Summa Theologiae

St. Thomas Aquinas

Selected Questions · 1265–1274

The Summa Theologiae is the masterwork of medieval philosophy and theology—an encyclopedic synthesis of Christian doctrine, Aristotelian philosophy, and rational argument. Written as a manual for theological students, it addresses over 500 questions across three parts, using the scholastic method of objection, contrary, response, and reply.

Summa Theologiae est opus praecipuum philosophiae et theologiae mediaevalis—synthesis encyclopaedica doctrinae Christianae, philosophiae Aristotelicae, et argumenti rationalis. Pro manuale studiosorum theologiae scripta, plus quam quingentas quaestiones per tres partes tractat, methodo scholastica obiectionis, contrarii, responsionis, et replicationis utens.

"Grace does not destroy nature but perfects it."

"Gratia non tollit naturam, sed perficit."

Part I: The One God

Question 2

The Existence of God

Whether the existence of God is self-evident? Whether it can be demonstrated? Whether God exists?

Utrum Deum esse sit per se notum? Utrum sit demonstrabile? Utrum Deus sit?

Article 3

Whether God Exists?

Objection 1

Obiectio 1

It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word "God" means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.

Videtur quod Deus non sit. Quia si unum contrariorum fuerit infinitum, totaliter destruetur aliud. Sed hoc intelligitur in hoc nomine Deus, scilicet quod sit quoddam bonum infinitum. Si ergo Deus esset, nullum malum inveniretur. Invenitur autem malum in mundo. Ergo Deus non est.

Objection 2

Obiectio 2

Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence.

Praeterea, superfluum est ponere plura, ubi pauciora sufficiunt. Sed videtur quod, Deo non existente, omnia quae apparent in mundo possint compleri per alia principia: quia ea quae sunt naturalia, reducuntur in principium quod est natura; ea vero quae sunt a proposito, reducuntur in principium quod est ratio humana vel voluntas. Nulla igitur necessitas est ponere Deum esse.

On the Contrary

Sed contra

It is said in the person of God: "I am Who am." (Exodus 3:14)

Dicitur ex persona Dei: "Ego sum qui sum." (Exodus 3:14)

I Answer That

Respondeo dicendum

The existence of God can be proved in five ways.

Deum esse quinque viis probari potest.

The First Way: Motion

Prima Via: Ex Motu

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act.

Prima autem et manifestior via est quae sumitur ex parte motus. Certum est enim, et sensu constat, aliqua moveri in hoc mundo. Omne autem quod movetur, ab alio movetur. Nihil enim movetur, nisi secundum quod est in potentia ad illud ad quod movetur: movet autem aliquid secundum quod est actu.

For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.

Movere enim nihil aliud est quam educere aliquid de potentia in actum: de potentia autem non potest aliquid reduci in actum, nisi per aliquod ens in actu: sicut calidum in actu, ut ignis, facit lignum, quod est calidum in potentia, esse actu calidum, et per hoc movet et alterat ipsum.

Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself.

Non autem est possibile ut idem sit simul in actu et potentia secundum idem, sed solum secundum diversa: quod enim est calidum in actu, non potest simul esse calidum in potentia, sed est simul frigidum in potentia. Impossibile est ergo quod, secundum idem et eodem modo, aliquid sit movens et motum, vel quod moveat seipsum.

Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover.

Omne ergo quod movetur, oportet ab alio moveri. Si ergo id a quo movetur, moveatur, oportet et ipsum ab alio moveri: et illud ab alio. Hic autem non est procedere in infinitum: quia sic non esset aliquod primum movens; et per consequens nec aliquod aliud movens, quia moventia secunda non movent nisi per hoc quod sunt mota a primo movente.

Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

Ergo necesse est devenire ad aliquod primum movens, quod a nullo movetur: et hoc omnes intelligunt Deum.

The Second Way: Efficient Cause

Secunda Via: Ex Ratione Causae Efficientis

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.

Secunda via est ex ratione causae efficientis. Invenimus enim in istis sensibilibus esse ordinem causarum efficientium: nec tamen invenitur, nec est possibile, quod aliquid sit causa efficiens sui ipsius; quia sic esset prius seipso, quod est impossibile.

Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one.

Non est autem possibile quod in causis efficientibus procedatur in infinitum, quia in omnibus causis efficientibus ordinatis, primum est causa medii, et medium est causa ultimi, sive media sint plura sive unum tantum.

Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false.

Remota autem causa, removetur effectus. Ergo, si non fuerit primum in causis efficientibus, non erit ultimum nec medium. Sed si procedatur in infinitum in causis efficientibus, non erit prima causa efficiens: et sic non erit nec effectus ultimus, nec causae efficientes mediae: quod patet esse falsum.

Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

Ergo est necesse ponere aliquam causam efficientem primam: quam omnes Deum nominant.

The Third Way: Possibility and Necessity

Tertia Via: Ex Possibili et Necessario

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be.

Tertia via est sumpta ex possibili et necessario, quae talis est. Invenimus enim in rebus quaedam quae sunt possibilia esse et non esse: cum quaedam inveniantur generari et corrumpi, et per consequens possibilia esse et non esse.

But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence.

Impossibile est autem omnia quae sunt, talia esse, quia quod possibile est non esse, quandoque non est. Si igitur omnia sunt possibilia non esse, aliquando nihil fuit in rebus.

Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence—which is absurd.

Sed si hoc est verum, etiam nunc nihil esset: quia quod non est, non incipit esse nisi per aliquid quod est. Si igitur nihil fuit ens, impossibile fuit quod aliquid inciperet esse: et sic modo nihil esset: quod patet esse falsum.

Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes.

Non ergo omnia entia sunt possibilia: sed oportet aliquid esse necessarium in rebus. Omne autem necessarium vel habet causam suae necessitatis aliunde, vel non habet. Non est autem possibile quod procedatur in infinitum in necessariis quae habent causam suae necessitatis, sicut nec in causis efficientibus, ut probatum est.

Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

Ergo necesse est ponere aliquid quod sit per se necessarium, non habens causam necessitatis aliunde, sed quod est causa necessitatis aliis: quod omnes dicunt Deum.

The Fourth Way: Gradation

Quarta Via: Ex Gradibus

The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest.

Quarta via sumitur ex gradibus qui in rebus inveniuntur. Invenitur enim in rebus aliquid magis et minus bonum, et verum, et nobile: et sic de aliis huiusmodi. Sed magis et minus dicuntur de diversis secundum quod appropinquant diversimode ad aliquid quod maxime est: sicut magis calidum est quod magis appropinquat maxime calido.

So that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in the book On the Causes.

Est igitur aliquid quod est verissimum, et optimum, et nobilissimum, et per consequens maxime ens: nam quae sunt maxime vera, sunt maxime entia, ut dicitur in libro De Causis.

Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.

Quod autem dicitur maxime tale in aliquo genere, est causa omnium quae sunt illius generis: sicut ignis, qui est maxime calidus, est causa omnium calidorum. Ergo est aliquid quod omnibus entibus est causa esse, et bonitatis, et cuiuslibet perfectionis: et hoc dicimus Deum.

The Fifth Way: Governance of the World

Quinta Via: Ex Gubernatione Rerum

The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result.

Quinta via sumitur ex gubernatione rerum. Videmus enim quod aliqua quae cognitione carent, scilicet corpora naturalia, operantur propter finem: quod apparet ex hoc quod semper aut frequentius eodem modo operantur, ut consequantur id quod est optimum.

Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer.

Unde patet quod non a casu, sed ex intentione perveniunt ad finem. Ea autem quae non habent cognitionem, non tendunt in finem nisi directa ab aliquo cognoscente et intelligente, sicut sagitta a sagittante.

Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

Ergo est aliquid intelligens, a quo omnes res naturales ordinantur ad finem: et hoc dicimus Deum.

Reply to Objection 1

As Augustine says: "Since God is the highest good, He would not allow any evil to exist in His works, unless His omnipotence and goodness were such as to bring good even out of evil." This is part of the infinite goodness of God, that He should allow evil to exist, and out of it produce good.

Reply to Objection 2

Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle.

The Divine Attributes

Question 3

The Simplicity of God

When the existence of a thing has been ascertained there remains the further question of the manner of its existence, in order that we may know its essence. Now, because we cannot know what God is, but rather what He is not, we have no means for considering how God is, but rather how He is not.

Therefore, we must consider: first, how He is not; secondly, how He is known by us; thirdly, how He is named. Now it can be shown how God is not, by denying Him whatever is opposed to the idea of Him—namely, composition, motion, and the like.

Therefore let us inquire first, concerning His simplicity, whereby we deny composition in Him.

"God is absolutely simple, having in Himself no composition whatsoever."

"Deus est omnino simplex, nullam in se compositionem habens."

God is not a body. God is not composed of matter and form. In God there is no composition of essence and existence. God is not in a genus. There is no accident in God. God is altogether simple.

Question 4

The Perfection of God

God is absolutely perfect. For there are perfections of all kinds in Him. He is absolutely perfect insofar as all perfections exist in Him—not merely some particular perfections, but all perfections universally.

Every perfection existing in an effect must be found in its cause—either in the same formality, as when a man generates a man, or in a higher formality, as when the sun generates fire. Since God is the first efficient cause, the perfections of all things must pre-exist in God in a more eminent way.

Part I-II: Human Acts

Question 1

The Last End of Human Life

Man must, of necessity, desire all whatsoever he desires for the last end. Now I say two things must be considered: first, the thing itself which we desire as a last end; secondly, the attainment or possession, the use or enjoyment, of that thing desired. In the first sense, the last end of man is an uncreated good, namely God, Who alone by His infinite goodness can perfectly satisfy man's will.

In the second sense, the last end of man is something created, existing in him, and this is nothing else than the attainment or enjoyment of the last end. Now the last end is called happiness. Therefore happiness is the last end of man.

"Happiness is a perfect operation."

"Beatitudo est operatio perfecta."

Question 2

In What Does Happiness Consist?

Happiness does not consist in wealth. For wealth is sought for the sake of something else, because of itself it brings us no good, but only when we use it, whether for the support of the body, or for some similar purpose. Now that which is the supreme good cannot be sought for the sake of something else.

Happiness does not consist in honors. For honor is not that reward of virtue for which the virtuous works: but he receives honor from men by reason of his virtue, as something less than virtue. Therefore happiness does not consist in honor.

Happiness does not consist in fame or glory. For glory consists in being well known and praised. But fame has no stability; rather it is changeable without any concurrence on our part. Therefore happiness does not consist in fame or glory.

Happiness does not consist in power. For power has the nature of a beginning; whereas happiness has the nature of an ultimate end. Moreover, power is open to good and evil: but happiness is man's proper good.

Happiness does not consist in any bodily good. For it is impossible for man's happiness to consist in goods of the body; since a thing cannot have its last end in that which is directed to something else as its end; and the body is directed to the soul as its end.

Happiness does not consist in pleasure. For the delightful and the useful do not transcend the good; but the useful is sought for the sake of something else, and the delightful on account of itself. That which is essentially the good is better than that which is good by participation.

Man's happiness does not consist in any good of the soul. For happiness is the soul's perfect good; but the soul is only perfectible by something better than itself. Therefore, happiness consists in something outside the soul.

"Final and perfect happiness can consist in nothing else than the vision of the Divine Essence."

"Ultima et perfecta beatitudo non potest esse nisi in visione divinae essentiae."

For perfect happiness the intellect needs to reach the very Essence of the First Cause. And thus it will have its perfection through union with God as with that object in which alone man's happiness consists.

The Virtues

Question 55

The Nature of Virtue

Human virtue is a habit. It is clear that virtue is a habit of a good quality, whereby we live righteously, of which no one can make bad use, which God works in us without us. Human virtue is an operative habit.

Virtue is that which makes its possessor good, and his work good likewise. Now for a thing to be good it must be fitting, and this in a twofold sense. First, in relation to itself; and this fitness comes to a thing from its disposition. Secondly, in relation to something else; and this fitness comes from the thing's order to that other thing.

"Virtue is a mean between two extremes."

"Virtus est medium inter duo extrema."

Question 61

The Cardinal Virtues

Four virtues are called cardinal, or principal: prudence, justice, fortitude, and temperance. A principal virtue is one that is concerned with something principal wherein reason has its full force. Now reason is perfected in its nature by prudence; the will by justice; the irascible appetite by fortitude; and the concupiscible appetite by temperance.

Prudence is the virtue that perfects reason in its practical operations. It is the virtue that enables us to discern in every circumstance what is truly good, and to choose the right means of achieving it. "Prudence is right reason applied to action."

Justice is the constant and perpetual will to render to each one his right. It is concerned with our dealings with others, whereby we give to each man his due, neither taking what belongs to another, nor refusing what is owed.

Fortitude strengthens the mind against the greatest dangers, which are those of death. It enables us to face difficulties and dangers and to overcome fear, not rashly or foolhardily, but with reason and purpose.

Temperance moderates our desires for sensible pleasures, keeping them within the bounds of reason. It perfects the part of the soul that is most subject to the pull of bodily desires.

Part II-II: Charity

Question 23

The Nature of Charity

Charity is friendship. For since there is a communication between man and God, inasmuch as He communicates His happiness to us, some kind of friendship must needs be based on this same communication. Now the love which is based on this communication is charity: wherefore it is evident that charity is a friendship of man for God.

"Charity is the form, mover, and root of all the virtues."

"Caritas est forma, motor, et radix omnium virtutum."

Charity is more excellent than faith or hope. For the excellence of faith and hope is to be measured according as they approach to charity, which unites man to God immediately. Faith makes us adhere to God as the source whence we derive the knowledge of truth; hope as the source whence we obtain the enjoyment of happiness; but charity attains God Himself that it may rest in Him, and not that something may accrue to us from Him.

Charity is a virtue. For charity is not merely a passion or feeling, but a habit of the will by which we love God above all things and our neighbor as ourselves for God's sake. It is the highest of all the theological virtues, for while faith and hope are about God as He is known and desired, charity is about God as He is loved in Himself.

Part III: Christ and the Sacraments

Question 1

The Fittingness of the Incarnation

It was fitting that God should become incarnate. For to each thing, that is befitting which belongs to it by reason of its very nature; thus, to reason befits man. Now the very nature of God is goodness. Hence, what belongs to the essence of goodness befits God. But it belongs to the essence of goodness to communicate itself to others.

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.

The work of the Incarnation was chiefly ordained to the restoration of the human race by the remission of sins. For Christ came into this world to save sinners, according to 1 Timothy 1:15. The Incarnation was also fitting for the full participation of the Divinity, which is the true bliss of man and end of human life.

"The Son of God became man that man might become God."

"Filius Dei factus est homo ut homo fieret Deus."

Question 60

The Nature of the Sacraments

A sacrament is a sign of a sacred thing insofar as it sanctifies men. Sacraments are needed for the sanctification of man in three ways. First, for the perfecting of man in those things which pertain to the worship of God according to the Christian religion. Secondly, as a remedy against the defects caused by sin. Thirdly, that the signs may serve as exercises beneficial to the virtues.

Sacraments are necessary for man's salvation for three reasons. The first is taken from the condition of human nature which is such that it has to be led by things corporeal and sensible to things spiritual and intelligible. The second reason is taken from the state of man who in sinning subjected himself by his affections to corporeal things. The third reason is taken from the fact that man is prone to direct his activity chiefly towards material things.

On Grace

Question 109

The Necessity of Grace

In the state of intact nature, man could by his natural power perform the good proportionate to his nature, without the addition of any supernatural gift; but he could not perform good that exceeded his nature, such as meritorious good.

But in the state of corrupt nature, man falls short even of what he could do by nature, so that he is unable to fulfil the whole of that good by his natural powers. Yet because human nature is not altogether corrupted by sin, so as to lose all natural good, even in the state of corrupted nature man can, by virtue of his natural power, work some particular good, as to build dwellings, plant vineyards, and the like.

Yet he cannot do all the good natural to him, so as to fall short in nothing; just as a sick man can of himself make some movements, yet he cannot be perfectly moved with the movements of one in health, unless by the help of medicine he be cured.

"Grace does not destroy nature but perfects it. Therefore natural reason should minister to faith as the natural inclination of the will ministers to charity."

"Gratia non tollit naturam, sed perficit. Unde ratio naturalis subservire debet fidei: sicut et naturalis inclinatio voluntatis obsequitur caritati."

Hence in the state of corrupted nature man needs a gratuitous strength superadded to natural strength for one reason—namely, in order to do and wish supernatural good. But for two reasons—namely, in order to be healed, and furthermore in order to carry out works of supernatural virtue, which are meritorious. Beyond this, in both states man needs the Divine help that he may be moved to act well.

The Unfinished Work

Aquinas left the Summa incomplete. On December 6, 1273, during Mass, he had a mystical experience after which he ceased writing entirely. When his secretary urged him to continue, he replied:

"I cannot, because all that I have written seems like straw to me."

"Non possum, quia omnia quae scripsi videntur mihi paleae."

He died three months later, on March 7, 1274, at the age of forty-nine. The Third Part of the Summa breaks off in the middle of the treatise on Penance. His friend Reginald of Piperno later compiled a supplement from Aquinas's earlier writings to complete the work.

The Summa Theologiae comprises 512 questions, 2,669 articles, and roughly 10,000 objections with their replies. It remains the most comprehensive and influential work of systematic theology in the Western tradition.