13.1 The sources

To understand the development of his thought correctly, we should list chronologically the works in which he treats of the existence of God: (1)

Scriptum super libros Sententiarum, I, d.3,q.1,a.1 1252-1256, during first Paris regency
De veritate, q. 5, a. 2 1256-1259
Summa contra gentiles, I & II 1259-65
De potentia, III, a. 5 1265-66
Compendium theologiae ad fratrem Reginaldum 1265-67
Summa theologiae, I, q. 2, a. 3 1266-68
Sententia super Physicam, Books 7 & 8 1268-69
Sententia super Metaphysicam, XII 1271-72

13.2 Thomas' rejection of wrong approaches to the question

13.2.1 The opinion that God's existence is self-evident (Cf. Summa contra Gentiles, I, 10)

Some persons consider superfluous and impossible any attempt to demonstrate that God exists because, they say, his existence is self-evident, in such a way that the contrary cannot be entertained in the mind, as may be seen from the following arguments:

St. Anselm argues as follows: Those propositions are said to be self-evident that are known immediately upon the knowledge of their terms. Thus, as soon as you know the nature of a whole and the nature of a part, you know immediately that every whole is greater than its part. The proposition God exists is of this sort. For by the name God we understand something than which nothing greater can be thought. This notion is formed in the intellect by one who hears and understands the name God. As a result, God must exist already at least in the intellect. But he cannot exist solely in the intellect, since that which exists both in the intellect and in reality is greater than that which exists in the intellect alone. Now, as the very definition of the name points out, nothing can be greater than God. Consequently, the proposition that God exists is self-evident, as being evident from the very meaning of the name God.

Another argument is that, since God's being is his essence, the question What is he? and the question Is he? have the same answer. Thus, in the proposition God exists, the predicate is either identical with the subject or at least included in the definition of the subject. Hence, that God exists is self-evident.

Again, what is naturally known is known through itself, for we do not come to such propositions through an effort of inquiry. But the proposition that God exists is naturally known since, as will be shown later on, the desire of man naturally tends towards God as towards the ultimate end. The proposition that God exists is, therefore, self-evident.

There is also the consideration that that through which everything else is known ought itself to be self-evident. Now, just as the light of the sun is the principle of all visible perception, so the divine light is the principle of all intelligible knowledge, since the divine light is that in which intelligible illumination is found first and in its highest degree. That God exists, therefore, must be self-evident.

By these and similar arguments some think that the proposition God exists is so self-evident that its contrary cannot be entertained by the mind.

13.2.2 A refutation of the above opinion (Ibid., 11)

The above opinion arises partly from the fact that people, right from childhood, hear about God and are taught to call on his name. As a result, the mind holds on to the existence of God very firmly, as something known naturally and self-evidently.

This opinion also partly arises from a failure to distinguish between what is self-evident in an absolute sense and what is self-evident in relation to us. God's existence is most evident in itself, since he is his own being. But we do not see this being; so God's existence is not self-evident to us.

Contrary to the first argument, it does not follow immediately that, as soon as we know the meaning of the name God, that the existence of God is known. First of all, not even all those who admit that God exists accept that God is that than which nothing greater can be thought. After all, many ancients said that this world itself was God. What is more, granted that everyone should understand by the name God something than which nothing greater can be thought, it will still not be necessary that there exist in reality such a thing. From the fact that we have such an idea, it only follows that it exists in the intellect. Denying God's existence is a problem only for someone who holds that in reality there is something than which nothing greater can be thought. And that must be proved.

As for the second argument, just as it is evident to us that a whole is greater than a part of itself, so to those seeing the divine essence in itself it is supremely self-evident that God exists because his essence is his being. But, because we are not able to see his essence, we arrive at a knowledge of his being, not through God himself, but through his effects.

The answer to the third argument is likewise clear. For man naturally knows God in the same way as he naturally desires God. Now, man naturally desires God in so far as he naturally desires happiness, which is a certain likeness of the divine goodness. On this basis, it is not necessary that God considered in himself be naturally known to man, but only a likeness of God. It remains, therefore, that man is to reach the knowledge of God through reasoning from the likenesses of God found in his effects.

As for the last argument, God is indeed that by which all things are known, not in the sense that they are not known unless he is known (as obtains among self-evident principles), but because all our knowledge is caused in us through his influence.

The position of St. Anselm that God's existence is self-evident is sometimes presented as "the ontological proof": It would be impossible to have an idea of an infinite, perfect being if there were no such being really existing. The argument was taken up by Descartes, Leibnitz and Hegel. Its refutation by Thomas Aquinas, given above, is sufficient. Today no school of philosophical thought upholds the ontological argument.

13.2.3 The opinion that God's existence can be known by faith alone (Ibid., 12)

A contrary opinion to the above also makes any proof for the existence of God useless. It is that we cannot arrive at the existence of God through reason; it is received by way of faith and revelation alone. This opinion is most common today among philosophers of religion, who take it as a dogma that the existence of God cannot be proved by reason.

This opinion originated from the weakness of some of the arguments advanced to prove that God exists. It also originates from a general scepticism about the power of the human intellect to know anything. Idealist philosophers deny that we can know the essence of anything or the causal connection between one thing and another. Their denial of our ability to prove the existence of God is just one application of their position that we cannot prove anything about reality.

Some of these philosophers argue that, since all our knowledge takes its origin from the senses, and God transcends all sense and sensible things, his existence must be indemonstrable.

Others, following the teaching of philosophers and theologians that we cannot know what God is and that we cannot define him, conclude that we cannot prove his existence. That is because every demonstration is based on the definition of a thing.

This opinion goes against all common sense and science and the art of logic, which teaches us to arrive at causes from their effects. If there is no knowable substance higher than sensible substance, there will be no science higher than physics [Cf. Aristotle, Metaphysics, IV, 3]. But from ancient times philosophers have tried to prove that immaterial substances exist and God is the cause of all existence. Likewise we read: "The invisible things of God are clearly seen, being understood by the things that are made" (Romans 1:20). "Blessed be He who set lights in the sky and set their a lamp and an illuminating moon. He is the one who made night and day succeed one another, for whoever wishes to reflect or be thankful" (Qur'ân 25:21-22).

We assume that we can know what created things are and their causal connections. Yet we do not argue from a knowledge of what God is to the fact of his existence. Rather, we argue from his effects to the fact of his existence. His effects give us an imperfect knowledge of what he is, since divine names are derived either by negating creaturely imperfections of him or by relating God in some way to his effects.

Although God transcends all sensible things and sense knowledge, his effects, on which the demonstration proving his existence is based, are nevertheless sensible things. Knowledge of these sensible things leads us to knowledge of God who transcends sense.

13.2.4 The Ash`arite argument for the existence of God

Another argument, presented by Ash`arite Muslim theologians, is based on the premise that the world must have had a beginning in time. It could not begin to exist by itself, but must have been produced by an eternal all-powerful being, which we call God. They attempt to prove the premise that the world had a beginning in time from the fact that a world that existed from eternity would imply an infinite series of night and day and of generation of men and animals. Such an infinite series, they say, is impossible. Therefore the world began in time.

Thomas Aquinas argues against the premise that an infinite temporal succession is impossible, by pointing out that such a series is not infinite in act, but only in potency. Here and now only a finite number of things exist. He concludes that we cannot prove by reason that the world either had a beginning or did not have a beginning, but its creation in time can only be known from revelation. (Cf. Summa contra gentiles, II, 38.)

Let us now look at Thomas' own attempt to prove the existence of God, following the five ways of the Summa theologiae, and presenting along with each way the corresponding texts from his other writings.

13.3 The arguments from motion and efficient cause

Summa contra gentiles, I, ch. 13 (the first three of five ways):

1) Everything that is being moved is being moved by something else. But it is evident through sensation that something is being moved, for instance the sun. Therefore it is being moved by something else moving it. [Thomas means a "pusher", not thinking to bring in his notion of impetus.] This mover will accordingly either be being moved or not. If it is not being moved, we have therefore what was proposed, that it is necessary to posit an immobile mover. And this we call God. but if it is being moved, it is being moved by something else moving it. The process will therefore either go on indefinitely, or it will arrive at some immobile mover. But it cannot go on indefinitely. Therefore it is necessary to posit a first immobile mover.

But in this proof there are two premises to be proved; namely, that everything that is being moved is being moved by another and that in movers and things one cannot proceed indefinitely.

Thomas goes on to explain the first, that everything that is being moved is being moved by another by three reasons: (a, from Physics 7) Where anything moves itself, it is by reason of one part moving another, (b, from Physics 8) by induction: that violent motion must come from an outside agent; natural motion comes from the form: the soul in the case of animals, the form given by the generator in the case of the gravitational falling of bodies when an impediment is removed, and (c, from Physics 8) by the general principle that to be moved is to go from potency to act; yet nothing can be at the same time in act and in potency with respect to the same thing; therefore to be moved must mean to be moved by another. Only the last of these arguments is retained in the Summa theologiae.

For the second premise, that in movers and things one cannot proceed indefinitely, he also gives three reasons: (a, from Physics 7) that infinite things cannot be moved in finite time, (b, from Physics 8) that motion has to be primarily caused by a first mover, while in an infinite series there is not first, (c, from Physics 8) that an instrument cannot move unless there is a first principal mover.

(2) The second way, from Physics 8, is here summarized: Not every mover is being moved. The unmoved mover is either entirely immobile or else self-moving. But if self-moving, it is being moved by an immobile part; Thomas then goes on to summarize Aristotle's arguments that the first mover is entirely immobile. Thomas later comes back to the same arguments in Summa contra gentiles, I, ch. 15 [5]:

We find in the world certain beings, those namely that are subject to generation and corruption, which can be and non-be. But what can be has a cause because, since it is equally related to two contraries, namely, being and non-being, it must be owing to some cause that being accrues to it. Now, as we have proved by the reasoning of Aristotle, one cannot proceed to infinity among causes. We must therefore posit something that is a necessary being. Every necessary being, however, either has the cause of its necessity in an outside source or, if it does not, it is necessary through itself. But one cannot proceed to infinity among necessary beings the cause of whose necessity lies in an outside source. We must therefore posit a first necessary being, which is necessary through itself. This is God, since, as we have shown, he is the first cause. God, therefore, is eternal, since whatever is necessary through itself it eternal.

(3) In Metaphysics II [2, 994a 1] Aristotle also uses another argument to show that there is no infinite regress in efficient causes and that we must reach one first cause, God. In all ordered efficient causes, the first is the cause of the intermediate cause, whether one or many, and this is the cause of the last cause. But, when you suppress a cause, you suppress its effect. Therefore, if you suppress the first cause, the intermediate cause cannot be a cause. Now, if there were an infinite regress among efficient causes, no cause would be first. Therefore, all the other causes, which are intermediate, will be suppressed. But this is manifestly false. We must, therefore, posit that there exists a first efficient cause. This is God.

Compendium theologiae ad fratrem Reginaldum, chapter 3:

Concerning the unity of the divine essence, first it must be believed that God exists, which can also be seen by reason. For we see that all that is moved is moved by others, the lower by the higher, such as elements by heavenly bodies, and among elements and heavenly bodies the lower by the higher. This process cannot be traced back into infinity. For everything that is moved by another is a sort of instrument of the first mover. Therefore, if a first mover is lacking, all things that move will be instruments. But if the series of movers and things moved is infinite, there can be no first mover. In such a case, these infinitely many movers and things moved will all be instruments. But even the unlearned perceive how ridiculous it is to suppose that instruments are moved unless they are moved by some principal agent. This would be like fancying that when a chest or a bed is being built, the saw or the hatchet performs its functions without the carpenter. Accordingly there must be a first mover that is above all the rest; and this being we call God.

Sententia super Metaphysicam, XII:

2521 Now it is said that the first mover causes motion as something appetible because the motion of the heavens has this mover as its end or goal, for this motion is caused by some proximate mover which moves on account of the first unmoved mover in order that it may be assimilated in its causality to the first mover and bring to actuality whatever is virtually contained in it. For the motion of the heavens does not have the generation and destruction of lower bodies as its end, since an end or goal is nobler than the things ordained to it. Therefore the first mover causes motion as something appetible.

2529 He now relates the first unmoved mover to the first sphere. He says that, since the first unmoved mover causes motion as something loved, there must be something which is first moved by it, through which it moves other things. This is the first heaven. Therefore, since we suppose motion to be eternal, the first sphere must be moved eternally, and it in turn must move other things. And it is better to speak of it as something loved rather than as something desired, since there is desire only of something that is not yet possessed, but there is love even of something that is possessed.

2534 It is on this principle, i.e., the first mover viewed as an end, that the heavens depend both for the eternality of their substance and the eternality of their motion. Consequently the whole of nature depends on such a principle, because all natural things depend on the heavens and on such motion as they possess.

2557 The stars are eternal and are substances. Hence their mover must also be eternal and a substance; for a mover is prior to the thing moved, and that which is prior to a substance must be a substance. It is clear, then, that there must be as many substances as there are motions of the stars [i.e. planets], and that these substances must be by nature eternal and essentially immovable and without magnitude, for the reason given above, i.e., because they move in infinite time and therefore have infinite power. Hence it is evident that there are immaterial substances which are as numerous as the motions of the stars, and that they have the same order as the motions of the stars.

2558 Now it must be borne in mind that after the first motion Aristotle computes only the motions of the planets, because at his time the motion of the fixed stars had not been detected. Hence he thought that the eighth sphere, in which the fixed stars are located, was the first one to be moved, and that is mover was the first principle. But later on astronomers perceived that the motion of the fixed stars was in an opposite direction to the first motion, so that above the sphere of the fixed stars it was necessary to posit another sphere, which surrounds the entire heavens and turns the whole in its daily motion. This is the first sphere, which is moved by the first mover of which Aristotle speaks. (2)

2613 The first mover is its own act of understanding... 2615 His act of understanding must be most perfect. Therefore he understands himself most perfectly. Now the more perfectly a principle is known, the more perfectly is its effect known in it; for things derived from principles are contained in the power of their principle. Therefore, since the heavens and the whole of nature depend on the first principle, which is God, God obviously knows all things by understanding himself.

Summa theologiae, I, q. 2, a. 3, the first two ways:

The first and most obvious way is based on motion. Some things in the world are being moved: this we plainly see. Now anything that is being moved is being moved by something else. For nothing is moved except as it is in potency to that towards which it is being moved, whereas something moves another as it is in act. That is because moving is nothing else than bringing something from potency to act. But nothing can be brought from potency to act except through an actual being: thus something actually hot, like fire, makes wood, which is potentially hot, to be actually hot, and in this way moves and alters it. 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. Therefore, whatever is moved is moved by another. If that by which it is moved be itself moved, then this also must be moved 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, since subsequent movers move only in as much as they are put in motion by the first mover, as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, moved by no other, and this everyone understands to be God.

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

13.4 Critique of the first two ways based on motion and efficient causality

Of the preceding texts, Contra gentiles, ch. 13, and the Compendium theologiae do not go beyond the cosmological arguments proposed in Physics 7 & 8. Metaphysics XII makes a significant addition in proposing, besides the souls or separate spirits that push the spheres, a distinct set of spirits that act as final causes for each of these motions, the first of which is God. These two were not distinguished in the Physics, where Thomas simply says that the mover of the first sphere is God.

The Summa theologiae, like Contra gentiles, ch. 15, presents the same argument in the more general terms of potency and act, without making any reference to a cosmological system. Although Thomas' words could be accommodated to what later Thomists called "divine pre-motion", in a way that the existence of all motion and all things depends immediately upon God, this would be a forced reading of the text. The text is clearly a summary of the cosmological argument from motion, deriving from Aristotle, as proposed in Thomas' other works. Thomas thought this was a solid argument, and never for once doubted its validity.

I have translated Thomas' principle "Quidquid movetur ab alio movetur" as "Whatever is moved is moved by another". Some translations have: "Whatever is in motion is put in motion by another" (English Dominicans); "Anything in the process of change is being changed by something else" (Timothy McDermott, vol. 2 of Gilby edition). One thing Thomas never intended to assert, as the last translation implies, is that natural movement depends on a mover here and now; the only movers he admits are the generator (responsible for the substantial form, from which motion flows), a thrower (who gave a transient accidental impetus), and accidental causes such as one who removes an obstacle (here and now).

The argument from efficient cause, as proposed in Contra gentiles, ch. 13 (3) and Summa theologiae (2), though expressed in more general terms and seemingly referring properly to the substantial generation of things, shows nothing that really goes beyond the argument from motion.

The meaning of Thomas' argument is clear from Aristotle's Physics VII and VIII, his Metaphysics XII, and Aristotle's cosmological treatise, On the heavens. There Aristotle makes it clear that all life on earth depends on the sun. Its changing positions bring the wind, rain and heat that bring about all motion on the earth. Since he believed in an eternal universe, he held that the sun is incorruptible; it heats, but is not hot or on fire. Likewise he maintained that the sun's motion around the earth (according to the Ptolemaic theory) was not a natural motion, like gravity, but required constantly renewed energy to keep it going. Aristotle had no idea of impetus (or inertia), whereby an agent can communicate to a projectile a transient accidental form {resembling the permanent form of gravity} that keeps it in motion until this form is corrupted by resistance. Aristotle realized that no power could keep fuelling the sun, moon and the planets on their daily course around the earth for eternity unless it had infinite energy. Infinite energy cannot be contained in any body. Therefore the movers of these heavenly bodies must be spirits. These spirits carry out this task in service of the earth below out of love of the supreme principle of the universe, God himself. One only needs to study De potentia q. 5, a. 7-9 to see how tightly Thomas makes the action of all living and non-living things on earth depend on heavenly bodies. "In these things there is no principle of motion not dependent on the first mobile [the highest heavenly sphere], since the very souls of animals and plants are totally subject to impressions of the heavenly bodies. Thus, if heavenly motion stops, these things cannot move or live" (a. 9).

Aristotle's universe consisted of a chain of movers depending here and now on a spiritual source. But once we introduce the notion of impetus {to say that the heavenly bodies are no different from man-launched satellites kept in motion by the two vectors of gravity and an impetus perpendicular to gravity, which need no refuelling but require only their initial propulsion} then there is no need to postulate spiritual forces to push the moon and other heavenly bodies. Once they were initially set in motion, they go on by themselves, just like natural motion, which Aristotle and Thomas Aquinas said requires no efficient cause here and now.

By the principle "Whatever is moved is moved by another" Aristotle and Thomas never meant that "Whatever is in motion is moved by another," but "Whatever is set in motion is set in motion by another." Modern physics shows that the heavenly bodies were set in motion in the distant past and need no additional energy to keep on going. Thus the argument from motion, as Aristotle constructed it, carries no weight. And once we try to construct a chain of movers going into the past we arrive at nothing certain.

In his article, "The conclusion of the prima via", (3) Joseph Owens argues that in the prima via of the Summa theologiae Thomas, in contrast to Aristotle, "approaches the question from the viewpoint of the essential imperfection of motion. Such an approach, based on the relation of the imperfect to the perfect, is on the metaphysical level." (4) I reply that, although Thomas' reasoning overlaps with metaphysics, it cannot be formally metaphysical, since metaphysics presupposes that the existence of immaterial substances has been demonstrated; then it considers these and what is in common between material and immaterial substances. If, as I have maintained, Aristotle's argument from motion failed to establish the existence of an immaterial first mover, then the other arguments given by Thomas must be taken still as part of natural science seeking to demonstrate the existence of an immaterial cause for the physical world.

Owens goes on to argue that Aristotle's argument from motion (if valid) arrives only at a multitude of separated or angelic substances, none of which is the Christian God. He explains:

The root of the difficulty undoubtedly lies in the ambiguous conception of "pure act". The argument from motion, by eliminating all potency, finally reaches pure act. But if with Aristotle potency is equated with matter and act with form, the pure act reached will be form only. If, on the other hand, the form toward which motion tends is treated as part of an essence in potency to existential act, then the elimination of all potency whatsoever will result immediately in a subsistent existential act. Such a pure act cannot at any stage be looked upon as indifferently an intellectual soul or a finite separate substance or as the Christian God. It is seen at once to be identified with the "I am who am" of Exodus. (5)

I reply that there is no evidence that Thomas understood the argument from motion differently from Aristotle or that he ever doubted the validity of Aristotle's argument. Yet in the way he phrased the argument in the Summa theologiae, he prepared the way for the following arguments:

13.5 The argument from contingency

Scriptum super libros Sententiarum, I, d.3, q.1, a.1:

Thomas' commentary on the Sentences of Peter Lombard already has the three ways of knowing God in this life later propounded in his Summa theologiae (I, q. 12, a. 12): by causality, by removal, and by eminence. With regard to these he says:

(1) The first reason is taken by way of causality, and takes this form: Everything that has being from nothing [i.e. from non-being, preceding generation] must exist by means of something from whom its being flowed. But all creatures have being from nothing, which is manifested from their imperfection and potentiality. Therefore they must exist by some first being, and this is God.

(2) The second way is taken by way of removal, and is like this: Beyond every imperfect thing there most be something perfect which is not mixed with any imperfection. But a body is something imperfect, because it is limited and defined by its dimensions and is mobile. Therefore, beyond bodies, there must be something which is not a body. Likewise, every non-bodily thing which is changeable is imperfect by is very nature. Therefore, beyond all changeable species, such as souls and angels, there must be a being that is non-bodily, immobile and completely perfect, and that is God.

Summa contra gentiles, I, ch. 16 [7]

7) We see something in the world that emerges from potency to act. Now, it does not educe itself from potency to act, since that which is in potency, being still in potency, can therefore not act. Some prior being is therefore needed by which it may be brought forth from potency to act. This cannot go on to infinity. We must, therefore, arrive at some being that is only in act and in no wise in potency. This being we call God.

Summa contra gentiles, II, ch. 15

4) Then, too, the order of causes necessarily corresponds to the order of effects, since effects are commensurate with their causes. Hence, just as effects are referred to their appropriate causes, so that which is common in such effects must be reduced to a common cause. Thus, transcending the particular causes of the generation of this or that thing is the universal cause of generation: the sun; and above the particular governors of the kingdom, as, indeed, of each city in it, stands the king, the universal cause of government in his whole realm. Now, being is common to everything that is. Above all causes, then, there must be a cause whose proper action is to give being. But we have already shown in Book I that God is the first cause. Everything that is must, therefore, be from God.

Summa contra gentiles, II, ch. 15

1) Now, because it has been proved that God is the source of being to some things, it must be demonstrated further that everything besides God derives its being from Him.

(2) For whatever does not belong to a thing a such appertains to it through some cause, as white to man; that which has no cause is primary and immediate, so that it must needs be through itself and as such. But no single entity can as such belong to two things and to both of them; for what is said of a thing as such is limited to that very thing; the possession of three angles equal to two right angles is proper to the triangle exclusively. So, if something belongs to two things, it will not belong to both as such. Therefore, no single thing can possibly be predicated of two things so as to be said of neither of them by reason of a cause. On the contrary, either the one must be the cause of the other {as fire is the cause of heat in a mixed body, and yet each is called hot} or some third thing must be the cause of both, as fire is the cause of two candles giving light. But being is predicated of everything that is. Hence, there cannot possibly be two things neither of which has a cause of its being, but either both of them must exist through a cause, or the one must be the cause of the other's being. Everything which is in any way at all must then derive its being from that whose being has no cause. But we have already shown that God is this being whose existence has no cause. Everything which is in any mode whatever, therefore, is from Him. Now, to say that being is not a univocal predicate argues nothing against this conclusion. For being is not predicated of beings equivocally, but analogically, and thus a reduction to one must be made.

(3) Furthermore, whatever a thing possesses by its own nature, and not from some other cause, cannot be diminished and deficient in it. For, if something essential be subtracted from or added to a nature, another nature will at once arise, as in the case of numbers, where the addition or the subtraction of the unit changes the species of the number. If, however, the nature or quiddity of a thing remains integral, and yet something in it is found to be diminished, it is at once clear that this diminution does not derive simply from that nature, but from something else, by whose removal the nature is diminished. Therefore, whatever belongs to one thing less than to others belongs to it not by virtue of its own nature alone, but through some other cause. Thus, that thing of which a genus is chiefly predicated will be the cause of everything in that genus. So we see that what is most hot is the cause of heat in all hot things; and what is most light, the cause of all illuminated things. But as proved in Book I, God is being in the highest mode. Therefore, He is the cause of all things of which being is predicated.

Summa contra gentiles, II, ch. 43 [8]

Also, just as the act of being is first among effects, so, correspondingly, is it the proper effect of the first cause. But it is by virtue of form and not of matter that this act exists. Therefore, the first causation of forms is to be attributed above all to the first cause.

De potentia

q.3, a.3: Creation is not a motion leading up to a term, but a fact that is; thus creation is not a progress towards existence or a change by the Creator, but merely a beginning of existence and a relationship to the Creator from which the thing holds existence; thus creation is really nothing else than a relationship to God with a temporal beginning.

q.3, a.5: First, if in a number of things we find something that is common to all, we must conclude that this something was the effect of some one cause: for it is not possible that this common something belong to each one by reason of itself, since each one by itself is different from the others: and diversity of causes produces a diversity of effects. Seeing then that being is found to be common to all things, which are by themselves distinct from one another, it follows of necessity that they must come into being not by themselves, but by the action of some cause. Seemingly this is Plato's argument, since he required every multitude to be preceded by unity not only as regards number but also in reality.

q.3, a.14, ad 10: The action by which God brings things into being should not be understood to be like that of a workman who makes a box and then abandons it, but God continually sustains existence... Thus there is no need to suppose an instant when he made things, before which they were not made, apart for the reason that Faith tells us this is so.

Read also q. 5, a. 1 (& ad 2 & 18).

Summa theologiae, I, q. 2, a. 3

(3) 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. 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. 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. 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. 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.

The way that Thomas formulated his argument from motion in the Summa theologiae constitutes a bridge to the argument from contingency. That is because this argument also starts from motion and substantial change, but then, instead of looking for a series of proper causes, it takes this motion or change as a sign of a radical instability of all changeable things, such that they do not have existence from themselves but depend constantly on a necessary being to keep them in existence and operation. The commentary on the Sentences (1) expresses this as regards coming into being; the same commentary (2) expresses this as regards the continued duration of things. Both these arguments are summarized as one argument, stressing that existence as the most generic effect must come from the most general Cause, in the Summa contra gentiles, I, ch. 16 (the same thing looked at from God downwards in II, chs. 15 & 43) and De potentia (3, as coming from Ibn-Sînâ), and are succinctly presented in Summa theologiae (3).

13.6 The argument from degrees of perfection

Scriptum super libros Sententiarum, I, d.3, q.1, a.1:

(3) Two other reasons are taken by way of eminence. But eminence can be taken in two ways: as to being or as to knowing. Therefore the third reason is taken by way of eminence in being, and is like this: The good and the better are spoken of in reference to the best. But in substances we find that a body is good and a created spirit is better, although it does not have goodness from itself. Therefore there must be something best from which exists the goodness in the first two.

(4) The fourth reason is taken by way of eminence in knowledge, and is like this: In whatever things can be found a greater and lesser degree of beauty, there is to be found a principle of beauty, by approximation to which one thing is said to be more beautiful than another. But we find that bodies are beautiful with a sensible form, while spirits are more beautiful with an intelligible form. Therefore there most by something from which each of them are beautiful, which created spirits more approximate.

Summa contra gentiles, I, ch. 13

4) In Metaphysics II [1, 993b 30] he shows that what is most true is also most a being. But in Metaphysics IV [4, 1008b, 31] he shows the existence of something supremely true from the observed fact that of two false things one is more false than the other, which means that one is more true than the other.

Summa contra gentiles, II, ch. 43 [9]

Furthermore, since every agent produces its like, the effect obtains its form from that reality to which it is made like through the form acquired by it; the material house acquires its form from the art which is the likeness of the house present in the mind. But all things are like God, who is pure act, so far as they have forms, through which they become actual; and so far as they desire forms, they are said to desire the divine likeness. It is therefore absurd to say that the formation of things is the work of anything other than God the Creator of all.

De potentia, III, a. 5

Later philosophers, such as Plato, Aristotle and their disciples, attained to the study of universal being: and hence they alone posited a universal cause of things, from which all others came into being, as Augustine states (The city of God, 8, 4). This is in agreement with the Catholic Faith, and may be proved by the three arguments that follow:

(1) First, if in a number of things we find something that is common to all, we must conclude that this something was the effect of some one cause: for it is not possible that this common something belong to each one by reason of itself, since each one by itself is different from the others: and diversity of causes produces a diversity of effects. Seeing then that being is found to be common to all things, which are by themselves distinct from one another, it follows of necessity that they must come into being not by themselves, but by the action of some cause. Seemingly this is Plato's argument, since he required every multitude to be preceded by unity not only as regards number but also in reality.

(2) The second argument is that whenever something is found to be in several things by participation in various degrees, it must be derived by those in which it exists imperfectly from that one in which it exists most perfectly: because where there are positive degrees of a thing so that we ascribe it to this one more and to that one less, this is in reference to one thing to which they approach, one nearer than another: for if each one were of itself competent to have it, there would be no reason why one should have it more than another. Thus fire, which is the extreme of heat, is the cause of heat in all things hot. Now there is one being most perfect and most true: which follows from the fact that there is a mover altogether immovable and absolutely perfect, as philosophers have proved. Consequently all other less perfect beings must derive being from him. This is the argument of the Philosopher.

(3) The third argument is based on the principle that whatsoever is through another is to be reduced to that which is of itself. Wherefore if there were a per se heat, it would be the cause of all hot things, that have heat by way of participation. Now there is a being that is its own being: and this follows from the fact that there must be a being that is pure act, in which there is no composition. Hence from that one being all other beings that are not their own being, but have being by participation, must proceed. This is the argument of Ibn-Sînâ (Comm. on Metaphysics, 8, 6; 9, 8).

Thus reason proves and faith holds that all things are created by God.

Summa theologiae, I, q. 2, a. 3

4) 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; 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 Metaphysics II. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum of 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.

All these ways look at what is imperfect, potential, finite and mobile as pointing to what is perfect, actual, infinite and immobile. They indicate a here-and-now dependence of the sensible and even the spiritual world upon what is Being itself, and do not refer to or depend on the cosmological assumptions of Aristotle (which Thomas nevertheless accepted).

While the argument of contingency focuses on the limited, labile and dependent existence of things, the argument from degrees of perfection focuses on limitation found in the essences of things, manifesting this by comparing things which vary in perfection. The commentary on the Sentences (3) shows that things which are imperfect with regard to being or (4) [= Contra gentiles, I, ch. 13 (4)] beauty and truth (because of potentiality) depend on something which has these perfections perfectly and independently of all else. Summa contra gentiles, II, ch. 43 presents briefly Thomas' teaching that God is the creator of all species (whereas individuals are simply the proper causes of multiplication of individuals within the species).

The very fact of diversity of being in the world, as argued in De potentia (2, repeated because it applies to both ways 3 & 4), following Plato, implies limitation and imperfection; all these scattered perfections, then, must be found united in one perfect being. The fact that in every category there are degrees of perfection, as argued in De potentia (2), following Aristotle, points to the same limitation and dependence on a perfect being. The same arguments are presented in De potentia (3) looking from God downwards. All these lines of thought are summarized in Summa theologiae (4).

13.7 The argument from purpose or design

De veritate, q. 5, a. 2

Providence is concerned with the direction of things to an end. Therefore, as the Commentator [Ibn-Rushd] says, whoever denies final causality should also deny providence... In that case all the harmony and usefulness found in things would be the result of chance. This was actually what Empedocles held. He asserted that it was by accident that the parts of animals came together in this way through friendship --and this was his explanation of an animal and of a frequent occurrence! This explanation, of course, is absurd, for those things that happen by chance, happen only rarely; we know from experience, however, that harmony and usefulness are found in nature either at all times or at least for the most part. This cannot be the result of mere chance; it must be because an end is intended. What lacks intellect or knowledge, however, cannot tend directly toward an end. It can do this only if someone else's knowledge has established an end for it, and directs it to that end. Consequently, since natural things have no knowledge, there must be some previously existing intelligence directing them to an end, like an archer who gives a definite motion to an arrow so that it will wing its way to a determined end. Now, the hit made by the arrow is said to be the work not of the arrow alone but also of the person who shot it. similarly, philosophers call every work of nature the work of intelligence.

Consequently, the world is ruled by the providence of that intellect which gave this order to nature; and we may compare the providence by which God rules the world to the domestic foresight by which a man rules his family, or to the political foresight by which a ruler governs a city or a kingdom, and directs the acts of others to a definite end. There is no providence, however, in God with respect to himself, since whatever is in him is an end, not a means to it.

Summa contra gentiles, I, ch. 13

(5) Damascene proposes another argument for the same conclusion taken from the government of the world. Ibn-Rushd likewise hints at it. The argument runs thus: Contrary and discordant things cannot, always or for the most part, be parts of one order except under someone's government, which enables all and each to tend to a definite end. but in the world we find that things of diverse natures come together under one order, and this not rarely or by chance, but always or for the most part. There must therefore be some being by whose providence the world is governed. This we call God.

Summa theologiae, I, q. 2, a. 3

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

The fact that things that have no intelligence act for intelligible purposes defined in their natures points to an intelligence that designed their natures and keeps them operating according to the intelligent purpose he has set for them. This argument is set out in De veritate, Contra gentiles (5) and Summa theologiae (5).

The argument from design is based on three quite distinct phenomena: One is the design found in composition and form of things, particularly complex things, like the design of an eye or of the human body. Secondly there is the design found in the operation of natural things for their own survival, whether we look at something so basic as gravity or the behaviour of plants and animals to ensure the survival of their species. Thirdly there is the design found in the interaction of diverse things in the world to form a harmonious ecological system; for instance, it is natural for a banana plant to produce bananas (the second type of design), but that bananas should be food for animals and men is an extrinsic purpose accidental to the banana plant, but part of a cosmic design, which we call God's providence.

13.8 Critique of these other arguments

All of these arguments, just like that from motion, are based on the potentiality and, hence, limitation and dependency of things. The different "ways" are not different arguments, but simply ways of looking at the various manifestations of potentiality. That of contingency points at the potentiality of essence to existence. Those of degrees of perfection and design point at limitations within the essences of things. These arguments do seem valid, and make up for the weakness of the argument from motion, based as it is on a Ptolemaic universe with no notion of impetus in the movement of heavenly bodies.

Thomas says at the conclusion of his commentary on the Metaphysics that "Aristotle's conclusion is that there is one ruler of the whole universe, the first mover, and one first intelligible object, and one first good, whom he called God, who is blessed for ever and ever. Amen." We have seen that Aristotle may not have been so successful as Thomas would like to believe, but where Aristotle failed, Thomas succeeded.


1. See J.A. Weiseipl, Friar Thomas d'Aquino, his life, thought and works (N.Y.: Doubleday, 1974), pp. 355 ff. Dates are revised according to Jean-Pierre Torrell, Initiation à saint Thomas d'Aquin (Editions Universitaires de Fribourg, 1993), pp. 483 ff.

2. This 'ninth" orb or sphere of which St. Thomas speaks was postulated by the astronomers in order to account for the motion which the celestial pole was discovered to be describing every 36,000 years. Since it encompassed all the other spheres, it was considered to be a ninth or outermost sphere, and therefore the first in order of all the spheres. The increasing complications found in the Ptolemaic system made it more and more implausible from a mathematical point of view.

3. The Modern Schoolman, 30 (1952-3), 33-53, 109-121, 203-215.

4. Ibid., p. 42.

5. Ibid., p. 207.