: “The soul is the cause or source of the living body.”[46] St. Thomas repeats Aristotle’s teaching when he says that “the intellect which is the prin­ciple of intellectual operation is the form of the human body.”[47] This view is based, of course, on Aristotle’s general teaching of “hylomorphism,” according to which all physi­cal entities are composed of matter (Greek, hyle) and form (Greek, morphe): “‘[N]ature’ has two senses, the form and the matter. . . .”[48] We cannot go here into all the reasons why Aristotle’s account of human nature is superior to Plato’s, but one can see that it is so even on prima facie grounds, as Aristotle’s position can explain the fact that everyone experiences himself as a whole and not simply as a soul or mind trapped in a body.[49] As indicated above, it is necessary to have a sound grasp of human nature, that is, of the nature of the knowing subject, before one can examine whether and how knowl­edge is possible. But this means that the investigation into human nature and human knowing are necessarily “pre-critical,” but it does not in fact beg the question of the critical problem, though it does perhaps show that the problem runs into serious compli­cations at the very outset. Furthermore, if the critical problem should prove to be a genuine problem after all, in the “post-critical” stage, any conclusions about the nature of human knowing and the processes of knowledge obtained in the pre-critical analysis can still be revised after­wards. Thus, there is no reason to reject a pre-critical investigation into human nature and human cognition.


For our purposes, we will only consider empirical knowledge, that is, knowledge of sensible objects, since this is the kind of knowledge most fundamentally under attack by the radical skeptics. Later on, in our direct response to the skeptical arguments, we must also briefly discuss a priori knowledge, i.e., first epistemological principles and the like. But first, we must turn to the question of the workings of empirical knowledge, even if the scope of this paper forces us to be brief.

Empirical knowledge is necessarily obtained through perception, which, in turn, is based upon sensation. Unfortunately, nowadays the notion of sensation is often reduced to that of mere sight, as though the other four senses—touch, hearing, smell, and taste—were somehow less significant, when in fact, at least in the case of touch, quite the oppo­site is the case.[50] St. Thomas, like Aristotle, was adamant that there is nothing in the intel­lect that was not first in the senses;[51] that is, anything that is received into the mind has arrived there through sense-perception. This is a far cry, however, from saying that everything in the mind is limited to sense-experience, as modern empiricists like Hume would contend. Perception makes empirical knowledge possible because the very notion of perception implies that there is epistemic access to the extra-mental world, as Mortimer Adler (1902-2001) points out:

The statement “I perceive X” is inseparable from the assertion “X exists.” If X did not exist, it would be imperceptible, and the statement “I perceive X” would be false. In its place, there should be a true statement about me—namely, “he is hallucinating X.”[52]


To examine how empirical knowledge works, then, it is expedient to examine the process of knowledge that moves from sensation via perception to intellection.

Sensation, Aristotle says, is “the receiving of forms without matter,” which he likens to the way in which “wax receives a seal without the iron or gold of the signet-ring. It receives an imprint of the gold or bronze, but not as gold or bronze.”[53] The sensed object acts upon the sense organ and thus causes an alteration in it, as the signet-ring alters the wax, thus causing a sense impression. In the example of my touching of a table, the table receives some sort of existence in me, but not material existence, since the table still exists outside me as before. Aristotle’s analogy illustrates what kind of existence it is instead, namely, a formal existence. The form of the sense-object is received by—and thus becomes present in—the knower, but “the form having, in the sense, a different mode of being from that which it has in the object sensed,” as Aquinas explains. In the object, the form is present in a “material mode of being (esse naturale),” whereas in the sense, the form is present in a “cognitional and spiritual mode.”[54] Since the sensed object does not undergo any change through my sensing it (except perhaps for the sense of touch, which can modify more delicate objects), we cannot say that sensation receives the matter of the object; instead, it can only be the form. This is the essential basis for the process of knowledge.

Sensation is the basis of perception. Perception differs from sensation inasmuch as perception is a process involving the internal senses and is dependent on the sensation of the outer senses.[55] In fact, it is “the internal senses, taken together, that bring the sensing process to completion.”[56] The four internal senses—“the common sense, imagination, memory, and the cogitative sense”[57]—are responsible for unifying the different sen­sa­tions into one coherent whole that lets us truly perceive an object. Whereas sen­sation gives us green, spherical, and sweet, it is the internal senses which put these quali­ties together and present them as belonging to one and the same object, which is later, through intellection, identified as an apple. We can say, then, that perception is more elaborate and advanced than mere sensation and perfects it.

The likenesses received in sensation are called “impressed species” or “sensible species,” whereas those attained in perception are called “phantasms.”[58] Neither sensible species nor phantasms must be confused with the Cartesian and post-Cartesian “ideas,” which are complete representations of actual objects. Rather, sensible species and phan­tasms are likenesses similar to the way in which a mirror-image is a likeness. A man who looks in the mirror sees himself, not a representation of himself, but he does so by means of the mirror. So it is with the sensible species and the phantasms and also the intelligible species, as we will see later. They are not objects of knowledge or perception or sen­sa­tion, but the means which make knowledge, perception, and sensation possible.[59]

The phantasm is an essential component to empirical knowledge, for it “links,” as it were, perception and intellection. It does not represent the concrete object, as the moderns would have it; it simply presents it. As such, the phantasm is necessary, but not sufficient, for intellectual knowledge; it is a vehicle that makes intellectual knowledge possible; and so the phantasm is to the intellect as the sense object is to the sense faculty. Hence Aristotle and St. Thomas teach that “the soul cannot understand without phan­tasms.”[60]

Now, in the Aristotelian tradition, the intellect is a particular power, or “part,” of the soul: “By the powers of the soul, we mean the vegetative, the sensitive, the appetitive, the locomotive, and the intellectual.”[61] This intellectual power is both active and passive; that is, we must distinguish between an active and a passive component in the intellect in or­der to explain the process of knowledge. By “passive intellect” is meant the intellect’s receptive power, which can be demonstrated to exist by the simple fact that we all experi­ence not knowing something at one point and knowing it later, e.g., the rules for playing tennis. The knower is said to pass from the potential state of knowing the rules for play­ing tennis to the actual state of knowing them—before, he could know them; then he does know them. This is what St. Thomas and Aristotle mean when they say that “with us [humans] to understand is to be passive.”[62] Since the mind has the capacity to understand all earthly things, even though at no point does it ever actually understand them all, we can say that “the passive intellect is all things potentially. . . .”[63]

In addition to the passive component of the intellect, we must also posit an active component, however. St. Thomas Aquinas explains why this is:

Since then the intellectual part of the soul alternates between potency and act, it must include these two distinct principles: first, a potentiality within which all intelligible concepts can be actualized . . . and then, also, a principle whose function it is to actualize those concepts.[64]


Without the active component, the human intellect would be reduced to a mere receptacle of sensations, as in the case of brute animals. We could know particular trees, stones, and cabbages, but we could know them only as particular; we could not know what it is to be a tree, a stone, or a cabbage, and thus that these objects actually are trees, stones, and cabbages. More abstractly, it would be the purpose of such an active component of the intellect to render intelligible to the mind the concrete singular object. The mind itself cannot grasp the object in its singularity because the mind is geared towards universals only, whereas the sense is exclusively geared towards singulars (and for which reason brutes, which do not have intellection, cannot “know” anything but can merely sense things). But each perceived object is particular, that is, singular and concrete. It is also necessarily material. The intellect being an immaterial power, however, it cannot receive that which is material.

How, then, can the immaterial receive that which is material? Properly speaking, it cannot. St. Thomas teaches that “according to the mode of immateriality is the mode of knowledge.”[65] That is, the intellect must somehow consider the immaterial aspects of the sensed object in order to know it, and this it can do only by means of an active compo­nent, as nothing can act that is not itself in act.[66] The phantasm cannot reduce the passive intellect to act because it is not in act but only in potency with respect to the intelligible order, since it is enveloped in material—and therefore, properly speaking, “non-intelli­gible”[67]—conditions. Hence the passive intellect cannot be actualized by the phantasm, since the intellect can only receive what is actually intelligible, much as the nose can only smell what actually—and not just potentially—has odor.[68]

The mind thus needs what the Aristotelian tradition calls the “active intellect,” a fac­ulty of the mind that illuminates the phantasm and abstracts from it the intelligible spe­cies and thus feeding the passive intellect with immaterial and therefore actually intelligi­ble information: “[T]hrough the light of the agent [=active] intellect the forms abstracted from sensible things are made actually intelligible so that they may be received in the possible [=passive] intellect.”[69] Phantasm and active intellect thus work together to pro­vide the mind, i.e., the passive intellect, with the necessary intelligible species it needs in order to know the object, as St. Thomas points out: “[T]he possible intellect receives forms whose actual intelligibility is due to the power of the agent intellect, but whose determinate likeness to things is due to cognition of the phantasms.”[70] The intelligible species is thus impressed, as it were, on the passive intellect.

However, since our intellect considers only what it has abstracted (and therefore what is universal) in the intelligible species, it cannot know the singular directly but must do so in a roundabout way. This it does by turning again to the phantasm, which presents the singular in its singularity:

Our intellect cannot know the singular in material things directly and primarily. The reason of this is that the principle of singularity in material things is individual matter, whereas our intellect, as we have said above (Q. 85, A. 1), understands by abstracting the intelligible species from such matter. Now what is abstracted from individual matter is the universal. Hence our intellect knows directly the universal only. But indirectly, and as it were by a kind of reflection, it can know the singular, because, as we have said above (Q. 85, A. 7), even after abstracting the intelligible species, the intellect, in order to understand, needs to turn to the phantasms in which it understands the species, as is said De Anima iii. 7. Therefore it understands the universal directly through the intelligible species, and indirectly the singular represented by the phantasm.[71]


This “conversio ad phantasmata,” or “turning to the phantasms,” is therefore necessary for our knowledge of singulars. It is something we are not conscious of, but Aquinas mentions that we can see for ourselves how we turn to singulars again and again by noting that it is easier for us to understand something when we are given concrete exam­ples rather than arguing totally in the abstract.[72] This is somewhat analogous to how the intellect turns again to the phantasms to know singulars, albeit indirectly.

This, then, is how empirical knowledge works in the Aristotelian-Thomistic tradition. The knower who thus possesses the intelligible species is said to know the object. In knowledge, knower and thing known are identical; the knower “becomes” the object of knowledge, but he does so in an “intentional” manner. This is why Aristotle says that “knowledge in act is the same as the thing itself.”[73] The scholastics called this mode of existence “intentional” to distinguish it from “natural” or “entitative” existence, which is existence in reality apart from being known. Intentional existence[74] is peculiar to knowl­edge and only denotes existence of objects as known. Interestingly enough, during the modern era of Descartes and his intellectual heirs, the doctrine of intentionality was com­pletely abandoned, and it was not until the 19th century that it was revived by Franz Brentano, who heavily influenced Edmund Husserl and various schools of phenomenol­ogy that remain influential to the present day.




Finally, we must touch upon the question of knowledge of the existence of sensible things in the Aristotelian-Thomistic paradigm, especially because this is the kind of knowledge denied or at least attacked by the radical skeptics. Strictly speaking, the exis­tence of anything is not an object of knowledge because it is nothing concrete and cannot be conceptualized. Existence is affirmed or denied in a judgment, which is the second operation of the intellect. Since concrete existence is always existence of a singular, such existence can only be discerned by the senses, and then only implicitly. It is necessarily implicit because discernment of concrete existence comes by way of inference. The senses perceive a concrete singular because it exists. If it did not exist, the senses could not perceive it. The reality of sense-perception guarantees that the perceived object in fact exists, since no one can perceive what is not actually there;[75] at best, one can dream or hallucinate it.[76]

The intellect itself gets a glimpse of existence when turning to the phantasm, because, as we said before, the phantasm presents the sensed object in its concrete singularity, and hence existence is implied. But since even the senses know existence only implicitly and the intellect knows the singular only indirectly, we have to conclude that the intellect only has “indirectly implicit” knowledge of concrete existence. Since, as we saw, the proper object of the intellect is the universal, and since the universal is never concrete, the intellect cannot know concrete existence directly.

Keeping all of these things in mind, we are now ready to look critically at the chal­lenge posed by the idealists, a challenge that basically began with Descartes’ hyperbolic doubt, the doubt which he himself had invented to free himself forever from skepticism, but which he was really unable to overcome despite his confidence to the contrary.




“St. Thomas took direct realism for granted. However valid that realistic position is, we cannot do that today. Both Neo-Thomism and modern philosophy in general need an epistemology that vindicates direct realism and rejects both rationalism and empiricism.”


—Robert J. Henle, S.J.[77]


As the foregoing section showed, the Aristotelian-Thomistic tradition teaches a direct realism in epistemology. It is precisely this approach to knowledge that is heavily under attack today, but it is certain to the present writer that no other approach can satisfy the human mind inquiring into the nature of the workings of human knowledge, because it alone is true. Since St. Thomas Aquinas did not have to battle with the issues and ques­tions that were raised when Descartes set his axe to the tree of knowledge, we cannot easily find the answers Thomas himself would have given to Descartes if he had lived in the seventeenth century. Nevertheless, we have a solid foundation in the Aristotelian-Thomistic tradition from which we can draw to evaluate the critical problem and answer directly the challenges put forth by today’s radical skeptics.  

“It is certain,” says Jacques Maritain (1882-1973), “that today reason can work use­fully at the general reform everyone feels so necessary only if it first of all cures itself of Cartesian errors.”[78] The Cartesian errors are numerous, of course, but the most funda­mental of them is Descartes’ irrational desire for absolute certainty, which has driven modernity into this vicious predicament of never-ending skepticism, to which all of the post-Cartesian philosophies are a response in one way or another.

Opening his Posterior Analytics, Aristotle writes: “All teaching and all intellectual learning come about from already existing knowledge.”[79] When Descartes, however, began his critique of knowledge by tearing down, as it were, the house of knowledge, doubting anything he was logically able to doubt, he prided himself on investigating the foundations of knowledge without presuppositions of any kind. “Presuppositionless phi­losophy” came to be an object of desire in modernity, and it is still very much taken to be a high ideal in certain circles today. That, however, is pre­cisely a grave error in Cartesian philosophy: the attempt to presuppose nothing.



The reason it is erroneous to try to have absolutely no presuppositions or assumptions whatsoever is that it is not possible. Moreover, it is not necessary. It is not possible because human intellection would be without foundation and therefore could not operate if there was nothing whatsoever that grounds and guides intellectual judgment. Even if someone were to try as hard as possible to presuppose nothing at all and as a result would remain entirely idle and not do anything, his inaction would still betray his holding to some assumed premises or principles, because he would be judging that it is better to remain idle than to do anything if one assumes nothing, and this judgment is a conclusion that has come about as the result of an inferential chain. But every conclusion is based on premises, which are necessarily presupposed.[80] So, the fact that he makes the judgment to be idle reveals that he holds to some presupposed premises, or else he could not judge that he should be idle—and a judgment he must make, one way or another, namely, the judgment of whether to be idle or not. It is akin to the choice of not choosing.

Seeing that it is impossible to be totally “presuppositionless,” we need not panic, however, for it is simply not necessary to start philosophical inquiry without any assumptions whatsoever.[81] The reason for this is that presupposing certain things is not an obstacle to sound philosophical reasoning—what is an obstacle is presupposing the wrong things. If we assume dubious contentions, half-truths, or downright errors, then philosophical inquiry will be inhibited, and we will most probably end up with a false conclusion.[82] Thus, rather than purging our intellects of all presuppositions indiscrimi­nately, we ought to take care to eradicate only any false or unjustifiable presuppositions we might have.

In order to be able to distinguish true presuppositions from false ones, we must first understand the nature and necessity of first principles. The necessity of first principles arises out of the fact that all reasoning needs a foundation: “[I]f it were necessary to prove every statement, an infinite series of reasons would be required.”[83] This should be obvious not only to the realist but also to the idealist. If the idealist wishes to deny this foundationalism, he can only do so by giving reasons for his denial—and this would in­volve him in giving tacit approval to the very foundationalism he denies. He would thus be trapped in a vicious circle. This shows that anyone who argues against foundation­alism does so by using arguments that, if they are to be forceful at all, presuppose its truth. Thus we can see the necessity for a foundation of all reasoning, and this basis is provided in first principles and certain axioms and postulates[84] that are self-evident.

For our purposes, we need to enumerate only two of the first principles, namely, the principle of non-contradiction and the principle of identity. “[T]he firmest [i.e., most certain] of all principles,” says Aristotle, “is that about which it is impossible to make a mistake,”[85] and that is the principle of non-contradiction. It states that “the same attribute cannot both belong and not belong to the same subject at the same time and in the same respect. . . .”[86] On this, all other principles depend.[87] An easier way of putting it is that nothing can be and not-be at the same time in the same sense. Everyone believes this, even if he denies it in his speech.[88] In fact, his speech already betrays him, for he can only speak against the prin­ciple of non-contradiction if the words he uses have the intended meaning and not the opposite, and so he who claims he denies the principle of non-contradiction could never speak against it. In fact, he could not even be silent, for even understanding what is meant by the principle of non-contradiction presupposes its truth. This is what Aristotle means when he says that “he who destroys reason upholds reason.”[89] It is really and truly impos­sible to deny this fundamental principle of all being and thought.

The principle which follows from that of non-contradiction is the principle of iden­tity, which simply says that a thing is what it is, that everything is identical to itself. In mathematical terms it is simply the necessary truth that A = A. It would be literally absurd to deny this, and it would also be impossible, for to deny it one must understand it, and if it were not true, it could not be understood, for then it would not be what it is but it would either be nothing at all or whatever anyone might want it to be. Like the principle of non-contradiction, it is therefore likewise impossible to deny the principle of identity.

These two principles are “first” because they cannot be positively demonstrated,[90] and this is so because there is nothing more basic from which these principles could be deduced. It would be entirely wrong, however, to think that simply because these princi­ples allow of no demonstration, therefore we merely “assume” them, if by “assume” we mean a sort of arbitrary adhesion in which we might as well not engage. First princi­ples—which, with Bittle, we may simply wish to call “primary truths”[91]—, though not demonstrable, are nevertheless not without any verification or proof. This is so because it is false to think that unless something is demonstrable, it is believed only arbitrarily and unwarrantedly. As Bittle points out:

We must remember . . . that only those truths demand a demonstration which are so obscure in themselves or so doubtful to our mind that we need another and clearer idea to manifest them to us. If they are perfectly clear and self-evident, we do not need a demonstration to make them clear, because the very purpose of a demonstration is already fulfilled. Under such circumstances, therefore, a truth would not be a gratuitous assumption nor an unwarranted presupposition. And that, precisely, is the case with the . . . primary truths: they need no demonstration, because they are self-evident.[92]


We can see here that, ultimately, all inferential chains terminate in self-evident principles, and this is exactly what the foundation of all knowledge is: self-evidence. St. Thomas Aquinas concurs: “[W]e naturally know some things as self-evident. We examine all other things with reference to these, judging of them according to these.”[93] Self-evidence is what provides the basis for any reasoning and makes it possible.

The radical skeptic cannot deny this foundation, because in order to do it he must give or at least have reasons for his denial, and these reasons are part of an inferential chain, and they are thus based upon the very foundation the skeptic denies—or, to be more cor­rect, the foundation he professes to deny, as it is impossible really to deny it. It is impos­sible because his arguments have merit only if foundationalism is true, which amounts to a contradiction because then he is right only if he is wrong. But he cannot hold to a con­tradiction because the only way he could so is by saying that the principle of non-contra­diction is false, an assertion which itself would be meaningless if the principle were false and which would then be impossible to make, as we have seen. Therefore, it is false and absurd to assert, as the skeptic does, that self-evident principles can and must be called into question.

This can be further illustrated by T. V. Fleming’s following example:

To begin with, by seriously affirming “I doubt everything,” one implies that one knows (1) what doubt is, and (2) that doubt differs from knowledge and therefore (3) one knows what knowledge is. Further, the sceptic knows (4) the meaning of the proposition about which he doubts, and (5) the reason why he doubts. Again, by suspending his judgment about the proposition under consideration, he knows (6) that the reasons put forward for it are insufficient to win his assent, and therefore (7) he knows what these reasons are. Finally, since he holds on to his scepticism lest he should fall into error, (8) he knows what error is, and (9) that he does not wish to err, and also (10) that he experiences a desire for truth, etc. . . .[94]


There is perhaps no better example than this that it is impossible to presuppose nothing and to doubt everything. Of course it would be pure folly to seek to do what is impos­sible. We see, therefore, that one of the errors of the Cartesian enterprise, from which modern-day skepticism derives, was to seek to do the impossible. It should be of no sur­prise, therefore, that it precipitated nothing but confusion in the form of a chaotic plu­ralism.

We can see, then, that not everything can be questioned, not everything can be put into doubt, as Descartes thought. Hence, it is not surprising that we should also find that not everything can be demonstrated. The foundation upon which all demonstration is based, and without which it would be impossible, [95] are the primary truths, which are self-evident, because they are such that, as soon as the mind understands them, it necessarily gives its assent. They are not, as Plato thought, “innate” to the mind, as the mind begins as a tabula rasa, a “sheet of paper on which no word is yet written,”[96] which is simply the “default position” of the passive intellect unless and until actualized. Self-evidence, thus, is the ultimate “criterion of truth,” not Descartes’ “clear and distinct ideas.”[97]

It is clear, therefore, that first principles are self-evident and necessarily true. We can and must admit of them before we can engage in any rational exercise, but by this we are not simply “assuming” them, as though we arbitrarily decided to accept them without foundation. As we have seen, the truth of these first principles cannot be demonstrated, but it can nevertheless be justified. The first principles are presuppositions which cannot be false and therefore can and must be presupposed for any philosophical inquiry.

Lest anyone accuse us of presupposing a gratuitous notion of truth, however, we must now briefly turn to the nature and meaning of truth. It would be fair to assert, however, that, at least implicitly, everyone knows what is meant by “truth” and everyone agrees on the definition of the term, since people talk about what is true and false all the time. In fact, simply by offering a definition of “truth,” we already presuppose that we know what we mean by the term to be defined, as we tacitly claim that our definition is in fact true. This only poses a problem, however, to the one who would deny the traditional definition of “truth,” which is simply “conformity of the mind to the thing,”[98] as Bittle puts it, based on Isaac Israeli’s defi­nition endorsed by Aquinas.[99] This really means only that my judg­ment is true if and only if what it expresses corre­sponds to the way things are in reality. This is why Aristotle says that “to say that what is, is, or that what is not, is not, is true.”[100]

This is, of course, a definition entirely in agreement with common sense, something no one would contest, except, perhaps, a sophist masquerading as a philosopher. In fact, as pointed out above, any disagreement with this definition must presuppose its veracity in order to be meaningful. Thus, the skeptic cannot argue that the epistemological realist is only “presupposing” his definition of truth to be correct without demonstrating it—since any requested demonstration would necessarily rely on that very definition under dispute. We can say, therefore, that it cannot be denied without self-contradiction that truth is conformity of the mind to things and that its ultimate criterion is self-evidence.



With all of this in mind now, we are well equipped to answer the skeptical attacks on the possibility and justification of knowledge. By faithfully applying the principles, notions, and doctrines expounded by the Aristotelian-Thomistic tradition, as well as using common sense, we can now counter the arguments brought forth against epistemological realism since the time of Descartes.

When we look closely at Descartes’ own words, we see that as soon as he sets his methodic doubt in motion, he does so on the basis of the representative theory of per­cep­tion: “I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which [some malicious demon] has devised to ensnare my judgment.”[101] The experience we have of the external world is here tacitly relegated to a knowledge not of the world but of mere images, i.e., represen­tations, of this world, as though we were sitting in a theater, directly aware only of the movements on the screen in front of us. Descartes seems to accept this representational theory merely on the grounds that he has found himself to have believed to be perceiving some­thing when in fact he was only sleeping: “How often, asleep at night, am I convinced of just such familiar events—that I am  here in my dressing-gown, sitting by the fire—when in fact I am lying undressed in bed!”[102]

Interestingly enough, the only “proof” that is ever offered for the representative theory of perception is negative in nature—its “truth” is inferred from apparent diffi­culties that are encountered with epistemological realism. If this is really all the repre­sentationalist has to offer, then it follows that if realism can explain and properly account for these apparent difficulties, as we will show below, then the grounds for the represen­tational theory are undermined. Of course, this is entirely crucial because the validity of the critique of knowledge and radical skep­ticism depends upon this very theory to a large extent.

A good way to start, then, is by first rebutting this representative theory of perception. We can do so by appealing to the fact that there are no justifiable grounds to believe in the theory in the first place and by answering the objections to realism that represen­ta­tionalists bring up, as we shall do in the next section; as well as by showing that even if it were true, we could not know it. Since the representationalist cannot simply assert the truth of the theory without any proof,[103] it will suffice to show that even if it were true, we could not know it. This we will do as follows:

If we say that the ideas in our minds are accurate representations of the world outside our minds, then this position is necessarily arbitrary, because it is entirely impossible to verify that the representations in our minds are indeed true compared to the world out­side. Just like a man who has only seen paintings of a relative of his does not know whether the paintings are accurate renditions of the actual person—he simply assumes it—so we could never verify whether the ideas in our minds correspond to the external world, as this world is only accessible through those very ideas. We are thus stuck in a circle and can only arbitrarily “assume” or “hope” that these representations are true—but we could never know it. Hence, any claim that this theory is true is a gratuitous guess at best—and it can therefore be reasonably dismissed.

John Locke was one of the foremost proponents of the representative theory of per­ception, and he invoked the notion of causality to explain how it is that our ideas could accurately represent external reality:

It is . . . the actual receiving of ideas from without [i.e., outside] that gives us notice of the existence of other things, and makes us know that something doth exist at that time without us which causes that idea in us. . . .[104]


In other words, the things outside the mind somehow cause these representational ideas in us. But here the unspoken—though absolutely crucial—premise is that effects necessarily resemble their causes, so much so that the caused ideas are an exact representation of their cause, the thing.

But is this so? It could be so, but there is no necessity for it to be so, and this is what is required in order for the theory to work. Some effects resemble their causes (e.g., a baby resembles mother and father), but some do not (e.g., a cake does not resemble the woman who made it). It is plainly false, then, that all effects resemble their causes.[105] Given this, however, we have no way of knowing whether the things outside the mind produce effects which not only resemble them but in fact reproduce them with total accu­racy. Again we see that representationalism is without justification and hence arbitrary.

Another unjustified and absolutely crucial presupposition the representative theory of perception makes is that man has immediate, unrestricted access to his mind. It is with good reason that the Aristotelian-Thomistic tradition differs with this view, as it is clearly false according to the testimony of our experience. Children, though they are too young still to engage in self-reflection, nevertheless know and interact with the world, which they clearly perceive as outside of themselves. As soon as they are mature enough to reflect on their own knowledge, however, they can know not only things in the world but also the fact that they know those things; i.e., they can know their intellect.

This shows that reflection on the process of knowledge and the intellect and its con­tents does not occur until we have actual knowledge of things. This is so because the intellect is entirely passive until actualized in the process of knowledge:

Everything is knowable so far as it is in act, and not, so far as it is in potentiality. . . . [Therefore,] the intellect, so far as it knows material things, does not know save what is in act. . . . Now the human intellect is only a potentiality in the genus of intelligible beings, just as primary matter is a potentiality as regards sensible beings; and hence it is called possible [=passive]. Therefore in its essence the human mind is potentially understanding. Hence it has in itself the power to understand, but not to be understood, except as it is made actual. But as in this life our intellect has material and sensible things for its proper natural object, . . . it understands itself according as it is made actual by the species abstracted from sensible things, through the light of the active intellect, which not only actuates the intelligible things themselves, but also, by their instrumentality, actuates the passive intellect. Therefore the intellect knows itself not by its essence, but by its act.[106]


In other words, “it is in and through the act of knowing something that the mind becomes aware of its capacity to know.”[107] Our experience bears this out, and everyone can verify it for himself. First there is always knowledge of things, i.e., extra-mental being, and it is only after we have that, that we can reflect on the process of knowledge; and only then can people come up with different epistemological theories, like the representative theory of perception. As Maritain says: “Realism is lived by the intellect before being recog­nized by it.”[108] The rub for representationalists is that if we directly and immediately knew only the contents of our own minds, then we would experience ourselves as doing just that at first, and we would admit of the existence of extra-mental reality only as a sort of “reasonable inference” that is drawn some time later. But this is obviously not the case. In fact, unless we first thought about things, we could not have reflective knowl­edge, since there would be nothing to reflect on: “The first thing thought about is being independent of the mind.”[109] Author Christopher Derrick says as much: “The prime ob­ject of the intellect is reality, not ideas about reality.”[110] For the idealist, on the other hand, it is an axiom that all thought is always about thought. Our experience, however, does not confirm this assumption—on the contrary.

The modern skeptic faces an even more difficult challenge, for he has to realize that  unless we had knowledge of things, he couldn’t even begin to reason his way to the rep­resenta­tive theory of perception, because then he would not have the experience of knowledge. We can see this more clearly when we consider that he asks us to start phi­losophy by making

a double abstraction or removal from things as [we] find them prior to the act of philosophizing . . . The critical philosopher first abstracts this act of knowing from the man possessing the act. Secondly, this abstracted act of knowing turns about and finds it can throw into doubt both the existence of the man who has the act of knowing and the existence of the thing known in the act of knowing.[111]


This is entirely ironic because if it were not for both the knowing man and the thing known, there would be no “knowing” at all, nothing the idealist could analyze and inves­tigate. Paradoxically, starting philosophy with this double abstraction certainly does not qualify as “presuppositionless.”

So, when Descartes decided that the intellect was simply a container closed in on itself to which alone we have direct access, he was putting forth a novelty without justifi­cation. This position is not corroborated by experience, and if it were true, we could not know it, as pointed out above.

While the representative theory of perception may perhaps seem plausible to some at least prima facie, this plausibility diminishes as soon as we consider cases that involve any of the senses other than sight. When I perceive solid ground under my feet as I walk through the field, am I supposed to believe that what I am directly aware of is not the solid ground but merely some “representation” thereof? We can all fathom a visual repre­sentation, but what about tactile images? What about auditory, gustatory, and olfactory images? What are those? How much sense does it make to say that when I taste some­thing, I don’t actually taste the thing but only its representation? The prima facie plausi­bility of the representative theory of perception depends entirely on the sense of sight. But it is entirely reasonable to conclude—and entirely unreasonable to deny, at least absent any evidence—that just as the senses of touch, smell, taste, and hearing put us in touch with the things themselves, so does the sense of sight.

Despite these problems, the representative theory of perception has enjoyed a lot of popularity. We must therefore look at some of the objections broached against realism that have been used to justify the representational theory.



In his First Meditation, René Descartes tells us what sets him onto this course of hyperbolical doubt, namely, the possibility of sensory illusions and dreams:

Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses. But from time to time I have found that the senses deceive me, and it is prudent never to trust completely those who have deceived us even once. […]

How often, asleep at night, am I convinced of just such familiar events—that I am  here in my dressing-gown, sitting by the fire—when in fact I am lying undressed in bed! . . . I see plainly that there are never any sure signs by means of which being awake can be distinguished from being asleep. [112]


In short, Descartes asks: “The senses have deceived me before; how do I know they are not deceiving me now?” and: “I have been asleep dreaming before, and the dream seemed real; how do I know I am not dreaming now?” It is these questions that we will now answer.

We will first tackle the issue of “misleading sensations,” e.g., seeing that a stick looks bent in water, even though it is actually straight. To do this, we shall consider the nature of sensation,  the origin of truth and falsity, and how to distinguish the true from the false.

We have already talked about the nature of sensation at length. “The senses in the act of sensing are always truthful; they cannot err about their proper objects.”[113] This is so because there is simply no room for error, no time at which a mistake could be made. The senses simply report that which is sensed; it is only in the judgment that a mistake can be made regarding what it is that is sensed, for truth and falsity are only found in the judg­ment, as we said earlier. If my senses tell me “hard, brown, and spherical,” then I may mistakenly judge “coconut” instead of “basketball,” but this is not the fault of my senses; it is the judgment that makes the mistake. Though something may look smooth that is in fact rough, the eyes do not deceive us—they faithfully report what there is to be seen. Aquinas makes an important point about this:

Truth and falsity exist principally in the soul’s judgment; . . . Hence, a thing is not said to be false because it always of itself causes a false apprehension, but rather because its natural appearance is likely to cause a false apprehension.[114]


So here again we see that it is the judgment that makes the mistake, not the act of sensing, which simply lets us know how the object appears.[115] It is the judgment that rashly con­cludes that an object is smooth merely because it looks that way. Hence, it is simply false to suggest that the senses can deceive—they cannot; only a failure to consult them prop­erly can, which may or may not always be within our ability (e.g., bad lighting condi­tions, damaged sense organs, etc.[116]).

The real question is therefore: “How can I make sure that I am not making a wrong judgment?” And the answer to this lies once again in the senses. To be sure that some­thing doesn’t only look round but actually is round, we can consult the sense of touch, which, as already mentioned, is the basis for realism.[117] The senses are the only means through which we have contact with the world. That means if something were ever unlike the senses report it to us, we could not know it. Obviously, we can only know things as we know them—but that is through the senses. This is why we can say that “sensation is a first prin­ciple of human knowledge.”[118] Someone could, of course, gratuitously claim that things are totally different from the way in which we experience them, but there is absolutely no reason to believe him because the claim cannot be demonstrated, nor is it self-evident. Such an unfounded hypothesis can simply be ignored, and the only thing that can be said in its favor is that it is logically possible.

At this point it is prudent to note that, interestingly enough, the idealist who claims that his senses have deceived him and are therefore not reliable can know this only on account of the very senses whose reliability he disputes. The only way we know that a stick that looks bent in water is in fact straight is by perceiving it to be straight at all other times, and by touching it in the water and thus verifying through the sense of touch that it is in fact straight. Obviously, then, the idealist does give enough credence to the senses sometimes—it is this credence which lets him realize that at other times the senses “deceive him” (as he would mistakenly say).

In order to be consistent, then, the skeptic could not say that he is ever deceived (either by his senses or his judgment about reality)—all he could say is that he has differ­ent experiences and he doesn’t know which of them are true. Hankinson makes this point well:

The concept of illusion itself, some . . . have urged, is parasitic on that of veridicality, and hence there could be no illusion unless there were truth, just as there can be no fake Monets unless there are genuine ones. These arguments, I think, ultimately fail: at most they show that we could have no concept of illusion unless we had antecedently a notion [i.e., concept] of veridicality – but they do not establish the stronger conclusion that [global] illusions could not occur.[119]


So, no skeptic can say that he is deceived about things in the world—he can only say that he sometimes has experiences of the world that do not correspond to other experi­ences of the world. We can conclude, therefore, that while global illusions are logically possible, we could never know when or whether we are ever the victim of one without falling into contradiction.

But what about dreams? Couldn’t it be that I am only dreaming the world around me? Skeptics say that this is a possibility, but, of course, no one believes that, not even the skeptic; that is, he doesn’t really believe it. No one who thinks that he might only be caught in a dream does the things the skeptic does: he goes to work, takes out the trash, pays the rent, goes to the dentist, applies for a mortgage. This is already plentiful—albeit tacit—testimony that the skeptic does not ascribe any meaningful significance to his proposition that he could only be dreaming. But why should the realist take seriously an objection that not even the skeptic thinks plausible enough to act in accordance with?

Nevertheless, let us examine this issue rationally. We certainly experience ourselves as being awake; this much not even the skeptic doubts. But given this fact, it is reason­able to put the burden of proof on the skeptic—what evidence can he come up with to show that we are in fact asleep and not awake? None, really. He certainly cannot demonstrate it, for if he could, it would no longer be a matter of doubt whether one is asleep or not; it would be a matter of discernible fact. Of course, probably everyone has had a dream which was so lucid, so seemingly real that he mistook it for reality.[120] In fact, St. Thomas says that “there is no one who while sleeping does not regard some of the images formed by his imagination as though they were real.”[121] But in such dreams, can we really do proper rational reflection? Can we really think about dreaming and waking and sufficient evidence for either position as we are doing now? Fleming makes this point quite vividly by showing that

it would be completely baseless and irrational to assert that dreams could produce such highly complex arrangements [as reality does] (e.g., the clues of a “Crossword Puzzle,” or a book of difficult mathematical problems with correct answers).[122]


All evidence simply points to the common-sense supposition that the fact that one is awake is simply self-evident. There is no reason to believe that I am not awake now but dreaming, and given this absence of reasons, the most rational thing one can do is to stick with what is clearly experienced—and that is the state of being awake. If it should indeed be false—and this is logically possible, of course—then, as before, there is no way to prove it. I have every reason to believe that I am awake, because this is what my senses, my reason, my reflections tell me; I have no reason to suppose that I am dreaming. While I have indeed had lucid dreams before, never before have I dreamt with such clarity and vividness as I have when I experience myself to be awake. St. Thomas says that this is so because when we are awake, our intellect controls our imagination, which is not the case when we are dreaming.[123] There is no reason, therefore, to suppose to be dreaming; in fact, all the evidence points against it.

Having dealt with apparent sense illusions and dreams, we must necessarily examine now the issue of hallucinations, for this is another common skeptical argument made against realism. A hallucination may be defined as “an abnormal mental experience in which a person sees or hears things for which there are no external stimuli.”[124] Since halluci­nations seem very real, how do we know we are not hallucinating now? To answer this question, we must consider what we know about hallucinations and their origin.

St. Thomas Aquinas was quite aware, of course, that the intellect can be presented with a phantasm that doesn’t immediately come from the sense organs but in fact from the imagination. He candidly states: “Imagination . . . apprehends a sensible species when the thing is absent. So . . . the imagination usually apprehends a thing as it is not, since it apprehends it as present though it is absent.”[125] Hence, a hallucination, like a dream, would clearly fall into the category of illusions having their origin in the imagination. But the imagination does not simply produce phantasms out of nothing; rather, these phan­tasms have their ultimate origin in sensation, the only open channels man has to the world. St. Thomas says that Aristotle correctly identifies “an affinity between the imagi­nation and the senses, in that imagination presupposes sensation and is found only in sentient beings. . . .”[126] The phantasms, or images, that the imagination presents during hallucinations or dreams, are phantasms the parts of which have ultimately been derived from sensation, their only possible origin: “[M]an cannot sense whatever he pleases; not possessing sense-objects inwardly, he is forced to receive them from outside.”[127]

In other words, while someone may say that he is only hallucinating the world, this hallucination must, at least in its parts, have its origin in the senses presenting reality, i.e., the external world. Otherwise, the hallucination would be bereft of contents, for it is parasitic on true and actual sensations. Therefore, a skeptic cannot argue that he has or has had absolutely no contact with anything other than his own mind. It is this very con­tact which makes hallucinations possible.

Secondly, we must consider that we know of the existence and nature of halluci­nations only from our own sense experience, from our own contact with the world. But it is that same sense experience that tells us that hallucinations occur only extremely rarely, do not last very long, and only occur in people with a mental problem (e.g., schizo­phrenia) or under certain unfavorable circumstances (e.g., being extremely thirsty in the hot desert). So, we must ask: If the skeptic relies on sense experience to admit of halluci­nations deceiving people into mistaking the false for the true, why does he not rely on the same sense experience that tells him that hallucinations are only temporary and occur almost never and only under very restrictive circumstances? On what basis does he extend these conditions to a global scenario? Of course, he does so only on the basis that it is logically possible.

But the truth is, just as we saw concerning dreams, there are no reasons that we should suppose to be hallucinating right now. In the face of the evidence given that we are not hallucinating, it would be entirely irrational and imprudent to suppose the oppo­site merely because it’s a logical possibility. Hence, the skeptic needs to be put on the defensive again and explain to us just why we should believe we are hallucinating. He has no answer.



Several centuries have passed since Descartes’ “evil demon” argument first appeared. Since then, this argument has been updated to appeal to secular modern man, and so in 1983, Hilary Putnam published a new version of the argument in his book Realism and Reason. The argument asks how we can know that we are not simply a brain in a vat being fed the experiences and beliefs we have and which we attribute to extra-mental reality. Brendan Sweetman (b. 1962) has modified Putnam’s argument a bit to make the problem the skeptic presents even clearer.[128]

In essence, we are to imagine an evil scientist, Dr. Frankenstein, who has found a way to take live human brains out of their skulls and put them in vats and keep them alive this way. By means of cleverly-devised computer programs, he can feed each brain all sorts of experiences and beliefs, such that the brain itself receives the impression that it is having actual sense experiences. Worse yet, Dr. Frankenstein has also devised a program called “Logic II,” which is

a new system of logic . . . that . . . enables the brain to validly arrive at what it thinks are true conclusions from premises that are, in fact, false. The conclusions present themselves to the brain-in-the-vat with the same power, sense of certainty, etc., as true conclusions would which were arrived at in ordinary experience.[129]


In short, Dr. Frankenstein has the power to cause in the brain the exact same experiences that the brain would have if it were not in a vat but seated in a skull of a normally-functioning human  being. The problem that seems to pose itself to us now is simply this: How do we know that we are not such a brain in a vat being fed all of our experiences and beliefs? This is very similar to Descartes’ evil demon theory, but it moves it up a notch by attacking also the reliability of logic.

In response, we should first note that this argument, whatever it could logically prove, cannot prove that there is not an external world, as this is definitely excluded by the premises of the argument itself, for the brain, the vat, the evil scientist, etc. all exist in the external world. At best, this argument could be used to show that we cannot know whether the external world is in fact as we experience it.

Secondly, we can easily dismiss the hypothesis of being rigged on “Logic II.” The reason for this is that, as we have implicitly shown already, it is impossible that there be “another logic.” A different logic makes as much sense as a square circle. It is a total contradiction, and, Hegel notwithstanding, it cannot be admitted any more than a denial of the principle of non-contradiction can be admitted. In fact, by even speculating about whether Logic II is possible or not, we are excluding its possibility, since our theorizing is based on true logic (which we may call “Logic I”).[130] Just as a verbal denial of the prin­ciple of non-contradiction presupposes that very principle, so exploring the possi­bility (or even conceiving the notion) of Logic II presupposes the truth of Logic I. The reason Logic I can never rationally be assailed is that, like mathematics, logic is not something we make up or determine but something we discover and discern as present in reality.

Thirdly, we must once again point out that there is absolutely no reason to believe that we are a brain in a vat and that the scenario Sweetman describes is in fact true.[131] While Sweetman himself uses an example in which the main character has good reason to believe that he might be a brain in a vat, we know that keeping a human being alive by keeping the brain alive is impossible,[132] and hence the scenario could never obtain. So, once again the skeptic must resort to supporting his challenge only on the grounds that it is logically possible.



We must now turn to the issue of logical possibility. To say that something is logi­cally possible only means that asserting it does not involve a contradiction. But this includes even the most absurd of scenarios:

For example, it is logically possible for objects to fall faster or slower depending on their color; for you to chin yourself 6 million times in quick succession; for a man to live to the age of a million years; for your dogs to give birth to kittens.[133]


Yes, it is logically possible that your ears fly to the moon and that your house tap-dances on your forehead, because none of this involves self-contradiction. But so what? It would be absurd to suggest that from a mere absence of self-contradiction we can deduce any­thing relevant about reality.

It should be easily apparent here that mere logical possibility does not suffice to provide us with good reason to take the possibility seriously, for “the notion of logical possibility is much too broad to function as a criterion for deciding what possibilities to take seriously in human experience.”[134] Thus, the skeptic’s argument that what he proposes is logically possible is virtually irrelevant if he has nothing else to say to con­vince us that we should take his proposed scenario seriously. George Mivart draws out this point quite clearly:

If any one chooses to assert that stones are living things, accidently [sic] prevented by circumstances from showing forth their latent life, and that all plants are sensitive beings, accidently [sic] hindered from making their sensitivity manifest, we cannot, of course, refute him; but we also cannot but regard him as . . . credulous.[135]


Therefore, the mere asset that his suggested scenarios are “logically possible” really does not lend any significant credibility to the skeptic’s claim.

There is one more point concerning this issue that needs to be addressed. If, as we have said, it is self-evident that there exists a world outside of our mind that is inde­pendent of us and what we think, then it follows that it is not possible, not even logically so, that we should be dreaming or being deceived by an evil scientist. Sweetman makes this point clearly: “If I am here now, I cannot (logically cannot) be in a vat, or dreaming, or the subject of deception by an evil genius, etc.”[136] The question, of course, is, “Do I know I am here now?” but we have already seen that this is self-evident. I perceive myself to be here now, and perception always entails the existence of the thing perceived. The skeptic will say that the dreamer thinks the same thing and is yet dreaming, but we have already seen that no dream could produce as highly complex and accurate experi­ences as we encounter in reality. Besides, any dream could probably be identified as such if only one could properly reflect on it and contemplate its contents critically, as we are doing now examining the issue of dreaming.

We have already mentioned that aside from all arguments, everyone is a realist in practice. Even the skeptic is a realist as soon as he forgets to act the part. This is strikingly illustrated by Christopher Derrick’s recounting of an experience he had with two young philosophy students who claimed that the mind could know nothing outside of itself:

[S]oon it was time for them to go, and they started worrying about the time of their train. I pointed out, mildly, that since there was no real and knowable world within which their train could have any objective “out there” existence, their anxiety was misplaced. This irritated them a little: philosophy (I was given to understand) was one thing, but the practical business of daily life was another.

. . . When pressed, they admitted that for them and their instructors too, philosophy amounted to little more than a word-game, making no real claim to yield “truth”.[137]


The skeptic may reply, however, that he acts the way he does because while he is very much convinced that we cannot know that there is an external world and that things are the way we perceive them, it is still very “probable” that this is so, and, therefore, in his daily actions he relies on probability or “probable knowledge.”

But is this a good defense or only a copout? In ancient times, St. Augustine was faced with the same problem when he battled the Academics. The first step we must take to evaluate this “probability thesis” is to define what is meant by “probable.” The Academics defined it as “what-is-like-truth,”[138] and this is exactly what the word means.[139] Augustine quickly pointed out the folly into which the Academics had thus put them­selves, saying to Licentius: “Your Academics [ought to be] laughed at, since they say that in practical matters they follow ‘what-is-like-truth,’ although they do not know what truth itself is.”[140] This response is straight to the point: someone who says he does not (and perhaps cannot) know what truth is, cannot turn around and say that he does know, however, what is similar to truth. If I do not know the standard by which to judge, I cannot know whether anything is similar to that standard; if I do not know what my grandmother looks like, I cannot say who resembles her.

Nevertheless, the skeptic may have a rejoinder. He may assert that when he says that something is probable, he simply means that he has good reason to believe it. But even this will not do, as the skeptic also insists that we have good reason to be skeptics, to doubt the external world, etc., and so he is actually saying that we have good reason for two contradictory positions at the same time. But in this case, neither of them can be con­sidered “probable” but only “plausible” at best. Besides, if we are stuck in Cartesian skepticism, as the skeptic says we must be, then we must also agree with Descartes regarding the senses that “it is prudent never to trust completely those who have deceived us even once.”[141] This point is made even stronger by epistemologist Myles Burnyeat:

If the evidence of our senses is really shown to be unreliable and the inferences we ordinarily base on this evidence are unwarranted, the correct moral to draw is not merely that we should not claim to know things on these grounds but that we should not believe them either.[142]


Thus we can see that the skeptic who holds to the probability thesis has no grounds to stand on, and his position is indeed a mere copout. He is forced to admit either that his non-skeptical behavior in practice is hypocritical or that we do indeed know that skepti­cism is false.



We all know, of course, that the scenarios of brains in vats, evil geniuses, etc. are all made up. They were invented, contrived, and fabricated in order to pose a problem that naturally does not occur, that naturally does not impose itself on our minds as a real and significant issue. While we all wonder every now and then about whether a particular experience of ours in fact corresponds with reality, at no point does a given situation ever suggest to us that we should subject our own knowledge and first epistemological prin­ciples to a “critique.”

In short, the skeptical problem is artificial. Descartes himself, to whom all of these problems can be traced, admitted that he was raising the issue of knowledge of the external world only in a “make-believe” fashion; it was all a matter of method, not of sincere wonder or doubt. It was never meant as a serious problem to begin with. It was contrived from the very outset, and due to a weak knowledge of the justification of epis­temological realism and an obsession with applying mathematical certainty to non-mathematical knowledge, Descartes slipped into this predicament we now call the “critique of knowledge,” which has shaped all like philosophy since.

However, how surprising is it that an artificial problem should not have a natural response? If the challenge that is raised is itself only artificial, must we not also expect an artificial solution? Some such artificial solutions were given early on, such as the occasionalism of Malebranche, the infinite attributes of Spinoza, and the psycho-physical parallelism of Leibniz. Why does the skeptic not accept these solutions? Are they too artificial, too absurd? Indeed they are, but no more artificial or absurd as the challenge itself. If the skeptic is willing to entertain an absurd artificial problem, he should also be willing to accept an absurd artificial solution.

The realist, on the other hand, does not face this dilemma of the skeptic. The realist recognizes that there is no real problem to begin with. If there are reasons to doubt what he thinks he knows, he looks at these reasons and proceeds to eliminate them or to rectify his knowledge. Following the advice of St. Augustine and all other realists, he consults his senses and his reason if he suspects that something he seems to be perceiving is not the way he judges it to be. But never does he abandon his reason, the knowledge that reason is reliable, and his fundamental convictions that are not demonstrable but none­theless knowable because self-evident. The realist knows that both knowledge and skep­ticism are a matter of evidence, and there is nothing more basic than self-evidence, the ultimate and infallible criterion of knowledge.



“I think nobody can, in earnest, be so sceptical as to be uncertain of the existence of those things which he sees and feels. At least, he that can doubt so far . . . will never have any controversy with me: since he can never be sure I say any thing contrary to his opinion.”


John Locke[143]


As we conclude this study, it is appropriate to give a brief review of what we have done. We started out looking at the problem of skepticism in its historical context. Though skepticism existed already in ancient times, the problem as it has come to char­acterize modern philosophy began in the seventeenth century by René Descartes, who, though intending it only as a method and never as genuine, embarked on a course of radi­cal doubt that questioned not only abstract metaphysical truths but absolutely every­thing that was logically dubitable. The resulting “indubitable truth” of his own existence came to shift all of philosophy to the ego that Descartes defined only as a res cogitans, a “thinking thing,” not a living body whose form is a rational soul, as had hitherto been taught by the Aristotelian-Thomistic tradition. This bifurcation of the world of spirit and the world of bodies precipitated the “critical problem,” the question of whether there exists anything outside of the knowing subject and, if so, whether there can be true knowledge of these things.

Realizing that it is impossible to analyze the workings of empirical knowledge with­out having some kind of conception of the nature of the knower, man, we then proceeded to take the “pre-critical” attitude and investigate how sense knowledge is said to work in the realist tradition of Aristotle and St. Thomas Aquinas. We examined the first epistemo­logical principles taught by this tradition and found them anchored in unshak­able foundations. At the same time we saw that, quite ironically, the critical paradigm holds to gratuitous presuppositions and contains flaws that cannot stand up to close scrutiny, most especially the representative theory of perception.

Next, we took a critical look at the major arguments brought forth by the radical skeptics, i.e., the arguments from dreams, illusions, and artificial deceptions, and we saw that they cannot adequately refute the direct realist position of Aristotle and St. Thomas. They are not strong enough to force the conclusion the skeptic desires, which is further attested to by the skeptic himself, who acts like a realist in his daily affairs. Applying the insights of the Aristotelian-Thomistic tradition shows that the critical problem is unnatural, absurd, and not a genuine problem but rather an artificially invented problem for which, if taken seriously, there can only be an artificial (and untenable) “solution.”

We have thus shown that not only is there no reason to accept the critical paradigm, there is also ample evidence to reject it and instead to hold to the epistemological realist framework as exemplified in the philosophies of Aristotle and St. Thomas. It is, of course, ironic that the critique of knowledge, originating in an effort to purge the mind of all precon­ceived opinions and biases, should itself supervene on unwarranted presuppo­sitions; but it clearly reinforces the admonition of Pope Leo XIII about how easy it is for the mind to slip from doubt into error.[144] Since doubt is the very essence of the critical problem, we should not be astonished that this issue has caused centuries of dispute and debate of the most agonizing kind, so much so that contemporary nihilist Peter Unger reached the utterly erroneous and self-contradictory conclusion that “not only can nothing ever be known, but no one can ever have any reason at all for anything” and that “no one can ever believe, or even say, that anything is the case.”[145] In the face of such absurdity, one can only sympathize with St. Augustine, who stated in his disputes with the Academic skeptics: “I could no longer laugh, but was half angry and half sorrowful that men so learned and intelligent should have descended to such criminal and shameful doctrine.”[146]

Augustine rightly characterizes the sophistry of the skeptics as “criminal,” as it is indeed very dangerous and paralyzing poison for the inquiring mind, whose task it is to “seek with all [its] strength for truth.”[147] If the philosopher begins his inquiries in the wrong way, it is more than likely that his conclusions will be tainted by error. This is especially and clearly so when the starting point he takes is the critical paradigm, for, attempts of several confessed Thomists[148] notwithstanding, no one who starts with thought or mind can ever arrive at the extra-mental world, as Etienne Gilson has demon­strated in several of his works.[149]

In short, choosing the right starting point is crucial for proper philosophical investi­gation. Maritain likens the modern skeptic to a

logician who would deny reason, a mathematician who would deny unity and duality, a biologist who would deny life. From the moment they set out, they have turned their back on philosophic knowledge and philosophical research. They are not philosophers.[150]


What, then, are they? Maritain calls them “ideosophers”,[151] but perhaps we should call them neo-sophists. This is not meant to be derogatory but simply factual. The Cartesian legacy gives witness to this. At the dawn of the twenty-first century, the major points of disagreement in the philosophical pluralism of our day virtually dwarf the relatively minor quarrels of the medieval scholastics. Ironically, as we noted at the beginning, it was precisely those quarrels which, more or less directly, triggered the Cartesian enter­prise in the first place.

What more proof is needed to show that starting out “critically” is the wrong thing to do? The realist starts with what experience gives him, the same experience that the ideal­ist has prior to his “critique.” But, unlike the idealist, the realist sees no sufficient grounds for dismissing this “pre-critical” attitude because he realizes that the arguments brought forth against epistemological realism, such as the argument from supposed perceptual illusions and hallucinations, can be answered well within the realist paradigm. That is, if we simply start from our everyday experience—as “presuppositionless”[152] a starting point as it gets—we realize that at no point is there any justification for abandoning the realist attitude towards knowledge that everyone, even the skeptic, harbors within himself until he artificially and gratuitously rejects it for his representative theory of perception. Hence we can “validate” the pre-critical paradigm simply by pointing out that it is given in experience and that there are no sufficient grounds for adopting any other paradigm, cer­tainly not one that conflicts with experience as much as the representative theory of per­ception does. Thus denying the skeptic his starting point, we will be prevented from falling into the representationalist traps.

In sum, then, we can say that by temporarily presupposing the Aristotelian-Thomistic notion of knowledge and its workings, the critique of knowledge can be exposed to rest on unfounded premises and to supervene on a failure to understand the realist response to some common difficulties that can come up within the realist framework. Once we see that the idea of “critical” and “presupposi­tionless” philosophy is in fact merely a ruse, the problem of “validating” the Aristotelian-Thomistic model diminishes, since its teachings are based on sound “uncritical” reasoning applied to the evidence of experience and fundamental metaphysical truths. It is of no surprise, therefore, that Pope Pius XI advised the world that “if we are to avoid the errors which are the source and fountain-head of all the miseries of our time, the teaching of Aquinas must be adhered to more religiously than ever.”[153] More than 350 years of post-Cartesian philosophy bear this out.



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[1] Frederick D. Wilhelmsen, Man’s Knowledge of Reality: An Introduction to Thomistic Epistemology (Englewood Cliffs, NJ: Prentice-Hall, 1956), p. 40.

[2] Pope Leo XIII, Encyclical Letter Aeterni Patris, August 4, 1879 (Boston, MA: Pauline Books & Media, n.d.), par. 2, p. 4.

[3] Ibid., par. 31, p. 21.

[4] Martin Heidegger, Being and Time, trans. John Macquarrie and Edward Robinson (New York, NY: Harper & Row, 1962), p. 249; italics given.

[5] Leo XIII, Aeterni Patris, par. 31, p. 21.

[6] Etienne Gilson, The Unity of Philosophical Experience (San Francisco, CA: Ignatius Press, 1999), p. 49.

[7] See R. J. Hankinson, The Sceptics (New York, NY: Routledge, 1995), p. 31.

[8] See ibid., pp. 13-18.

[9] Sextus Empiricus, Outlines of Scepticism, ed. Julia Annas and Jonathan Barnes (New York, NY: Cambridge University Press, 2000), I:12, p. 6. In I:202, Sextus explains that he is speaking of accounts that allege to establish the truth about “something unclear.” That is, the Pyrrhonist does not question absolutely everything, e.g., his appearances or the fact that he is suspending judgment.

[10] See ibid., I:226, p. 59.

[11] Cf. Hankinson, The Sceptics, pp. 14-16.

[12] St. Augustine, Against the Academics [Contra Academicos], trans. John J. O’Meara (New York, NY: Paulist Press, 1978), III:18, p. 118.

[13] See Gilson, Philosophical Experience, pp. 69-72.

[14] Michel de Montaigne, “Apology for Raymond Sebond,” ll. 692-95, in Selections from the Essays, trans. Donald M. Frame (Arlington Heights, IL: AHM Publishing, 1972), p. 68.

[15] Ibid., ll. 811-14, p. 71.

[16] Gilson, Philosophical Experience, p. 74.

[17] René Descartes, Discours de la méthode/Discourse on the Method: A Bilingual Edition with an Interpretive Essay, trans. George Heffernan (Notre Dame, IN: University of Notre Dame Press, 1994), I:12, p. 21.

[18] Ibid.

[19] In fact, in his Meditations on First Philosophy, Descartes does say he merely withholds assent, but he does so “just as carefully as I do from those [opinions] which are patently false.” See René Descartes, Meditations on First Philosophy, trans. John Cottingham (New York, NY: Cambridge University Press, 1996), I, p. 12.

[20] Descartes, Discourse on the Method, II:2, p. 29.

[21] Descartes, Meditations, I, p. 12.

[22] This statement needs qualification, which will be given below.

[23] M. F. Burnyeat, “Idealism and Greek Philosophy: What Descartes Saw and Berkeley Missed,” Philosophical Review 90 (1982): p. 19.

[24] Descartes, Meditations, I, p. 15.

[25] Ibid.

[26] For instance, Descartes never made an effort to doubt the rules of logic or to doubt his very own methodical doubt as a proper method for arriving at certainty.

[27] Cf. Descartes, Discourse on the Method, III:6, p. 47.

[28] See Descartes, Discourse on the Method, IV:1, p. 51.

[29] Descartes, Meditations, II, p. 18.

[30] Ibid., VI, p. 54.

[31] Wilhelmsen, Man’s Knowledge of Reality, p. 14.

[32] Among others, Descartes used a modified version of St. Anselm’s ontological argument. See Meditations, III, pp. 24-36, and Discourse on the Method, IV:5, pp. 55-57.

[33] Descartes, Meditations, III, p. 24. See also Descartes’ replies to the seventh set of objections, where he nonchalantly claims that “nothing can be clearly and distinctly perceived . . . without being true” (ibid., p. 67).

[34] Burnyeat, “Idealism and Greek Philosophy,” p. 33.

[35] We will critically examine this theory in more detail in chapter 8.

[36] The one thinker most clearly endorsing it was perhaps John Locke (1632-1704). See John Locke, An Essay Concerning Human Understanding (Amherst, NY: Prometheus Books, 1995), IV:I.1, p. 424.

[37] Etienne Gilson, Methodical Realism, trans. Philip Trower (Front Royal, VA: Christendom Press, 1990), p. 29.

[38] For purposes of this paper, I am treating “representationalist,” “idealist,” and “(radical) skeptic” synonymously, unless otherwise indicated.

[39] See St. Thomas Aquinas, Truth, vol. 1, trans. Robert W. Mulligan (Indianapolis, IN: Hackett Publishing Co., 1994), q. 2, art. 6, ad. 3, p. 94. Cf. Aristotle, De Anima, 408b15.

[40] Cf. Celestine Bittle, Reality and the Mind (Milwaukee, WI: Bruce Publishing Co., 1936), pp. 83f.

[41] John F. X. Knasas, Being and Some Twentieth-Century Thomists (New York, NY: Fordham University Press, 2003), p. 34.

[42] Aristotle, Metaphysics, 980a21. Translation taken from St. Thomas Aquinas, Commentary on Aristotle’s “Metaphysics,” trans. John P. Rowan, rev. ed. (Notre Dame, IN: Dumb Ox Books, 1995).

[43] See, e.g., Bittle, Reality and the Mind, pp. 14f.; George P. Klubertanz, The Philosophy of Human Nature (New York, NY: Appleton-Century-Crofts, 1953), pp. 82f.

[44] Aquinas, Truth, vol. 1, p. 54, q. 2, art. 1, ad cont. 3.

[45] See Plato, Phaedrus, 250, in John M. Cooper, ed., Plato: Complete Works (Indianapolis, IN: Hackett Publishing Co., 1997), p. 527.

[46] Aristotle, De Anima, 415b8. Translation taken from St. Thomas Aquinas, Commentary on Aristotle’s “De Anima, trans. Kenelm Foster and Silvester Humphries, rev. ed. (Notre Dame, IN: Dumb Ox Books, 1994).

[47] St. Thomas Aquinas, Summa Theologica, vol. 1, trans. Fathers of the English Dominican Province (Allen, TX: Christian Classics, 1981), p. 370, I, q. 76, art. 1.

[48] Aristotle, Physics, 194a12. Translation taken from St. Thomas Aquinas, Commentary on Aristotle’s “Physics, trans. Richard J. Blackwell, Richard J. Spath, and W. Edmund Thirlkel, rev. ed. (Notre Dame, IN: Dumb Ox Books, 1995), p. 87.

[49] Even the skeptic will have to admit that if he is only a mind, then it cannot be he who speaks, types, and writes about skepticism, since these are all acts of his body.

[50] As Fr. Robert Henle argues, the sense of touch is in fact the basis for all epistemological and thus perceptual realism. See Robert J. Henle, “The Basis of Philosophical Realism Re-examined,” The New Scholasticism 56 (1982): pp. 1-29.

[51] See Aquinas, Summa Theologica, vol. 1, p. 58, I, q. 12, art. 12; St. Thomas Aquinas, Truth, vol. 2, p. 121, q. 12, art. 3, ad 2.

[52] Mortimer J. Adler, Intellect: Mind Over Matter (New York, NY: Macmillan Publishing Co., 1990), p. 96.

[53] Aristotle, De Anima, 424a18-21.

[54] Aquinas, Commentary on Aristotle’s “De Anima, p. 172, II, lectio 24, no. 553.

[55] See William A. Wallace, The Modeling of Nature: Philosophy of Science and Philosophy of Nature in Synthesis (Washington, DC: Catholic University of America Press, 1996), p. 134.

[56] D. Q. McInerny, Philosophical Psychology (Elmhurst, PA: Alcuin Press, 1999), p. 117.

[57] Ibid., p. 116.

[58] See Wilhelmsen, Man’s Knowledge of Reality, pp. 95f.

[59] Cf. Aquinas, Summa Theologica, vol. 1, pp. 433f., I, q. 85, art. 2.

[60] Aquinas, Commentary on Aristotle’s “De Anima, p. 230, III, lectio 12, no. 772. Cf. Aristotle, De Anima, 431a17.

[61] Aristotle, De Anima, 414a31-32.

[62] Aquinas, Summa Theologica, vol. 1, p. 397, I, q. 79, art. 2. Cf. Aristotle, De Anima, 429a13-24.

[63] Aquinas, Summa Theologica, vol. 1, p. 449, I, q. 88, art. 1; italics given.

[64] Aquinas, Commentary on Aristotle’s “De Anima, p. 219, III, lectio 10, no. 728.

[65] Aquinas, Summa Theologica, vol. 1, p. 72, I, q. 14, art. 1.

[66] See ibid., p. 452, I, q. 89, art. 1.

[67] Cf. ibid., p. 72, I, q. 14, art. 1; p. 431, q. 85, art. 1.

[68] Cf. Aristotle, De Anima, 429a22-24.

[69] Aquinas, Truth, vol. 2, p. 28, q. 10, art. 6.

[70] Ibid., p. 29, ad 7.

[71] Aquinas, Summa Theologica, vol. 1, pp. 440f., I, q. 86, art. 1.

[72] Ibid., p. 429, q. 84, art. 7.

[73] Aristotle, De Anima, 430a19.

[74] This has alternatively been called “intentional inexistence,” inasmuch as it is existence in the knower. See Herbert Spiegelberg, “Der Begriff der Intentionalität in der Scholastik, bei Brentano und bei Husserl,” Philosophische Hefte 5 (1936): p. 85.

[75] Cf. the quote by Mortimer Adler in chapter 5.

[76] For a response to anti-realist arguments from the possibility of dreaming and hallucinating, see Part IV of this paper.

[77] Robert J. Henle, “Apropos of From Unity to Pluralism by Gerald McCool, S.J.,” in John F. X. Knasas, ed., Thomistic Papers VI (Houston, TX: Center for Thomistic Studies, 1994), p. 148.

[78] Jacques Maritain, The Dream of Descartes, trans. Mabelle L. Andison (New York, NY: Philosophical Library, 1944), p. 10.

[79] Aristotle, Posterior Analytics, 71a1-2. Translation taken from Aristotle, The Complete Works of Aristotle: The Revised Oxford Translation, ed. Jonathan Barnes, vol. 1 (Princeton, NJ: Princeton University Press, 1984), pp. 114-66.

[80] They may not be presupposed immediately but simply be other conclusions of previous chains of reasoning; but ultimately, some premises must be accepted without proof, or else one would not be able to argue for—or even believe—anything. See also the discussion on self-evidence below.

[81] In fact, one may wonder whether the idealist’s conviction that it is necessary to start without any presuppositions is a presupposition in itself.

[82] An example of this would be Jean-Paul Sartre’s argument for the non-existence of God. Sartre claimed the notion of God was self-contradictory, but his argument is ultimately based on the unjustified—and false—assumption that consciousness is not-being. See Frederick Copleston, A History of Philosophy, vol. 9 (New York, NY: Doubleday, 1994), p. 364.

[83] T.V. Fleming, Foundations of Philosophy (London: Shakespeare Head, 1949), p. 13.

[84] Though we need not treat of them here, a number of self-evident axioms and postulates are provided, explained, and defended in Fleming, Foundations of Philosophy, pp. 13-20, 22-25.

[85] Aristotle, Metaphysics, 1005b11-12.

[86] Ibid., 1005b19-20.

[87] See Aquinas, Summa Theologica, vol. 2, p. 1009, I-II, q. 94, art. 2.

[88] Even in Aristotle’s times, people tried to deny this principle in their speech. But, Aristotle observed, “what a man says he does not necessarily accept” (Metaphysics, 1005b25-26), and this is especially so when his actions contradict what he professes to believe, as is the case with anyone who claims to reject the principle of non-contradiction.

[89] Aristotle, Metaphysics, 1006a26.

[90] It is possible, however, to demonstrate the truth of these principles negatively, i.e., by reductio ad absurdum, showing that assuming the opposite leads to an impossibility, as we will do below.

[91] Bittle uses the term specifically to refer to the three undeniable truths of our own existence, the principle of non-contradiction, and the general reliability of our reason (see Bittle, Reality and the Mind, p. 67), but we can nevertheless adopt this phrase and use it to refer to any indemonstrable, self-evident truth.

[92] Bittle, Reality and the Mind, pp. 69f.; italics given.

[93] Aquinas, Truth, vol. 2, p. 42, q. 10, art. 8.

[94] Fleming, Foundations of Philosophy, pp. 27f.; italics given. See also Bittle, Reality and the Mind, pp. 47-50.

[95] Aristotle realized this centuries ago: “It is not possible to understand through demonstration if we are not aware of the primitive, immediate, principles” (Posterior Analytics, 99b20-21).

[96] Aquinas, Commentary on Aristotle’s “De Anima, p. 217, III, lectio 9, no. 722.

[97] See Descartes, Meditations, III, p. 24.

[98] Bittle, Reality and the Mind, p. 19; italics deleted. Diametrically opposed to this traditional definition was Kant’s notion of truth, which we may candidly define as “conformity of the thing to the mind.”

[99] See Aquinas, Truth, vol. 1, p. 7, q. 1, art. 1. There are two other definitions of “truth” which St. Thomas gives in this article, but they need not concern us here.

[100] Aristotle, Metaphysics, 1011b27.

[101] Descartes, Meditations, I, p. 15.

[102] Ibid., p. 13.

[103] This is so because quod gratis affirmatur, gratis negatur—what is gratuitously asserted can be gratuitously denied. Cf. the quote by George Mivart on page 52.

[104] Locke, Essay, IV:XI.2, p. 537.

[105] Cf. Bittle, Reality and the Mind, pp. 188f.

[106] Aquinas, Summa Theologica, vol.1, p. 444, I, q. 87, art. 1.

[107] Copleston, A History of Philosophy, vol. 9, p. 262.

[108] Jacques Maritain, The Degrees of Knowledge, trans. Gerald B. Phelan (Notre Dame, IN: University of Notre Dame Press, 1995), p. 83.

[109] Ibid., p. 115.

[110] Christopher Derrick, Escape from Scepticism: Liberal Education as if Truth Mattered (San Francisco, CA: Ignatius Press, 2001), p. 89.

[111] Wilhelmsen, Man’s Knowledge of Reality, pp. 26f.

[112] Descartes, Meditations, I, pp. 12f.

[113] Aquinas, Commentary on Aristotle’s “De Anima, p. 198, III, lectio 5, no. 645.

[114] Aquinas, Truth, vol. 1, p. 45, q. 1, art. 10; italics added.

[115] See also St. Augustine, Against the Academics, pp. 127f.

[116] Cf. Aquinas, Truth, vol. 1, p. 49, q. 1, art. 11.

[117] See footnote 50.

[118] Wilhelmsen, Man’s Knowledge of Reality, p. 34; italics deleted.

[119] Hankinson, The Sceptics, p. 22; italics given.

[120] We may ask here that while everyone has mistakenly thought before that he was awake when he was in fact dreaming, who has ever mistakenly thought himself to be dreaming when he was in fact awake?

[121] St. Thomas Aquinas, Summa Theologica, vol. 4, p. 1813, II-II, q. 154, art. 5.

[122] Fleming, Foundations of Philosophy, p. 80; italics given.

[123] Aquinas, Commentary on Aristotle’s “De Anima, p. 203, III, lectio 6, no. 670.

[124] McInerny, Philosophical Psychology, p. 120; italics given.

[125] Aquinas, Truth, vol. 1, p. 49, q. 1, art. 11.

[126] Aquinas, Commentary on Aristotle’s “De Anima, p. 201, III, lectio 6, no. 657.

[127] Ibid. p. 121, II, lectio 12, no. 375.

[128] See Brendan Sweetman, “The Pseudo-Problem of Skepticism,” pp. 230-32, in Brendan Sweetman, ed., The Failure of Modernism: The Cartesian Legacy and Contemporary Pluralism (Mishawaka, IN: American Maritain Association, 1999), pp. 228-241. It should be noted here that Sweetman himself is not a skeptic but has devised this argument to demonstrate where the real problem with skepticism lies, as we will see.

[129] Ibid., p. 231.

[130] To prove this, recall simply Logic I’s principle of identity.

[131] This Sweetman readily admits. See ibid., p. 235.

[132] It is enough to remove a man’s stomach to make him die—not to mention removing everything but his brain. While we could imagine a scenario in which people in their bodily entirety are kept alive but their brains are manipulated by an evil scientist in the same manner as described by Sweetman, we likewise have no reason to believe that this scenario currently obtains or that even this is possible, for it would seem extremely difficult to imagine how anyone could verify or know the contents of someone else’s thoughts, emotions, and experiences, which would be necessary in order to control them.

[133] John Hospers, An Introduction to Philosophical Analysis, 3rd ed. (Englewood Cliffs, NJ: Prentice Hall, 1988), p. 133; italics given.

[134] Sweetman, “Skepticism,” p. 236.

[135] George J. Mivart, The Origin of Human Reason (London: Kegan Paul, Trench & Co., 1889), p. 13; qtd. in Fleming, Foundations of Philosophy, p. 24.

[136] Sweetman, “Skepticism,” p. 237, n. 8; italics given.

[137] Derrick, Escape from Scepticism, pp. 79f.

[138] St. Augustine, Against the Skeptics, p. 82.

[139] We can see this by looking, for instance, at the German equivalent, wahrscheinlich, which literally means “seemingly true,” or the French vraisemblable, literally, “resembling truth.”

[140] St. Augustine, Against the Skeptics, p. 85.

[141] Descartes, Meditations, I, p. 12.

[142] Myles Burnyeat, “Can the Skeptic Live his Skepticism?”, p. 27, in Myles Burnyeat and Michael Frede, eds., The Original Sceptics: A Controversy (Indianapolis, IN: Hackett Publishing Co., 1997), pp. 25-57.

[143] Locke, Essay, IV:XI.3, p. 537.

[144] Leo XIII, Aeterni Patris, par. 24, p. 18.

[145] Peter Unger, Ignorance: A Case for Scepticism (New York, NY: Oxford University Press, 2002), back cover.

[146] St. Augustine, Against the Academics, p. 139.

[147] Ibid., p. 97.

[148] E.g., T. V. Fleming, Cardinal Désiré Mercier, and Bernard Lonergan come to mind.

[149] E.g., Gilson, Methodical Realism, passim; Etienne Gilson, Thomist Realism and the Critique of Knowledge, trans. by Mark A. Wauck (San Francisco, CA: Ignatius Press, 1986), passim.

[150] Jacques Maritain, The Peasant of the Garonne: An Old Layman Questions Himself about the Present Time, trans. Michael Cuddihy and Elizabeth Hughes (New York, NY: Holt, Rinehart and Winston, 1968), p. 100.

[151] Ibid., p. 102; italics deleted.

[152] It may be called “presuppositionless” inasmuch as it takes as its data simply what is given in experience and works with it; it does not add to it or subtract anything from it.

[153] Pope Pius XI, encyclical letter Studiorum Ducem, June 29, 1923, Pius11/P11STUDI.HTM (accessed August 12, 2003), par. 27.