THE FIRST PRINCIPLES
The First principles are self evident or known through themselves (per se nota). They are primary truths immediately perceived by the intellect, i.e. without demonstration. In this sense, they are objects of simple intuition. The first principles are perceived in the idea of being, which is the formal object of our intellect. The four main ones are; identity, sufficient reason, causality and finality.
1. The Principle of Identity
Intelligible extra mental being, when apprehended, divides itself into two aspects: being given to the mind and being recognized by the mind. We identify these two objects conceptually different but really identical in formulating the principle of identity "ens est ens".
J. Maritain explains it as each being is what it is". i.e. "ens under two different aspects, being as simply existing and being with a particular essential determination. Fr. Garrigou-Lagrange explains it as "every being is of a determinate nature, i.e. "ens est res". in other words we are concerned with two different transcendentals. But both explanations are only slightly different.
The principle of identity can be formulated in a logical form called principle of non-contradiction: "it is impossible for the same thing to be affirmed and denied at the same time".
The principle of identity is not only a subjective law of thought, but also an objective law of reality. In other words a round square is not only unthinkable, it is also unrealizable. Not even God can create something absurd. He can make an exception to a physical Law (a miracle) but not a metaphysical law.
2. The Principle of Sufficient Reason
In this principle, being divides itself into a different manner than the principle of identity; on the one hand, there is being simply existing, ens. On the other hand, there is being as having a ground for its intelligibility, verum. e connect these two objects of thought in formulating the principle of sufficient reason everything which is possesses a sufficient reason for its existence.
This principle cannot be directly demonstrated, since it is a self evident intuition as all first principles. However it can be indirectly demonstrated by a "reductio ad absurdum" which will attach it to the principle of identity. The demonstration is as follows: the sufficient reason is that in virtue of which something is or that without which something is not. To admit an existing being without a sufficient reason is to admit a being, which exists and does not exist at the same time, since it lacks that without which it is not. A chair without a builder is absurd because this chair at the same time exists and does not exist (it does not have that without which it cannot exist).
The principle of sufficient reason can also be enunciated in this way: "everything must have a reason of being, either in itself or in something else". If a property does not necessarily flow from the essence of a thing (i.e. risibilitas from humanitas) then this property must have its explanation elsewhere. The union of things which are in themselves different must have an extrinsic reason. An uncaused contingent being is absurd since it is a thing, which at the same time exists and does not exist by itself.
3. The Principle of Efficient Causality
This principle is the immediate basis of the proofs for the existence of God. It can be attached to the principle of identity (ultimate basis of the 5 ways) through the principle of sufficient reason. The efficient cause is the sufficient reason as a realizing principle, i.e. that by which a thing is accomplished. The intellect once again divides being into two aspects: the one, contingent being which is not self-existent and the other, being which is caused, i.e. being which has a ground exterior to itself (ens ab alio). When we formulate the principle of causality "every effect must have a cause", we enunciate a self-evident truth, namely that a contingent being must have an extrinsic reason of being called efficient cause.
Once again, an effect without a cause is absurd because it is contradictory. It is a being that at the same time would not be dependent on oneself and dependent on oneself. It is therefore clear that we are compelled to identify "ens non a se" (Being not from itself) and "ens ab alio" (being from another) and to acknowledge the absolute necessity of this principle of causality. It is a universal principle, which is valid throughout the created world.
It is important to observe, against the nominalists, that the notion of efficient cause is a notion, which is per se intelligible and only per accidens sensible. The intellect alone can perceive directly the reason of being of the effect in its realizing cause. We also have to hold, against the idealists, that the notion of cause is an objective notion (flowing from the notion of being) having an ontological content and not a subjective category of our mind.
4. The Principle of Finality
It is also derived from the principle of sufficient reason. The end, i.e. the reason why an action takes place, is a definite perfection, which directly refers to the agent as its own good, and for the sake of which it acts. Now every agent produces a determinate effect rather than any other and this means that it is pre-ordained towards this particular effect. Common sense tells us that if a pear tree produces pears rather than apples, it a dog barks rather than meows (like a cat) it is not by chance. They are ordained towards their specific end. Thus the principle of finality "every agent acts for an end" is absolutely universal, whether for rational or non-rational or even inanimate agents.
Here also the intellect is combining two aspects of being, being as an agent (operation) and being as tending towards a good (end). In other words, being is love of a good. The love of this good, of this end is the reason of the agents action. It is in the this sense that we say "Omnem Formam sequitur inclinatio" It can be brought back to the principle of sufficient reason in the following way: If we maintain that an agent can produce a determined effect without being ordained for it, it means that we cannot explain why for instance the eye sees rather than hears. We are on the contrary, compelled to acknowledge that the perfections of the effect must be virtually present in the cause, inasmuch as it has a tendency to produce this effect rather than any other, i.e. it is pre-ordained towards this end. Otherwise we deny the principle of "the reason of being" and therefore of identity. The principle of finality is another of these self-evident truths that the intellect perceives in the light of being.
On these first principles rests the whole human knowledge. To doubt their ontological validity in order to reduce them to useful constructions of the mind leads us to deny that ens is the object of the intellect. if we admit that our intellect is able to know, through abstraction, the extra mental being (i.e. the intelligible nature of things by opposition to sensible phenomena) then we have to admit the first principles since they all flow from the primary intuition of being.