By Joseph Kenny OP


I. Islam in general

What did St. Thomas know about Islam? He refers explicitly to it only twice. The first is at the beginning of Summa contra gentiles. As you know, Thomas wrote this work at the request of the Master of the Order, Raymond de Peafort, for Dominicans who were going to preach in Muslim territories, especially where they would be in contact with the thinkers of Islam, the philosophers and theologians. Thomas excuses himself first by saying that he knew little about Islam. Nevertheless, he gave reasons why it is not reasonable to accept Muammad as a prophet: first because he worked no miracles in proof of his mission (Thomas did not take the Qur'n as a miracle), and secondly because he won followers by a show of force and by promising them sensual satisfaction in this life and the next.

After this severe prologue, which lacks any appreciation of the positive aspects of Islam, Thomas consecrates three of the four books of this Summa to an exposition of the foundations of the Catholic Faith which reason can demonstrate and revelation confirms. He follows this method because he recognized that Muslims do not accept the authority of the Bible, regarding it as a deformation of the Torah, the Zbr and the Injl recognized by the Qur'n. In the first three books of Contra Gentiles Thomas appeals to the intelligence of Muslims, proposing truths that any Muslim and any Christian can accept. It is, in fact, as I said in the title of a rsum I wrote of these books, "an excercise in philosophy of religion, common to Christians and to Muslims". As I did in my work, all one has to do is to add to the confirmatory Biblical verses parallel quotations from the Qur'n.

Another work is consecrated almost entirely to Islam. It is De rationibus fidei contra Saracenos, Graecos et Armenos ad Cantorem Antiochenum. Apart from a chapter on the Eucharist and another on Purgatory, directed to the Armenians and the Greeks, six chapters are answers to Muslim objections to the Faith. Here, no doubt well informed by the Dominican Cantor of the Middle East, Thomas makes no excuse that he is ignorant of Islam, but faces the principal classical objections which Muslims make against the Catholic Faith, such as the possibility of the Trinity and of the Incarnation, the wisdom of the Crucifixion and human liberty before divine predestination. Here Thomas gives sharp and ever valid answers to these questions. I published in Islamochristiana (1996) an annotated translation of this work: Reasons for the Faith against Muslim objections.


II The Muslim philosophers

The relationship of Thomas Aquinas is a vast and complicated subject. This contribution is only a provisional essay, in view of a more comprehensive study of the subject.


1. Methodology

Such a study should begin with an inventory of the passages which deal with the Arab philosophers. Towards this we have the series of articles by C. Vansteenkiste listing all of St. Thomas' citations of Arab philosophers. These are: "Avicenna-citaten bij S. Thomas," Tijdschrift v. Philo., 15 (1953), 457-507; "San Tommaso d'Aquino ed Averro," Scritte in onore di Giuseppe Furlani. RSO?? 32 (1957), 585-623; "Autori Arabi e Giudei nell'opera di San Tommaso," Angelicum, 37 (1960), 336-401. He also did "Il Liber de Causis negli scritti di San Tommaso, Angelicum, 35 (1958), 325-374. The first of these articles is an improvement on the list in Aim Forest, La structure mtaphysique du concret selon Saint Thomas d'Aquin. Paris: Vrin, 1931, 331-360. These articles only indicate the passages where Thomas mentions Arab philosophers by name, whereas there are many other passages where he clearly discusses their ideas simply under "quidam dixerunt".

The next step is to identify the works of these philosophers that Thomas refers to, their authenticity and their translators. Very useful is E. Gilson, "Avicenne en Occident au Moyen Age," Archives d'Histoire Doctrinale et Littraire du Moyen Age, 44 (1969), 89-121; this article however should be used with care, distinguishing between historical information and Gilson's peculiar interpretation of St. Thomas. Another useful article is L. Gardet, "Thomas et ses prdecesseurs arabes," in A. Maurer (ed.), St. Thomas Aquinas 1274-1974, Commemorative studies (Toronto: Pontifical Institute of Mediaeval Studies, 1974), I, 419-448. These works are also useful for the relatively different use Thomas made of Ibn-Sn and Ibn-Rushd in his earlier and later works; here too Gilson has to be read with caution.

Another job is to determine the balance of what Thomas knew and did not know of the Arab philosophers and their works, which the following section will consider, and finally to study Thomas' reaction to particular teachings of these philosophers.


2. Works that Thomas definitely knew first hand:

Each work is followed by the books in which Thomas refers to it.

2.1 Ibn-Sn (Avicenna 980-1037)

Sufficientia (= ash-Shif', as-Sam` a-ab`): Sent., ST, CG, QD de veritate, QD de malo, De principiis naturae, In Job, De mixtione elementorum, In Boetii De Trinitate, In Physicorum, In De generatione et corruptione

De caelo (There is the corresponding title, as-Sam' wa-l-`lam in ash-Shif', but the Latin version is a different spurious work): Sent., In De caelo

De anima/ VI de naturalibus: Sent., ST, CG, In Ethicorum, QD de malo, QD de veritate, QD de virtutibus in communi, In Job, Contra impugnantes, Quodl.

De animalibus (= ash-Shif', al-ayawnt): ST, CG, In Ethicorum, QD de veritate

Metaphysica (= ash-Shif', al-Ilhiyyt): Sent., ST, CG, In Ethicorum, QD de veritate, QD de malo, QD de anima, QD de spiritualibus creaturis, QD de potentia, QD de virtutibus, De ente et essentia, In Job, Compendium th., In Boetii De Trinitate, In Metaphysicorum, Quodl.

De intelligentiis (is probably by Dominic Gundissalinus, with modified Avicennian ideas (1)): Sent.

Canon medicinae (= al-Qnn f -ibb): In Ethicorum, QD de veritate, QD de malo, In Job

2.2 Al-Ghazl (Algazel 1058-1111)

Metaphysica/ Prima philosophia (= Maqid al-falsifa): Sent., ST, CG, QD de veritate, QD de malo, QD de spiritualibus creaturis, QD de potentia, De mixtione elementorum, De unitate intellectus, In Job, De aeternitate mundi, In Boethii De Trinitate, Quodlibet 3,7,9, De spe

2.3 Ibn-Rushd (Averroes 1126-1198)

Expositio Poeticae (= Mukhtaar ash-Shi`r): QD de malo

In Analyticorum posteriorum (= Shar Kitb al-burhn): QD de veritate, (not In Post.)

In Physicorum (= Shar Kitb as-sam` a-ab`): Sent., ST, CG, QD de veritate, QD de malo, QD de potentia, In Physicorum

In De caelo (a spurious work purporting to represent Shar kitb as-sam' wa-l-`lam): Sent., ST, QD de veritate, QD de malo, QD de potentia, In Job, In De caelo

In De generatione et corruptione (= Talkh Kitb al-kawn wa-l-fasd): QD de veritate

In De anima (= Shar Kitb an-nafs): Sent., ST, CG, QD de veritate, QD de spiritualibus creaturis, De ente et essentia, De unitate intellectus, In De caelo

In De sensu et sensibili (used for the whole of Parva naturalia = Jawmi` Kitb al-iss wa-l-mass): ST

In De divinatione per somnium (part of Parva naturalia): De veritate

In Metaphysicorum (= Shar Kitb m ba`d a-ab`a): Sent., ST, CG, In Ethicorum, QD de veritate, QD de malo, QD de spiritualibus creaturis, QD de potentia, In Job, Resp. de 43 articulis, Resp. de 36 articulis, Resp. de 30 articulis, In Physicorum

De substantia orbis (= Maqla f jawhar al-fulk): Sent., ST, QD de spiritualibus creaturis, QD de veritate, QD de malo, QD de potentia, In Job

In Moralia Nicomachia (= Talkh Kitb al-akhlq): In Ethicorum

2.4 Ibn-Gabirol (Avicebron c. 1021-1058) (2)

Fons Vitae: Sent., ST, CG, De ente et essentia, QD de veritate, QD de anima, QD de spiritalibus creaturis, QD de potentia, In De generatione et corruptione, In De anima, Quotlibet 11, De substantiis separatis

2.5 Moses ben Maimon (Maimonides/ Rabbi Moses 1135-1205)

Doctor perplexorum (Dallat al-'irn): Sent., ST, CG, QD de potentia, QD de anima, QD de spritualibus creaturis, In Psalmum 18, In Threnos, In De anima, In Boethii De trinitate


3. Authors Thomas probably knew second hand, or makes little use of

3.1 Al-Farghn (Alfraganus): In De Caelo

3.2 Al-Frb (Alpharabius 875-950)

Commentary on Aristotle's Ethics & De intellectu: Sent.

3.3 Ibn-Bjja (Avempace d. 1138)

No work named; his opinions would have been known through Ibn-Rushd who frequently quotes him: In Phys., Sent., ST, CG, In Boethii De Trinitate, QD de veritate, In Physicorum

3.4 Isq ibn-Sulaymn Isr`l (Isaac ben Solomon Israeli, c. 855-955)

De definitionibus: Sent., ST, QD de veritate

3.5 Al-Mutakallimn (Loquentes in lege Maurorum): CG

3.6 Qus ibn-Lq (Costa ben Luca, d. 913)

De differentia spiritus et animae: QD de spiritualibus creaturis


4. Arab philosophers not referred to at all by St. Thomas

4.1 Al-Kind (c. 800-866) - a forerunner of Mutakallimn; he had no notable contribution which Thomas could not find in later philosophers.

4.2 Muammad ar-Rz (c. 865-925) - a Platonist, challenged foundations of Islam. Thomas could only have supported his defence of the autonomy of human reason.

4.3 Ibn-Masarra (883-931) - a man of esoteric views.

4.4 Miskawayh (932-1030) - anticipates Ibn-Sn in several points, such as the real distinction in creatures of essence and existence (for him an accident), while they are identical in God.

4.5 Ibn-ufayl (d. 1185) - Interesting for the autonomy of human reason, but his idea that all things are one being would not find acceptance with Thomas.


5. Works Thomas did not know among the philosophers he quotes

5.1 Ibn-Sn

It is a complicated task to list the works of Ibn-Sn, whether in the original Arabic, where so many works attributed to him are spurious or doubtful, and some works attributed to al-Frb belong to him, or in Latin translations, where spurious works are also attributed to him.

Let us begin with the philosophical summa, ash-Shif'. This word properly means "healing", a title likely given to advertize the value of the whole collection; since medicine was always valued, the Arab philosophers could package the bitter pills of their other writings under this label so as to escape the censure of Muslim theologians. Thomas makes no reference to the four works of the quadrivium: al-isb, Ul al-handasa, Jawmi` `ilm al-msq, and `ilm al-hay'a. Of the nine voluminous works of the logical section, Thomas makes only rare references to his "Logic I" and "Logic II" (= ash-Shif', al-Ma`qlt, al-`Ibra). In his own commentaries on Aristotle's logic he makes no reference to Ibn-Sn. It seems Thomas only stumbled across some references to Ibn-Sn's logical works and never had a chance to study them.

Of the works on natural science, Thomas made extensive use of the important as-Sam` a-ab`, or Physics, which had the Latin title "Sufficientia", a mis-translation of the word shif'. As for the as-Sam' wa-l-`lam, or De caelo, Thomas made considerable use of a spurious work attributed to Ibn-Sn. There is no reference at all to al-Kawn wa-l-fasd (De generatione et corruptione), al-Af`l wa-l-infi`lt (De actionibus et passionibus), and al-thr al-`ulwiyya (Meterology). As for the important Kitb an-nafs (De anima), Thomas unfortunately had access only to a reworked abridgement of this work by Dominic Gundissalinus. (3) He skips over an-Nabt (On plants), but makes a number of references to al-ayawn (De animalibus).

Thomas frequently quotes from Ibn-Sn's al-Ilhiyyt (Metaphysics). This is important because, as Gilson remarks, Thomas's metaphysics was highly influenced by Ibn-Sn, while in natural science he mainly followed Ibn-Rushd.

Apart from these sections of ash-Shif', Thomas refers to the famous al-Qnn f -ibb and the spurious De intelligentiis.

Besides these, Ibn-Sn has numerous opuscula that Thomas knew nothing about.

5.2 Al-Ghazl

Thomas cites only one work of al-Ghazl, whereas he is credited with at least 80 authentic works. The very work Thomas does quote, Maqid al-falsifa (the intentions of the philosophers) gave Thomas the impression that al-Ghazl was a philosopher, whereas he was mainly a scholar of law and of theology and vigorously attacked the philosophers for their main positions.

It has recently been pointed out that the Maqid al-falsifa, far from being an original work of al-Ghazl, is in reality a loose translation of Ibn-Sn's Dnishnmah `alt, written in Persian.

5.3 Ibn-Rushd

Of the nine sections of logic for which Ibn-Rushd wrote more than one commentary, Thomas does not seem to have known any of his two writings on the Eisagoge, the three on the Categories, the three on Peri Hermeneias, the twelve on the Prior Analytics, the five smaller writings on the Posterior Analytics, the three works on the Topics, the three on the Rhetoric, or the second work on the Poetics.

In the area of natural science, Thomas did not know the two smaller works on the Physics, four of the five authentic writings on De caelo, the smaller writing on De generatione et corruptione, the three on the Meteorology, the two smaller works on the De anima, the single work on botany, the two on animals, and the six essays on the intellect.

Thomas did not know the smaller work on the Metaphysics. Nor did he know the famous Tahfut at-Tahfut, or the important Fal al-maql, amma and Kashf `an manhij al-adilla. In moral science he missed the summary of Plato's Politics. There is no reference to any of Ibn-Rushd's many medical and several legal writings.


6. Thomas' understanding of the philosophers he quotes

One of the problems about Thomas' references to Ibn-Sn is that most of them are not direct quotes and very often the source cannot be easily identified. For instance, Thomas attributes to Ibn-Sn the opinion that elements in a composite retain their substantial forms, i.e. that there is a plurality of substantial forms in a single individual. In ash-Shif', the book of the Soul, Ibn-Sn holds that every soul, even that of plants, is a substance (jawhar) and not an accident (`ara); it is distinct from the body and gives it its consistence and existence. But, he says, not every substance is necessarily separable. Speaking of the question of intermediate forms, Ibn-Sn holds that there is no other actual form but the soul, and that the soul of an animal is the cause of its specific animal activities, like sensation, and also of its vegetative functions. (4) Yet in the book Generation and corruption Ibn-Sn attacks those who hold that in a composite the elements lose their own forms to take on the sole form of the compound. Rather, he says that earth and fire retain their own substantial forms when they are part of flesh, and only their active qualities are modified. (5)

As we have seen, Thomas had access to an abridgement of the first work, and did not see the second at all. How then did he get to know Ibn-Sn's opinion? The answer is simple. He took it from the Latin of Ibn-Rushd's De generatione et corruptione (V, p. 370b), where Ibn-Rushd very clearly accuses Ibn-Sn of this very error. But if we consult the Arabic for the same passage, we find that whole paragraph attacking Ibn-Sn is missing. Since Ibn-Rushd was constantly revising his own works, and much of his writings have survived only in Latin or Hebrew, it is probable that the passage in question represents one stage of Ibn-Rushd's revision. It is too bad that the editors of the Arabic edition did not take note of this passage in the Latin.

Another case is Ibn-Sn's real distinction between essence and existence. Although it was one of the principal sources for Thomas' own teaching on the subject, Thomas differed from Ibn-Sn on the exact relationship between the two.

For more on Thomas and Ibn-Sn, see the (unpublished) Licentiate thesis of Albert Judy, O.P. (Toronto: Pontifical Institute of Medieval Studies).


7. Thomas' evaluation of some ideas of the Arab philosophers

7.1 The existence of God

All "five ways" of Thomas Aquinas are found with the Arab philosophers. Among the slightly different cosmologies of each, Thomas is closest to that of Ibn-Rushd, who simplifies the number of heavenly spirits. Nevertheless Thomas says that it is improbable that the heavenly bodies are animated. Nevertheless he swallowed the whole system of spiritual movers of these bodies, a system that collapsed after the discovery that these bodies are not incorruptible and that they are subject to the same inertia (or impetus in Thomas' terminology) which governs earthly bodies.

As for the eternity of the world, like Moshe ben Maimon, Thomas says that neither its necessity nor its impossibility can be demonstrated. Against the objection of Ibn-Sn and Ibn-Rushd that every possibility must be found in already existing subject, Thomas states that the power of God extends to all being that does not imply a contradiction of terms. (6)

Thomas' most important borrowing from the Arab philosophers is the explicit recognition of a real distinction between essence and existence outside of God, likewise that everything depends on an exterior cause for the continuation of its existence. But Thomas refined this distinction, rejecting the idea of Miskawayh (less clear with Ibn-Sn) that existence is an accident, and showing that its relationship to essence is that of act to potency. Thomas also insisted that this act of existence depends immediately on God, and that there are no intermediaries in creation, as posited in the system of al-Frb and Ibn-Sn.

7.2 God's knowledge of singulars

In contrast to Ibn-Rushd, who rejects any sort of universality in God's knowledge, Thomas Aquinas distinguishes the imperfection of human universal knowlege and angelic and divine knowledge which is the more perfect the more it is universal and simple. He also maintains that God knows an infinitude of possible things.

Also, according to Ibn-Sn and even Ibn-Rushd, God should know singular effects in their causes and not in themselves. Thomas Aquinas considers this opinion insufficient and teaches rather that the knowledge of God extends as far as his causality; and the active power of God extends not only to forms, but also to matter, by which forms are individualized. (7)

7.3 Determination

In contrast to Ibn-Sn who distinguishes between what is necessary by itself and what is necessary by a cause, Thomas distinguishes between what is necessary by another in the sense that it has no material potency and what is essentially contingent because it is material, even though it may be necessary in reference to the first cause who determines all things without taking away their intrinsic contingency.

Against Ash`arism, particularly that of al-Bqilln, Thomas teaches that God preserves the continued existence of things, since the being of things depends directly on him. (8) Against the philosophers, he says that no intermediary can confer the act of existence. (9) With the Ash`arites, he holds that God is the cause of the action of all things, since they all depend constantly on him for their existence. (10)

On the other hand, Thomas insists that creatures have their own causality. In taking this position, he is not only against the Ash`arites but also Ibn-Sn who attributed the generation of everything on earth to the Agent Intellect as the giver of forms. Ash`arite occasionalism goes contrary to the evidence of the senses, which bear witness that definite effects come regularly from definite things. And, instead of exaggerating the omnipotence of God, he says that the power of God is manifested in the perfection and fertility of what he makes, and not in their poverty and sterility. And, as Ibn-Rushd objected, such a position denies the order and inter-dependence of things in the universe, and consequently the wisdom of God. Thus one should admit the causality of creatures not only in producing accidental effects, like heat, but also in the generation of their like. (11)

7.4 The human soul

Ibn-Sn rejects "the impression of the soul in the body", and thus "matter designated by quantity" which Thomas maintains is the principle of individuation. Ibn-Sn rejects reincarnation, taking more or less the same line of argumentation that Saint Thomas takes.

Ibn-Rushd rejects spontaneous generation, saying that it has "no evident proof" (ghayr al-mushhada)," (12) but Thomas Aquinas accepts the idea without question.

As for the immortality of the human soul, the problem that Thomas Aquinas faced was to reconcile two facts: (1) that the human soul is the substantial form of man, and (2) that the act of intellection transcends matter and the subject of this act can survive without the body. Since act must correspond to potency, according to the first fact the soul should be a material form, but according to the second fact the act of intelligence requires an immaterial subject.

First of all, Thomas did not identify the rational soul with the intellect, as the Arab philosophers did, but distinguished the substance of the soul from its powers, as he distinguished these powers from their habits and acts. For him a single soul is the substantial form of the body. By its vegetative powers it is the source of the vital functions of the body; by its sensitive knowing and appetitive powers it is the source of its animal functions, and by the passive and active intellects and the will it exercises properly human activities.

Thus the soul has some activities that are purely material and others that are spiritual. Against Ibn-Sn, man is essentially soul and body; there is no room for dualism. To solve the problem how the form of matter can have an operation which transcends matter and can exist without matter, Thomas makes an exception to his general teaching that the act of existence is the act of the composite of matter and form. Since the human soul has an act which is not that of the body, the existence is attached first and directly to the human soul, and through the soul to the body which participates in it, being animated by the soul. Thus at death the soul retains its existence apart from the body. (13)

Another point of sharp difference between Thomas and the Arab philosophers was his position that the intellect, whether passive or active, is a personal power of every man. (14) Instead of Ibn-Sn's theory of continual dependence on an exterior agent intellect, Thomas holds that man retains a habitual knowledge; nevertheless he admits that man, apart from his normal knowledge acquired from sense experience, can receive angelic inspiration.

As for the origin of the human soul, Thomas is in agreement with Ibn-Sn that it is created with the body. (15)

As for heavenly spirits, Thomas holds that there are incorporeal intellectual creatures, each unique in its own species, whose number is not limited to the movers of the heavenly bodies. (16)

The perfection of human life, for Thomas Aquinas, is to know God. (17) Since this knowledge is not possible to achieve by philosophy, by faith, or by conjunction with separated intellects (as Ibn-Rushd taught), it is not possible for man to achieve it in this life. (18) Even in the future life, the vision of God cannot be acquired by knowing the angels or other separated souls, but only God himself can give it. That is through the gift of glory, which is an adaptation of the soul to see God. (19) This vision is not comprehensive, but it is available to every soul to the extent of its readiness. (20)

7.5 Reason and revelation

For Ibn-Sn, a prophet is someone whose intelligence is supremely developed and he can grasp much at once. That is because of his power of intuition (adas), but especially because he is open to the influences of the heavenly spirits. (21) This is exactly what Thomas Aquinas calls "natural prophecy". (22)

Against the tendency of the Arab philosophers to reduce prophecy to a completely natural phenomenon pertaining to those who are eminent in intelligence, Thomas Aquinas holds that true prophecy is a purely gratuitous gift out of the control of the prophet, which he cannot exercise whenever he wishes. As a gift, it has nothing to do with the natural intelligence of the prophet, but the adaptation of his intellect to receive divine enlightenment is a supernatural gift. (23)

Against the anbalite and Ash`arite theologians who so exalt revelation that they give little or no value to reason, and against Muammad ar-Rz who recognizes only human reason, Thomas agrees with the other Arab theologians and philosophers who recognize the autonomy of reason and of revelation. Each of them leads to areas of truth where the other cannot go, but they overlap when it comes to certain fundamental truths concerning God, man and creation in general. (24)

Can there be a conflict between the two? God has endowed us with reason by which we know certain truths so clearly that it is impossible to deny them. It is likewise illegitimate to deny the truths of faith, which are confirmed by divine authority. Thus anything that is contrary to the truths of reason or of revelation cannot come from God, but must come from wrong reasoning. The conclusions of such reasoning have no validity, but only the appearance of truth. (25)



1. Cf. Gilson, ibid.

2. Cf. Fernand Brunner, Platonisme et Aristotlisme, la critique d'Ibn Gabirol par Saint Thomas d'Aquin (Publications Universitaires de Louvain, 1963).

3. Cf. Etienne Gilson, "Avicenne en occident au moyen ge," Archives d'Histoire Doctrinasle et Littraire du Moyen Age, 44 (1969), 89-121.

4. an-nafs, maqla 1, fal 3; maqla 5, fal 7.

5. Ash-Shif': al-Kawn wa-l-fasd, fal 7.

6. Summa theologiae, I, q. 25, a. 3.

7. Summa theologiae, I, q.14, a.11.

8. Cf. Summa contra gentiles, III, 65.

9. Ibid., n. 66.

10. Ibid., n. 67.

11. Ibid., nos. 69-70.

12. II, p. 622; Tafsr m ba`d a-ab`a, pp. 1497 ff.

13. Cf. Contra gentiles, II, n. 69-72.

14. Ibid., II, nos. 59, 69, 73-78; De unitate intellectus contra Averroistas.

15. Contra gentiles, II, nos. 83-90.

16. Ibid., II, nos. 91-101.

17. Contra gentiles, III, nos. 25, 37.

18. Contra gentiles, III, nos. 38-48.

19. Ibid., III, nos. 49-54.

20. Ibid., nos. 55-58.

21. Kalimt a-fiyya, p. 168.

22. Quaestiones disputatae de Veritate, 12, a. 3.

23. Summa theologiae, II-II, qq. 171-174.

24. Contra gentiles, I, nos. 4-6.

25. Ibid., I, nos. 7-8.