a very few selections from
Summa Theologiae

St. Thomas Aquinas, the Angelic Doctor


BECAUSE the Master of Catholic Truth ought not only to teach the proficient, but also to instruct beginners (according to the Apostle: As Unto Little Ones in Christ, I Gave You Milk to Drink, Not Meat—1 Cor. 3:1, 2), we purpose in this book to treat of whatever belongs to the Christian Religion, in such a way as may tend to the instruction of beginners. We have considered that students in this Science have not seldom been hampered by what they have found written by other authors, partly on account of the multiplication of useless questions, articles, and arguments; partly also because those things that are needful for them to know are not taught according to the order of the subject-matter, but according as the plan of the book might require, or the occasion of the argument offer; partly, too, because frequent repetition brought weariness and confusion to the minds of the readers.

Endeavoring to avoid these and other like faults, we shall try, by God’s help, to set forth whatever is included in this Sacred Science as briefly and clearly as the matter itself may allow.



We proceed thus to the First Article:—

Objection 1. It seems that, besides philosophical science, we have no need of any further knowledge. For man should not seek to know what is above reason: Seek not the things that are too high for thee (Ecclus. 3:22). But whatever is not above reason is fully treated of in philosophical science. Therefore any other knowledge besides philosophical science is superfluous.
Obj. 2. Further, knowledge can be concerned only with being, for nothing can be known, save what is true; and all that is, is true. But everything that is, is treated of in philosophical science—even God Himself; so that there is a part of philosophy called theology, or the divine science, as Aristotle has proved (Metaph. vi). Therefore, besides philosophical science, there is no need of any further knowledge.

ON THE CONTRARY, It is written (2 Tim. 3:16): All Scripture inspired of God is profitable to teach, to reprove, to correct, to instruct in justice. Now Scripture, inspired of God, is no part of philosophical science, which has been built up by human reason. Therefore it is useful that besides philosophical science there should be other knowledge—i.e., inspired of God.

I ANSWER THAT, It was necessary for man’s salvation that there should be a knowledge revealed God, besides philosophical science built up by human reason. Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason: The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee (Isa. 66:4). But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation.

Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors.

Whereas man’s whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that, besides philosophical science built up by reason there should be a sacred science learned through revelation.

Reply Obj. 1. Although those things which are beyond man’s knowledge may not be sought for by man through his reason, nevertheless, once they are revealed by God they must be accepted by faith. Hence the sacred text continues, For many things are shown to thee above the understanding of man (Ecclus. 3:25). And in this the sacred science consists.

Reply Obj. 2. Sciences are differentiated according to the various means through which knowledge is obtained. For the astronomer and the physicist both may prove the same conclusion—that the earth, for instance, is round: the astronomer by means of mathematics (i.e., abstracting from matter), but the physicist by means of matter itself. Hence there is no reason why those things which may be learned from philosophical science, so far as they can be known by natural reason, may not also be taught us by another science so far as they fall within revelation. Hence theology included in sacred doctrine differs in kind from that theology which is part of philosophy.



We proceed thus to the Seventh Article:—

Objection 1. It seems that God is not the object of this science. For in every science the nature of its object is presupposed. But this science cannot presuppose the essence of God, for Damascene says (De Fid. Orth. 1. iv): It is impossible to define the essence of God. Therefore God is not the object of this science.
Obj. 2. Further, whatever conclusions are reached in any science must be comprehended under the object of the science. But in Holy Writ we reach conclusions not only concerning God, but concerning many other things, such as creatures and human morality. Therefore God is not the object of this science.

ON THE CONTRARY, The object of the science is that of which it principally treats. But in this science the treatment is mainly about God; for it is called theology, as treating of God. Therefore God is the object of this science.

I ANSWER THAT, God is the object of this science. The relation between a science and its object is the same as that between a habit or faculty and its object. Now properly speaking the object of a faculty or habit is the thing under the aspect of which all things are referred to that faculty or habit, as man and stone are referred to the faculty of sight in that they are colored. Hence colored things are the proper objects of sight.

But in sacred science all things are treated of under the aspect of God; either because they are God Himself; or because they refer to God as their beginning and end. Hence it follows that God is in very truth the object of this science.

This is clear also from the principles of this science, namely, the articles of faith, for faith is about God. The object of the principles and of the whole science must be the same, since the whole science is contained virtually in its principles. Some, however, looking to what is treated of in this science, and not to the aspect under which it is treated, have asserted the object of this science to be something other than God—that is, either things and signs; or the works of salvation; or the whole Christ, as the head and members. Of all these things, in truth, we treat in this science, but so far as they have reference to God.

Reply Obj. 1. Although we cannot know in what consists the essence of God, nevertheless in this science we make use of His effects, either of nature or of grace, in place of a definition, in regard to whatever is treated of in this science concerning God; even as in some philosophical sciences we demonstrate something about a cause from its effect, by taking the effect in place of a definition of the cause.

Reply Obj. 2. Whatever other conclusions are reached in this sacred science are comprehended under God, not as parts or species or accidents, but as in some way related to Him.



We proceed thus to the Second Article:—

Objection 1. It seems that the existence of God cannot be demonstrated. For it is an article of faith that God exists. But what is of faith cannot be demonstrated, because a demonstration produces scientific knowledge; whereas faith is of the unseen (Heb. 11:1). Therefore it cannot be demonstrated that God exists.
Obj. 2. Further, the essence is the middle term of demonstration. But we cannot know in what God’s essence consists, but solely in what it does not consist; as Damascene says (De Fid. Orth. i. 4). Therefore we cannot demonstrate that God exists.
Obj. 3. Further, if the existence of God were demonstrated, this could only be from His effects. But His effects are not proportionate to Him, since He is infinite and His effects are finite; and between the finite and infinite there is no proportion. Therefore, since a cause cannot be demonstrated by an effect not proportionate to it, it seems that the existence of God cannot be demonstrated.

ON THE CONTRARY, The Apostle says: The invisible things of Him are clearly seen, being understood by the things that are made (Rom. 1:20). But this would not be unless the existence of God could be demonstrated through the things that are made; for the first thing we must know of anything is, whether it exists.

I ANSWER THAT, Demonstration can be made in two ways: One is through the cause, and is called a priori, and this is to argue from what is prior absolutely. The other is through the effect, and is called a demonstration a posteriori; this is to argue from what is prior relatively only to us. When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because since every effect depends upon its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can be demonstrated from those of His effects which are known to us.

Reply Obj. 1. The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for

faith presupposes natural knowledge, even as grace presupposes nature, and perfection supposes something that can be perfected.

Nevertheless, there is nothing to prevent a man, who cannot grasp a proof, accepting, as a matter of faith, something which in itself is capable of being scientifically known and demonstrated.

Reply Obj. 2. When the existence of a cause is demonstrated from an effect, this effect takes the place of the definition of the cause in proof of the cause’s existence. This is especially the case in regard to God, because, in order to prove the existence of anything, it is necessary to accept as a middle term the meaning of the word, and not its essence, for the question of its essence follows on the question of its existence. Now the names given to God are derived from His effects; consequently, in demonstrating the existence of God from His effects, we may take for the middle term the meaning of the word “God.”

Reply Obj. 3. From effects not proportionate to the cause no perfect knowledge of that cause can be obtained. Yet from every effect the existence of the cause can be clearly demonstrated, and so

we can demonstrate the existence of God from His effects; though from them we cannot perfectly know God as He is in His essence.



We proceed thus to the Ninth Article:—

Objection 1. It seems that God wills evils. For every good that exists, God wills. But it is a good that evil should exist. For Augustine says (Enchir. 95): Although evil in so far as it is evil is not a good, yet it is good that not only good things should exist, but also evil things. Therefore God wills evil things.
Obj. 2. Further, Dionysius says (Div. Nom. iv. 23): Evil would conduce to the perfection of everything, i. e., the universe. And Augustine says (Enchir. 10, 11): Out of all things is built up the admirable beauty of the universe, wherein even that which is called evil, properly ordered and disposed, commends the good the more evidently in that good is more pleasing and praiseworthy when contrasted with evil. But God wills all that appertains to the perfection and beauty of the universe, for this is what God desires above all things in His creatures. Therefore God wills evil.
Obj. 3. Further, that evil should exist, and should not exist, are contradictory opposites. But God does not will that evil should not exist; otherwise, since various evils do exist, God’s will would not always be fulfilled. Therefore God wills that evil should exist.

ON THE CONTRARY, Augustine says (Qq. LXXXIII., 3): No wise man is the cause of another man becoming worse. Now God surpasses all men in wisdom. Much less therefore is God the cause of man becoming worse: and when He is said to be the cause of a thing, He is said to will it. Therefore it is not by God’s will that man becomes worse. Now it is clear that every evil makes a thing worse. Therefore God wills not evil things.

I ANSWER THAT, Since the ratio of good is the ratio of appetibility, as said before (Q. V., A. 1), and since evil is opposed to good, it is impossible that any evil, as such, should be sought for by the appetite, either natural, or animal, or by the intellectual appetite which is the will. Nevertheless evil may be sought accidentally, so far as it accompanies a good, as appears in each of the appetites. For a natural agent intends not privation or corruption, but the form to which is annexed the privation of some other form, and the generation of one thing, which implies the corruption of another. Also

when a lion kills a stag, his object is food, to obtain which the killing of the animal is only the means. Similarly the fornicator has merely pleasure for his object, and the deformity of sin is only an accompaniment.

Now the evil that accompanies one good, is the privation of another good. Never therefore would evil be sought after, not even accidentally, unless the good that accompanies the evil were more desired than the good of which the evil is the privation. Now God wills no good more than He wills His own goodness; yet He wills one good more than another. Hence He in no way wills the evil of sin, which is the privation of right order towards the divine good. The evil of natural defect, or of punishment, He does will, by willing the good to which such evils are attached. Thus in willing justice He wills punishment; and in willing the preservation of the natural order, He wills some things to be naturally corrupted.

Reply Obj. 1. Some have said that although God does not will evil, yet He wills that evil should be or be done, because, although evil is not a good, yet it is good that evil should be or be done. This they said because things evil in themselves are ordered to some good end; and this order they thought was expressed in the words that evil should be or be done. This, however, is not correct; since evil is not of itself ordered to good, but accidentally. For it is beside the intention of the sinner, that any good should follow from his sin; as it was beside the intention of tyrants that the patience of the martyrs should shine forth from all their persecutions. It cannot therefore be said that such an ordering to good is implied in the statement that it is a good thing that evil should be or be done, since nothing is judged of by that which appertains to it accidentally, but by that which belongs to it essentially.

Reply Obj. 2. Evil does not operate towards the perfection and beauty of the universe, except accidentally, as said above (ad 1). Therefore Dionysius in saying that evil would conduce to the perfection of the universe, draws a conclusion by reduction to an absurdity.

Reply Obj. 3. The statements that evil exists, and that evil exists not, are opposed as contradictories; yet the statements that anyone wills evil to exist and that he wills it not to be, are not so opposed; since either is affirmative.

God therefore neither wills evil to be done, nor wills it not to be done, but wills to permit evil to be done; and this is a good.



We proceed thus to the First Article:—

Objection 1. It would seem that the trinity of the divine persons can be known by natural reason. For philosophers came to the knowledge of God not otherwise than by natural reason. Now we find that they said many things about the trinity of persons, for Aristotle says (De Cœlo et Mundo i. 2): Through this number—namely, three—we bring ourselves to acknowledge the greatness of one God, surpassing all things created. And Augustine says (Conf. vii. 9): I have read in their works, not in so many words, but enforced by many and various reasons, that in the beginning was the Word, and the Word was with God, and the Word was God, and so on; in which passage the distinction of persons is laid down. We read, moreover, in a gloss on Rom. 1 and Exod. 8 that the magicians of Pharaoh failed in the third sign—that is, as regards knowledge of a third person—i.e., of the Holy Ghost—and thus it is clear that they knew at least two persons. Likewise Trismegistus says: The monad begot a monad, and reflected upon itself its own heat. By which words the generation of the Son and the procession of the Holy Ghost seem to be indicated. Therefore knowledge of the divine persons can be obtained by natural reason.
Obj. 2. Further, Richard of St. Victor says (De Trin. i. 4): I believe without doubt that probable and even necessary arguments can be found for any explanation of the truth. So even to prove the Trinity some have brought forward a reason from the infinite goodness of God, who communicates Himself infinitely in the procession of the divine persons; while some are moved by the consideration that no good thing can be joyfully possessed without partnership. Augustine proceeds (De Trin. ix. 4; x. 11, 12) to prove the trinity of persons by the procession of the word and of love in our own mind; and we have followed him in this (Q. XXVII., A. 1, 3). Therefore the trinity of persons can be known by natural reason.
Obj. 3. Further, it seems to be superfluous to teach what cannot be known by natural reason. But it ought not to be said that the divine tradition of the Trinity is superfluous. Therefore the trinity of persons can be known by natural reason.

ON THE CONTRARY, Hilary says (De Trin. i.), Let not man think to reach the sacred mystery of generation by his own mind. And Ambrose says (De Fide ii. 5), It is impossible to know the secret of generation. The mind fails, the voice is silent. But the trinity of the divine persons is distinguished by origin of generation and procession (Q. XXX., A. 2). Since, therefore, man cannot know, and with his understanding grasp that for which no necessary reason can be given, it follows that the trinity of persons cannot be known by reason.

I ANSWER THAT, It is impossible to attain to the knowledge of the Trinity by natural reason. For, as above explained (Q. XII., AA. 4, 12), man cannot obtain the knowledge of God by natural reason except from creatures. Now creatures lead us to the knowledge of God, as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to Him as the principle of all things, and we have cited this fundamental principle in treating of God as above (Q. XII., A. 12). Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the persons. Therefore, by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the persons.

Whoever, then, tries to prove the trinity of persons by natural reason, derogates from faith in two ways. Firstly, as regards the dignity of faith itself, which consists in its being concerned with invisible things, that exceed human reason;

wherefore the Apostle says that faith is of things that appear not (Heb. 11:1), and the same Apostle says also, We speak wisdom among the perfect, but not the wisdom of this world, nor of the princes of this world; but we speak the wisdom of God in a mystery which is hidden (1 Cor. 2:6, 7).

Secondly, as regards the utility of drawing others to the faith. For when anyone in the endeavor to prove the faith brings forward reasons which are not cogent, he falls under the ridicule of the unbelievers: since they suppose that we stand upon such reasons, and that we believe on such grounds.

Therefore, we must not attempt to prove what is of faith, except by authority alone, to those who receive the authority; while as regards others it suffices to prove that what faith teaches is not impossible. Hence it is said by Dionysius (Div. Nom. ii): Whoever wholly resists the word, is far off from our philosophy; whereas if he regards the truth of the word—i.e., the sacred word, we too follow this rule.

Reply Obj. 1. The philosophers did not know the mystery of the trinity of the divine persons by its proper attributes, such as paternity, filiation, and procession, according to the Apostle’s words, We speak the wisdom of God which none of the princes of the world—i.e., the philosophers—knew (1 Cor. 2:6). Nevertheless, they knew some of the essential attributes appropriated to the persons, as power to the Father, wisdom to the Son, goodness to the Holy Ghost; as will later on appear. So, when Aristotle said, By this number, etc., we must not take it as if he affirmed a threefold number in God, but that he wished to say that the ancients used the threefold number in their sacrifices and prayers on account of some perfection residing in the number three. In the Platonic books also we find, In the beginning was the word, not as meaning the Person begotten in God, but as meaning the ideal type whereby God made all things, and which is appropriated to the Son. And although they knew these were appropriated to the three persons, yet they are said to have failed in the third sign—that is, in the knowledge of the third person, because they deviated from the goodness appropriated to the Holy Ghost, in that knowing God they did not glorify Him as God (Rom. 1); or, because the Platonists asserted the existence of one Primal Being whom they also declared to be the father of the universe, they consequently maintained the existence of another substance beneath him, which they called mind or the paternal intellect, containing the idea of all things, as Macrobius relates (Som. Scip. iv.). They did not, however, assert the existence of a third separate substance which might correspond to the Holy Ghost. So also we do not assert that the Father and the Son differ in substance, which was the error of Origen and Arius, who in this followed the Platonists. When Trismegistus says, Monad begot monad, etc., this does not refer to the generation of the Son, or to the procession of the Holy Ghost, but to the production of the world. For one God produced one world by reason of His love for Himself.

Reply Obj. 2. Reason may be employed in two ways to establish a point: firstly, for the purpose of furnishing sufficient proof of some principle, as in natural science, where sufficient proof can be brought to show that the movement of the heavens is always of uniform velocity.

Reason is employed in another way, not as furnishing a sufficient proof of a principle, but as confirming an already established principle, by showing the congruity of its results, as in astrology the theory of eccentrics and epicycles is considered as established, because thereby the sensible appearances of the heavenly movements can be explained;

not, however, as if this proof were sufficient, forasmuch as some other theory might explain them. In the first way, we can prove that God is one; and the like. In the second way, reasons avail to prove the Trinity; as, when assumed to be true, such reasons confirm it. We must not, however, think that the trinity of persons is adequately proved by such reasons. This becomes evident when we consider each point; for the infinite goodness of God is manifested also in creation, because to produce from nothing is an act of infinite power. For if God communicates Himself by His infinite goodness, it is not necessary that an infinite effect should proceed from God: but that according to its own mode and capacity it should receive the divine goodness. Likewise, when it is said that joyous possession of good requires partnership, this holds in the case of one not having perfect goodness: hence it needs to share some other’s good, in order to have the goodness of complete happiness. Nor is the image in our mind an adequate proof in the case of God, forasmuch as the intellect is not in God and ourselves univocally. Hence, Augustine says (Tract. xxvii. in Joan.) that
by faith we arrive at knowledge, and not conversely.

Reply Obj. 3. There are two reasons why the knowledge of the divine persons was necessary for us. It was necessary for the right idea of creation. The fact of saying that God made all things by His Word excludes the error of those who say that God produced things by necessity. When we say that in Him there is a procession of love, we show that God produced creatures not because He needed them, nor because of any other extrinsic reason, but on account of the love of His own goodness. So Moses, when he had said, In the beginning God created heaven and earth, subjoined, God said, Let there be light, to manifest the divine Word; and then said, God saw the light that it was good, to show the proof of the divine love. The same is also found in the other works of creation. In another way, and chiefly, that we may think rightly concerning the salvation of the human race, accomplished by the Incarnate Son, and by the gift of the Holy Ghost.




We proceed thus to the First Article:—

Objection 1. It seems that it was not fitting for God to become incarnate. Since God from all eternity is the very essence of goodness, it was best for Him to be as He had been from all eternity. But from all eternity He had been without flesh. Therefore it was most fitting for Him not to be united to flesh. Therefore it was not fitting for God to become incarnate.
Obj. 2. Further, it is not fitting to unite things that are infinitely apart, even as it would not be a fitting union if one were to paint a figure in which the neck of a horse was joined to the head of a man (Horace, De Arte Poet.). But God and flesh are infinitely apart; since God is most simple, and flesh is most composite,—especially human flesh. Therefore it was not fitting that God should be united to human flesh.
Obj. 3. Further, a body is as distant from the highest spirit as evil is from the highest good. But it was wholly unfitting that God, Who is the highest good, should assume evil. Therefore it was not fitting that the highest uncreated spirit should assume a body.
Obj. 4. Further, it is not becoming that He who surpassed the greatest things should be contained in the least, and He upon Whom rests the care of great things should pass on to lesser things. But God—Who takes care of the whole world—the whole universe of things cannot contain. Therefore it would seem unfitting that He should be hid under the frail body of a babe in swathing bands, in comparison with Whom the whole universe is accounted as little; and that this Prince should quit His throne for so long, and transfer the government of the whole world to so frail a body, as Volusianus writes to Augustine (Ep. 135).

ON THE CONTRARY,, It would seem most fitting that by visible things the invisible things of God should be made known; for to this end was the whole world made, as is clear from the word of the Apostle (Rom. 1:20): For the invisible things of Him … are clearly seen, being understood by the things that are made. But, as Damascene says (De Fide Orthod. iii.), by the mystery of the Incarnation are made known at once the goodness, the wisdom, the justice, the power or the might of God—the goodness, for He did not despise the weakness of His own handiwork; the justice, since, on man’s defeat, He caused the tyrant to be overcome by none other than man, and yet He did not snatch men forcibly from death; the wisdom, for He found a suitable discharge for a most heavy debt; the power, or infinite might, for there is nothing greater than for God to become incarnate.

I ANSWER THAT, To each thing that is befitting which belongs to it by reason of its very nature; thus, to reason befits man, since this belongs to him because he is of a rational nature. But the very nature of God is goodness, as is clear from Dionysius (Div. Nom. i.). Hence, what belongs to the essence of goodness befits God. But it belongs to the essence of goodness to communicate itself to others, as is plain from Dionysius (Div. Nom. iv.).

Hence it belongs to the essence of the highest good to communicate itself in the highest manner to the creature, and this is brought about chiefly by His so joining created nature to Himself that one Person is made up of these three—the Word, a soul and flesh,
as Augustine says (De Trin. xiii.). Hence it is manifest that it was fitting that God should become incarnate.

Reply Obj. 1. The mystery of the Incarnation was not completed through God being changed in any way from the state in which He had been from eternity, but through His having united Himself to the creature in a new way, or rather through having united it to Himself. But it is fitting that a creature which by nature is mutable, should not always be in one way. And therefore, as the creature began to be, although it had not been before, so likewise, not having been previously united to God in Person, it was afterwards united to Him.

Reply Obj. 2.

To be united to God in unity of person was not fitting to human flesh, according to its natural endowments, since it was above its dignity; nevertheless, it was fitting that God, by reason of His infinite goodness, should unite it to Himself for man’s salvation.

Reply Obj. 3. Every mode of being wherein any creature whatsoever differs from the Creator has been established by God’s wisdom, and is ordained to God’s goodness. For God, Who is uncreated, immutable, and incorporeal, produced mutable and corporeal creatures for His own goodness. And so also the evil of punishment was established by God’s justice for God’s glory. But evil of fault is committed by withdrawing from the art of the Divine wisdom and from the order of the Divine goodness. And therefore it could be fitting to God to assume a nature created, mutable, corporeal, and subject to penalty, but it did not become Him to assume the evil of fault.

Reply Obj. 4. As Augustine replies to Volusianus (Ep. 137): The Christian doctrine nowhere holds that God was so joined to human flesh as either to desert or lose, or to transfer and as it were, contract within this frail body, the care of governing the universe. This is the thought of men unable to see anything but corporeal things.… God is great not in mass, but in might. Hence the greatness of His might feels no straits in narrow surroundings. Nor, if the passing word of a man is heard at once by many, and wholly by each, is it incredible that the abiding Word of God should be everywhere at once? Hence nothing unfitting arises from God becoming incarnate.