An Examination of Thomas Aquinas' Cosmological Arguments as found in the Five Ways
© Scott David Foutz

Introduction to the Topic

After a brief discourse on the scope of philosophical theology, Aquinas begins his Summa Theologica with a treatment of the existence of God 1 . The first point addressed in this treatment regards the question of whether or not the existence of God is self-evident 2 , to which Aquinas replies that there are two ways in which a thing may be self evident; (a) self-evident in itself and not to us, or (b) self-evident in itself and to us. The proposition "God exists" is indeed self-evident in itself, but is not so to us due to the fact that a priori knowledge of God's essence whereby we may understand the proposition 3 is beyond us. Therefore, in order for this to become self-evident to us, we require a demonstration by things which are better known to us, namely, effects.

The second point addressed pertains to whether or not God's existence can be demonstrated 4 . Aquinas concludes that such a demonstration is possible in two ways. We may either demonstrate God's existence through a priori [propter quid] argumentation through reference to the absolute prior cause. Or we may demonstrate His existence through a posteriori [quia] argumentation, proceeding from knowledge of the effect to the prior cause relative to us. Aquinas stresses the value of a posteriori argumentation, stating: "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... 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." 5

The third and last point addressed in the treatment of the existence of God is the question of whether or not God exists. Having thus concluded that the proposition "God exists" is self-evident in itself, and that a posteriori demonstration of this existence is possible, Aquinas now offers what has come to be known as the Five Ways, namely, five a posteriori proofs or demonstrations of the existence of God. The first three proofs are versions of the cosmological argument. The fourth proof is a somewhat unusual argument based on degree of being. The fifth proof is a teleological argument.

Aquinas views each one of these a sufficient proof in itself 6 and believes them to be proofs in the strongest sense of the word and not merely arguments for probability. Elsewhere he refers to "demonstrative arguments, by which our adversary may be convinced". In this same passage Aquinas distinguishes demonstrative arguments from "certain likely (i.e., probable) arguments which should be brought forth... for the training and consolation of the faithful, and not with any idea of refuting those who are adversaries. For the very inadequacy of the (probable) arguments would rather strengthen them in their error, since they would imagine that our acceptance of the truth of the faith was based on such weak arguments. 7 "

The relationship between arguments and the believer's acceptance of the truth by faith is for Aquinas a crucial one. He considers knowledge of certain truths about God as foundational to faith. He writes: "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 presupposes 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." 8

At the conclusion of the Five Ways, God emerges as First Mover, First Efficient Cause, Necessary Being, Maximum Being, and Intelligent End. This collective picture of God is far from complete in Aquinas' estimation. Rather, these truths of God, and especially the first three, provide the sole foundation for Aquinas' subsequent treatment of the nature or attributes of God. Therefore, the validity of his conclusions regarding God's simplicity 9 , perfection 10 , goodness 11 , infinity 12 , omnipresence 13 , immutability 14 , eternality 15 and unity 16 , depend predominantly upon the validity of his conclusions in the Five Ways. For this reason alone, the Five Ways deserve the attention of the student of theology.

But other reasons also present themselves through contemporary theological or philosophical discussions, where, for example, Aquinas' conclusion of divine simplicity is being seriously challenged by thinkers such as Alvin Plantinga 17 . Can such a challenge be limited to Aquinas' secondary conclusion of simplicity, or has Aquinas grounded simplicity in the Five Ways to such an extent that his opponents are forced to argue against the Five Ways' seemingly necessary implications? Such questions can only be answered once a thorough understanding is achieved regarding what Aquinas' Five Ways necessarily, plausibly and fail to imply. The aim of this paper is not do arrive at conclusions regarding specific critiques of Aquinas' formulations. What is proposed here is a thorough examination of Aquinas' three cosmological arguments, with the intent of clearly establishing the parameters of the foundation upon which Aquinas' subsequent treatment of God's nature rest. A restriction of this discussion to the three cosmological arguments 18 , given the intent of establishing Aquinas' the parameter of the entire foundation is in my view justified due to Aquinas' predominant use and mention of these three in his subsequent treatments. Let us now turn to the proofs themselves.

First Mover

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. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be moved from potentiality to actuality, except by something in a state of actuality. ...It is therefore impossible that in the same respect and in the same way a thing could be both mover and moved, i.e., that it should move itself. Therefore whatever is in motion must be put in motion by another. ...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; ...Therefore it is necessary to arrive at a first mover, put in motion by no other; and everyone understands this to be God. 19

Aquinas claims that this proof is the most manifest or obvious of the five ways. He also makes use of this particular argument more frequently than the others 20 . Aquinas adopts this proof from Aristotle, introducing it in Summa Contra Gentiles 1.13, as "the argument by which Aristotle proceeds to prove God exists". Aristotle's argument from motion can be found in Physics , 7.1. Aquinas' first way can be outlined thus:

1. We see that in the world some things are in motion [motus].

2. Anything in motion is put in motion by something else

a. since things being moved are only in potentiality to that toward which

they are in motion, whereas those things causing motion in others are

able to do so only insofar as they possess the act toward which they

move others.

i. because motion is simply the reduction of something from

potentiality to actuality,

ii. nothing can be thus reduced except by something in actuality.

b. Therefore, a thing cannot be both mover and moved with respect to the

same quality at the same time, viz., a thing cannot move itself.

3. The mover which puts another in motion must itself have been moved by

another, and that by another, etc..

4. This series of things moving and being moved cannot go on to infinity

a. for then there would be no first mover, and subsequently no other mover

i. since subsequent movers cause motion only inasmuch as they

have been put in motion by the first mover.

5. Therefore, there must be a first unmoved mover; and this everyone understands

to be God.

This argument begins with the fact that we are able to observe movement taking place within our sphere of reference. But one question arising from this argument has been, in what sense does Aquinas understand motion. The term both Aquinas and Aristotle use for "motion" is motus , which must be differentiated from the term mutatio for "change". In his Commentary on Aristotle's 'Physics' 21 , Aquinas defines motus as properly taking place in only three of the ten Aristotelian categories: quantity, quality, and place. Therefore, strictly speaking, motus will be confined to changes in location, color, texture, number and the like. Substantial change cannot be understood as motus , but rather by the broader mutatio 22 . This would seem to imply that Aquinas' First Mover will be so only in relation to the physical realm, as was Aristotle's.

However, some Thomists suggest that Aquinas intends his prima via to conclude with a metaphysical First Mover from which he is able to draw certain implications regarding the essence/existence distinction. Those arguing for this understanding tend to point to Aquinas' possible deviation from the purely Aristotelian argument which assumes the eternality of the world, the influence of the heavenly bodies, and a purely physical conclusion. Another line of argumentation points to the conclusions Aquinas later draws from the first way's discussion of the actuality/potentiality complex. The manner in which Aquinas will establish his argument that God's existence is identical with His essence will be through the conclusion that the First Being can possess no potentiality, a conclusion which, some argue, derives directly from the first way. If it can be shown that Aquinas draws metaphysical conclusions from his argument for a First Mover, it would indeed point to the probability that Aquinas understood motus as possessing a broader application than did his predecessor Aristotle.

But the evidence for a purely physical reading of the first way seems much more substantial than that which is suggested above. First, and perhaps foremost, Aquinas' own introduction of the argument points toward a physical understanding. He claims that this proof is the most manifest, implying that it will pertain to something apparent to most, if not all who encounter it. In similar manner he claims, "It is certain and evident to our senses, that in the world, some things are in motion" 23 . From this we are told that the motus under consideration is certain and evident to our senses, strongly evidencing a physical reading. Aquinas' examples of fire, the movement of the hand, the heavenly bodies 24 , also all point to his understanding of a purely physical motus .

A second point of evidence is found in Aquinas' own use of the argument. The most elaborate formulation of this argument is found in Summa Contra Gentiles , 1.13.. This formulation chronologically precedes the first way of the Summa Theologica , and may be viewed as Aquinas' conceptual basis for the latter. As noted, Aquinas here attributes this argument to Aristotle, mentioning him throughout its formulation. When compared to Aristotle's cosmological argument from motion presented in his Physics , it becomes evident that Aquinas' formulation as presented in Summa Contra Gentiles is developed in strict adherence to Aristotle's model. Each of the three proofs which Aquinas offers for the proposition everything that is moved is moved by another comes directly out of Aristotle's Physics , 6.4; 8.4; 8.5 respectively. Likewise, his three proofs for the proposition in movers and things moved one cannot proceed to infinity are derived directly from Physics , 7.1; 8.5; 8.5 respectively. Aquinas neither significantly adds nor innovates in his formulation of the argument 25 . The fact that the formulation found in Summa Theologica omits most of the rich references to physicality and Aristotle, points not to a change toward metaphysics in Aquinas' understanding of the terms of the argument, but to the fact that this Summa is intended as a text book for students of theology, rather than a commentary on Aristotle.

A third point of evidence for a physical understanding of the argument is derived from the way in which it has been understood up to Aquinas' time. As we have seen, Aristotle clearly understood it in physical terms. Maimonides also seems to have understood it is this way, and believed that he could derive from it an understanding of the God of the Bible 26 . And it was precisely due to the fact that it was deemed physic-bound that later scholastic thinkers such as John Duns Scotus rejected it. Aquinas nowhere seems to imply that this argument ought to be understood in any different terms than that through which it is presented, namely, physical motion.

From these considerations we may safely conclude that in the first way Aquinas is arguing for a first mover by which we can account for the movement of the physical universe. This first mover accounts for all motion or change in physical quality, quantity and location. Therefore it will also be in this manner that we must understand the potentiality/actuality complex. This fact will allow us to avoid a potential misunderstanding of Aquinas' example of the fire and the wood. He writes: "...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." 27

From this it could easily appear to the reader that what Aquinas is implying is that the mover first possesses that specific actuality toward which it moves another. This reading results from Aquinas' point that the fire as actually hot moves the wood from potentially to actually hot. But Aquinas cannot possibly be implying this if we are correct in assuming that he understands motus and thereby potentiality/actuality in strictly physical terms. For in this case, we would be required to conclude that God possesses each of the physical actualities of quality, quantity and location, a conclusion which clearly deniable. Therefore we must understand Aquinas to be implying solely that the mover's actuality moves another from potentiality to actuality. Perhaps he means to present heat as analogous to actuality which is passed on as actuality from mover to moved. But this is hardly the simpler or more immediate rendering. In any case, Aquinas intends to conclude this portion with the understanding that a thing cannot possess both actuality and potentiality with regard to the same quality at the same time, hence, a thing cannot move itself. We are therefore participants in a causal series whereby one thing moved by another (moved by another, etc.) moves another (which moves another, etc.).

But, Aquinas claims, this causal series cannot possibly go on to infinity 28 . By this claim Aquinas is not committing himself to the impossibility of an infinite series of finite things 29 . Instead, he is claiming the impossibility of an infinite series of moved movers or intermediate movers. The reason for this is that if there existed only intermediate movers (i.e., a moved something moving another, etc.), there could be no first mover, which would result in there being no other mover. Aquinas' full argument for this is derived from Aristotle's Physics , 8.5, and is found in Summa Contra Gentiles , 1.13., and is an argument based on what some term an essentially subordinated series of causes . The scenario here is not one in which cause A moves B which causes motion in C which continues ad infinitum. This merely linear perspective differs from the hierarchical perspective of Aquinas' and Aristotle's argument. Rather, first cause A moves final cause Z through the intermediate causes B-Y. In this scenario, B-Y serve only as simple instruments for the delivery of the final cause. From this perspective is becomes clear that the existence of B-Y depends solely upon the fact that there exists cause A from which final cause Z proceeds. Aquinas and Aristotle equate observable physical motion with B-Y, and conclude (a) that such intermediate physical motions cannot possibly go on to infinity so as to preclude the possibility of there being a first mover, and (b) a first mover necessarily exists.

This then brings us through the first way. Let us look at what we have gained by doing so. First and foremost, we have a Unmoved First Mover which accounts for the motion of the physical universe. This First Mover is not self-moved 30 , since this would violate the potentiality/actuality principle. Our First Mover, therefore must be both Unmoved and Pure Motion 31 . In addition, we also may conclude that God is not a physical body for at least these two reasons. First, all bodies are in some way moved (or changed) by causing movement 32 ; but the First Mover is by definition Unmoved, thereby requiring the conclusion that the First Mover is not a body. Second, whereas every body possesses some potentiality (for motion), the First Mover as Pure Actual Motion cannot possibly be a body.

When all is said and done, the first way proves only that God is an immaterial Unmoved First Mover in relation to the physical motion of the universe. Although in the Summa Contra Gentiles Aquinas claims to have proved the existence of a being which is immaterial, Unmoved Mover, eternal and one 33 . But the Summa Theologica 's terse argument does not allow these further conclusions. Given only the formulation of the Summa Theologica , we might easily conclude that God may be a finite spiritual being.

First Efficient Cause

The second way is from the nature of efficient cause. In the world of sense we find there is an order of efficient causes. There is in no case (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 cause of the ultimate cause, whether the intermediate cause be several or only one. 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 cause, 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. 34

As with the first way, Aquinas attributes this argument from efficient causality to Aristotle 35 . In his Metaphysics , Aristotle had analyzed the notion of cause into a four-fold typology and argued that an infinite regress of causes was impossible in any of the four 36 . Aquinas' second way represents Aristotle's argument regarding efficient causes. The argument may be outlined as follows:

1. We see in the world an ordered series of efficient causes.

2. No thing can be its own efficient cause,

a. for this would require it to be prior to itself, which is impossible.

3. This series of efficient causes cannot be infinite,

a. for all efficient causes follow an order: first, intermediate, ultimate.

b. if there is no first cause, there can be neither ultimate nor intermediate

i. for to remove the first cause is also to remove its effect

c. in an infinite series there could be no first efficient cause, no ultimate

effect, nor any intermediate cause; but this is plainly false.

4. Therefore, a first efficient cause is necessary; which we all call God.

As with the first way, we must first ask what Aquinas means by efficient cause . Aristotle's understanding of efficient cause limited it to a cause of motion [ motus ], restricting its causal efficacy to changes in quality, quantity and location. This view, if held by Aquinas, would make the intention and implications of the second way not much different from the first. However, one distinction is possible given the simple fact that the first way accounts for motion or change and the second way accounts for the causes of motion or change. Avicenna (ibn Sina) saw grounds for a distinction within Aristotle's conception of efficient cause between a moving cause and an agent cause 37 . From this observation, Etienne Gilson suggests that Aquinas likewise is thinking of an ordered series of agent causes which produce both change and agent . He writes, "While the first [way] brings us to God as the source of cosmic motion..., the second [way] leads us to Him as the cause of the very existence of things. We knew that God was moving cause. We know now that He is cause of being" 38 .

Aquinas himself seems to support this conclusion in his discussion of God as the source of being. He writes: "For in Book I of this work it was shown, by means of Aristotle's demonstration, that there is a first efficient cause, which we call God. But an efficient cause brings its effects into being. Therefore, God is the cause of being to other things." 39 From this statement by Aquinas we must at least conclude that when it comes to drawing implications based on the second way, he will not limit his considerations to Aristotle's understanding of efficient causes as moving causes pertaining to quality, quantity and location. We may also conclude from the fact that the previous statement by Aquinas was made prior to the formulation of the second way, that whether or not he defines or redefines efficient cause, he is aware of the moving/agent distinction. And unless we can posit some reason why he must restrict himself to Aristotle's narrow definition of efficient cause when the coherency of the argument is not threatened, we may proceed with the expectation that Aquinas will intend to derive an existential First Cause from the second way.

We have therefore, in the second way, an argument from efficient causes, through which Aquinas focuses on a series in which an agent whose existence has been caused by another agent, causes the existence of another agent, etc.. Whereas this series cannot possibly go on into infinity 40 , the second way will conclude with the necessity of a First Efficient Cause which is to be understood as not only the efficient cause of physical movement (pace Aristotle), but also of the existence of the agents of motion. But this raises the question: In what way does Aquinas understand an intermediate agent to cause the existence of a subsequent agent?

Aquinas cannot be suggesting that intermediate agents cause being [ esse ] in another agent. He discounts this possibility in his discussion on the "The Mode of Emanations of Things from the First Principle" 41 , where he attributes the creation of being solely to God. Here he concludes: "So it is impossible for any creature to create, either by its own power, or instrumentally - that is, ministerially." 42 By this we must understand that the intermediate agent, that is, the intermediate efficient cause of the second way, cannot even instrumentally participate in the creation of being. This is true, Aquinas concludes, due to the fact that, "the secondary instrumental cause does not share in the action of the superior cause, except inasmuch as by something proper to itself it acts dispositively in relation to the effect of the principal agent. ...But the proper effect of God creating is what is presupposed to all other effects, and that is being taken absolutely. Hence nothing else can act dispositively and instrumentally towards this effect, since creation does not depend on anything presupposed which can be disposed by the action of the instrumental agent." 43

In other words, for any intermediate cause to truly be instrumental, is must cooperate or add to the effect of the prior cause, such that the latter could not be complete without (and thereby the latter "presupposes") the contribution of the former. Only in this case may the intermediate cause be said to act instrumentally, for if the effect of the prior cause is fully realized without any contribution by the intermediate cause, the latter can be said to produce nothing 44 . Incorporating this into our discussion of the second way, we find that we could say that the intermediate agent participates instrumentally in the creation of being in the subsequent agent only if the former's contribution to the First Efficient Cause's (i.e., God's) effect (of creating being) is necessary for the realization of that effect (i.e., God's act of creating being). But God's creation of absolute being is the ground of existence of all other intermediate agents, which makes it impossible for any created agent's contribution to be necessary for the fulfillment of the creation of absolute being.

How then are we to understand any intermediate agent's causing the existence of another agent? Whereas Aquinas understands the being or existence of any agent as fully accounted for in the creation of absolute being by God, perhaps the intermediate agent participates in the facilitation of the transition from absolute being to being found in the subsequent agent. In this way, the intermediate agent cannot be attributed with the creation of the subsequent agent's being, but could be said to act in some causal manner toward the individuated being of the subsequent being. Rather than viewing the intermediate agent as creating being itself, we might view it as causing absolute being to be in this or that particular thing.

In Summa Contra Gentiles , this is precisely what Aquinas claims happens in human procreation. "So in the production of a being of some particular kind, what is made through itself is that particular being ; what is made by accident is simply a being ; when a human being is born, it is a man that comes to be in the unqualified sense; ...Now, it is the first being alone which is the cause of being as being ; other things are the cause of being, by accident, and of this particular being, through themselves." 45 . From this we may accurately conclude that Aquinas views intermediate efficient causes as causing, through use of pre-existing absolute being, the existence of this or that particular thing, which in turn goes on to do the same. This, however, ought not to be understood as a creative act, for. "since to create is to produce being from nothing pre-existing, it follows that this act is exclusively God's own" 46 .

Let me here only briefly mention that, given even this account of Aquinas' understanding of efficient causes, a controversy revolves around his understanding of "an order of efficient causes". As seen in the first way, Aquinas views causal series hierarchically, a perspective from which he argues against the possibility of their continuing into infinity. In the second way, Aquinas will also argue for the impossibility of an infinite regress of efficient causes using the same hierarchical perspective. Is this hierarchy, then, what Aquinas understands to be the order behind the causal series? If so, rather than viewing the causal series linearly (e.g., a parent producing a child, who in turn becomes a parent, etc.), we would have to look for a hierarchical causal structure, one lower level of which would be human procreation. Some see in this implication a dependence upon Aristotle's cosmology in which the heavenly bodies act as causal intermediaries in the affairs of men. And indeed, Aquinas is not ignorant of this cosmology, but makes reference to it in several passages 47 . Whether or not Aquinas' cosmology is sympathetic toward Aristotle's is beyond the scope of this paper. Let it be noted, however, that some have on this ground discarded the entire second way as "archaic fiction" 48

Moving further into the second way, we see that its remainder consist of arguments identical to those of the first. The claim that no efficient cause can be its own cause is true due to the actuality/potentiality complex, which states that any reduction from potentiality to actuality requires something in actuality producing the reduction. Therefore, a potentiality cannot cause itself into actuality, since it lacks the necessary causal actuality. In this way, no efficient cause can account for its own reduction from potentiality to actuality. This fact holds true for the first efficient cause as well, and therefore we cannot posit the existence of a self-caused first cause. Rather, we must conclude an uncaused first cause. This same conclusion is reached in the first way in terms of an unmoved mover rather than a self-moved mover.

The second way continues to parallel the first by its argument against the possibility of an infinite regress of efficient causes. This conclusion is grounded in Aquinas' hierarchical understanding of efficient causal series, and understanding, as we have seen, which he shares with Aristotle. In the second way Aquinas specifically mentions that the number of intermediate efficient causes is irrelevant. For in an essentially subordinated series of causes, which the hierarchical perspective assumes, all intermediate causes serve as mere tools of the first cause. Therefore, whether there be one or several, for all practical purposes, intermediate causes may be considered a single whole 49 . This characteristic of intermediate causes, namely, their lack of efficacy apart from a first cause, makes even an infinite number of them inadequate to support any effect in and of themselves. From this and the preceding line of argumentation, Aquinas concludes that we must necessarily posit the existence of an Uncaused First Efficient Cause from which the existence and movement of all other beings is ultimately derived, and this is what we understand to be God.

Given the existence of this First Cause, what else are we able to conclude about its nature? Whereas this First Cause must account for all activity, He must be of pure form and possessing no matter 50 , in other words, God is immaterial. This lack of matter, as we have seen, implies pure form. Now the essence of any immaterial being is the form alone 51 , from which follows that being pure form, God is His own essence or nature . We also have seen that a being's existence is derived from external or internal principles or is identical with the being itself. As explained above, self-causation is impossible due to the potentiality/actuality complex, thereby ruling out derivation via internal principles. By definition, the First Cause is the Uncaused First Cause, and therefore derivation via external principles is also impossible. We see, therefore, that God is His own existence .

Aquinas will draw out other implications based this cosmological argument alone, including the conclusions that God is beyond definition of genus and difference 52 , is without accidents 53 , is absolutely simple 54 , does not enter into composition with other things 55 , is perfect 56 , is good 57 , is infinite 58 , is omnipresent 59 , is immutable 60 , is eternal 61 , and is a unity 62 . We see therefore, that, compared to the first, the second way yields a much greater exposition into the nature of God.

Necessary Being

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 be. But it is impossible for these always to exist, for that which is possible not to be, then at one time is not. Therefore, if everything is possible not to be, then at one time there was 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 already been 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. 63

Although this argument may also be traced back to Aristotle, it appears from Aquinas' formulation that Maimonides' third proof for God's existence may be the more immediate source 64 . A similar argument appears in Summa Contra Gentiles , 1.15.5., which, William Craig contends, though commonly though to be a similar version of the same proof, is in fact nearly identical to Averroes' revision of Avicenna cosmological argument 65 . If, therefore, Aquinas adheres strictly to Maimonides in the third way and to Averroes in Summa Contra Gentiles , Aquinas' apparently similar versions will in fact be two distinct arguments. The third way may be outlined thus:

1. We find in nature possible things which are not necessary things,

a. for we observe things generated and things corrupted.

2. Not all things can be merely possible things.

a. for what is possible at some time is not.

b. if all things are possible things, then at one time there exists nothing.

c. if at one time there is nothing, then even now there would exist nothing,

i. because non-existent things begin to exist only through existing


ii. if at one time there exists nothing, it would be impossible for

something to begin to exist; then even now there would be

nothing in existence; but this is not true.

(d). Therefore, some thing must be necessary.

3. Therefore, some thing must be necessary.

a. Necessary things either owe their necessity to another, or not.

b. an infinite regress of things owing their necessity to others is impossible

i. for intermediate causes cannot exist without a first cause.

4. Therefore, there must be a necessary thing whose necessity is of itself, and who

causes in others their necessity. This we understand to be God.

The third way is visibly divided into two halves 66 , each of which constitutes a separate argument. The first half begins with possible beings and concludes with the necessity of at least one necessary being. The second half begins which causally dependent necessary beings and concludes with an uncaused necessary being who accounts for the necessity in others. The distinctions between the two halves do not end with their starting and endings. For, as we will see, the first half seems to argue from the visible generation and decay of physical objects. Whereas the second half is grounded in the metaphysical realm of caused and uncaused necessary beings. For our conceptual benefit we will examine the argument in its two parts.

Part One: Possibility & Necessity

We turn now to the first half of Aquinas' third way, of which we must ask from the beginning: What does Aquinas mean by possible and necessary ? From the usage in the passage, Aquinas seems to define possible as that which is susceptible to corruption, rather than that whose non-existence is logically possible. In the same way, necessary seems to simply refer to that which is not susceptible to corruption. Aquinas nowhere hints that the corruption of such things are logically impossible, only that they do not corrupt for reasons unmentioned. It would, therefore, seem safe to conclude that this argument is not one from logically possible and necessary things. Rather, possible and necessary are to be understood solely in terms of the presence or lack of corruptibility. Aquinas does not inform us as to how we might identify possible things other than by seeing them undergo substantial change. Therefore, possible things are transitory things. Necessary things are therefore non-transitory things which do not undergo substantial change. This fact, however, does not imply that necessary things are eternal things, since they may still be subject to creation ex nihilo or annihilation, both events which would imply potential non-existence yet would allow for no substantial change. From this we may conclude that Aquinas is here presenting a physical argument which will conclude the necessity of some non-transitory thing.

From an understanding of Aquinas' reasoning elsewhere we may understand possible things, that is, those things susceptible to substantial change, as necessarily material things, since only matter/form composites are capable of undergoing a change in substratum while maintaining their essence to the degree it exists in the remaining form. Whereas necessary things, being incapable of substantial change, must be understood as possessing an essence composed solely of form. Therefore we may conclude that any being whose essence is comprised solely of its form is necessary. This would include such things as God, angels and divine ideas. And regarding possible things, we may conclude that all possible things are material things. But an interesting question arises when we attempt to conclude the converse, namely, all material things are possible (i.e., transitory) things . Does Aquinas believe this?

Aquinas does not insist that all material things are transitory, by allowing for those material things whose form completely exhausts the capacity of the matter to receive new forms, rendering the thing incapable of further substratum change. An example of such can be found, in Aquinas' view, in the celestial bodies 67 , which do not have the potentiality for non-existence, but rather continue forever (following their creation ex nihilo ) unless annihilated. This fact, then, would place the celestial bodies under the category of necessary things. For whatever is susceptible to substantial change is possible, and whatever is not susceptible is a necessary being.

A classic question raised at this point regards what Aquinas would say in response to a thing susceptible to substantial change but which nevertheless in actuality does not undergo any substantial change, for example, due to some type of external support. Could such a thing possibly defy the reasoning this argument? If such a thing could be found, it would bring into question the claim that possible things cannot always exist, a claim which lies at the heart of the argument. But the very fact that Aquinas and his predecessors have made this claim may reflect the fact that they have a response. Aristotle addresses the possibility of such a thing, namely, a thing which by nature is potential in regard to corruption but nevertheless does not corrupt. In De Caelo , Aristotle claims that such a thing simply is not possible. He writes,

"If certain things have the power both of being and not being, an outside limit must be set to the time of their being and their not being, the length of time, I mean, for which the thing can be or not be. (We are speaking of being in all the categories - man, white, three-cubits-long, and the rest.) For if the time is not of a certain definite length, but is always more than any given time, and there is none laid down which must exceed it, then the same thing will have the power of being for an infinite time and not being for another infinite time; which is impossible." 68

Aristotle here concludes that it is impossible for a thing to possess at the same time the potential for non-existence and the potential for infinite existence. Therefore anything in whose nature is found the potential for non-existence will necessarily pass into non-existence. It would appear that it is upon such reasoning that Aquinas and his predecessors confidently claimed both a possible thing cannot always exist , and if a thing is possible, at one time it is not . And these claims are crucial to the argument, as they pave the way for concluding the existence of necessary things.

But the intermediate step to that conclusion is the claim that since the previous is true, and if all things were possible things, then at one time there is nothing. This claim has prompted accusations of the logical fallacy of a quantifier shift from the claim that all possible things at some time are not, to at one time all possible things are not. Others have seen in Aquinas a reasoning by composition in the apparent argument that because all things (parts) are possible, the whole is possible. If Aquinas' conclusion stood solely on such reasoning, those critical of the argument would be justified. But such is not the case, for the significant ground of Aquinas' claim is its temporal perspective.

The claim that if all things are possible things then at one time nothing exists stands, given the truth that a possible thing cannot always exist, and given enough time. If indeed possible things are of limited duration, then in an infinite span of time, there will be a time when all things cease to exist. But is this time in the past, referring to the state of potentiality possessed by possible things prior to their generation? Or does this time point to an inevitable fate of corruptible things in an infinite span of time? The greater evidence points to the latter, given two considerations. First, Aquinas' arguments generally assume the infinite duration of the universe due to his belief that its duration was beyond demonstration. This inability to be demonstrated also characterizes the initial creation of possible things within the universe. Therefore, Aquinas felt that any proof for the existence of God operating under such an assumption would clearly be superior to an argument assuming a beginning of the universe. Given this tendency, we see how Aquinas might here use the same assumption of infinite duration, rather than basing his argument upon an assumed prior point in time in which the material universe came into being.

A second consideration is found in Maimonides' argument from which Aquinas derives his own. Maimonides' proof understand this time to be the eventual corruption of all possible things 69 . This would also seem to make probable the conclusion that Aquinas here understands a point in an infinite span in which all possible beings inevitably cease to exist. Some have here suggested that an infinite series of finite possible things would defy this conclusion. But this infinite series would not be possible if Aquinas understands matter to be among those possible things whose non-existence is inevitable. For if matter were to cease to exist, no other possible thing could exist, for as we have seen, all possible things are material things. In this way, the sheer duration of an infinite span of time retains its ability to observe the non-existence of all possible things.

If we are thus able to posit a time in the infinite future in which all possible things pass into non-existence, we must also conclude that such would be the case in the infinite past. Therefore, if we show that no possible thing exists in either past or future, there could be no possible things existing before us now. Aquinas' argument for this conclusion is the same as was used in the previous two ways, namely, the reduction of a thing from potentiality to actuality requires another actuality, thereby making it impossible that a thing cause itself. In contrast to his use of the actuality/potentiality complex if the previous arguments, Aquinas here utilizes primarily its temporal implications. That is, rather than stressing the inability for a thing to cause itself into actuality due to lack of the same, the point here is that a thing cannot temporally precede itself in order to actuate its own potentiality. This emphasis cooperates with Aquinas' stress on the temporal throughout this half of the third way. And it is with this temporal conclusion that Aquinas completes the first half of the third way. We must therefore postulate the existence of some necessary thing which is not subject to substantial change and which may give account for the otherwise impossible rise of the existence of possible things.

But it becomes quickly apparent that having established this conclusion, we have in fact established very little. For we know only that there exists necessary things which give account to the existence of possible things. We do not know the number or nature of these necessary things, only that they do not undergo substantial change. We do know from other portions of his writings that Aquinas counts God, angels and human souls among the necessary beings. Therefore, we have at least established our own soul's existence. And despite the previous attempt to place matter itself under the category of possible things, thereby making impossible an infinite series of finite beings, Aquinas himself will not cooperate with this effort. For as we have likewise seen, he understands the celestial bodies to be composed of matter so exhausted by their form so as not to be susceptible to substantial change, making them also necessary beings. Therefore, by the end of the first half of the third way, even the materialist is able to justify his view that matter is that necessary thing which gives account of the existence of all other possible things 70 . For this reason, the theist ought to rejoice that there yet remains a second half.

Part Two: Necessity Caused and Necessity Per Se

The second half of the third way presents an argument beginning with the claim that a necessary being has its necessity either from another or from itself, and ending with the existence of a necessary being whose necessity is from itself and who causes the necessity in other necessary beings. The conclusion of the first half was that a necessary thing must be postulated in order to account for the existence of possible things, but of the number and nature of these necessary things we do not know. For this reason, the second half will aim at providing conclusions about this realm of necessary beings.

Aquinas' first claim is that every necessary thing either has its necessity caused by another or not . We must ask what Aquinas means by cause of necessity . As we have pointed out above, the immediate source from whom Aquinas' derives this argument is Maimonides, who understood the issue in terms of an essence/existence distinction 71 . Those necessary beings whose existence depends upon another is possible per se , whereas a necessary being whose existence is from itself is necessary per se . Is this how Aquinas' understands the argument? If so, Aquinas here seems to be following Maimonides into adopting a metaphysical perspective which he has been unwilling to choose up until now. In De Potentia , Aquinas specifically rejects Avicenna's definitions of necessary and possible in terms of the essence/existence distinction, and instead follows Averroes' explanation of these in terms of susceptibility to substratum change 72 . In like manner, the third way has thus far defined necessary solely in terms of insusceptibility to substantial change, and has not yet introduced the essence/existence distinction. In addition, our discussion of the second way showed that Aquinas' understanding of the cause of existence among intermediate agents could not be in terms of the addition of essence to existence, for the act of creating essence belongs solely to God. How are we to understand these differences between Maimonides and Aquinas?

A closer look at Aquinas' position in context will render a possible solution. In De Potentia , the question under discussion is the definitional distinction between possible and necessary beings. In this context, Aquinas claims that such a distinction is not possible based upon the essence/existence distinction. For this distinction is not adequate to set the possible and necessary apart. To claim that possible beings can be defined by their existence's causal dependence would incorrectly imply that necessary beings' existence cannot be causally dependent. But Aquinas does not agree with this conclusion, since a necessary being's existence may be caused ex nihilo , as in the case of angels and human souls. Therefore, the position taken by Aquinas in De Potentia should not be viewed as a denial of the essence/existence distinction in necessary beings, but rather an insistence for a definition of possible and necessary not based on causality. This fact also influences Aquinas' definitions in the first half of the third way where he is concerned with the distinction between necessary and possible beings. In such a context, the introduction of the essence/existence distinction would be both unnecessary and confounding, whereas the definition based on susceptibility to substantial change proves fruitful.

Aquinas' use of cause of existence in the second way precluded the possibility of understanding this in terms of an intermediate agent's creative ability to add existence to essence. As we saw, Aquinas elsewhere argued that the creation of absolute being belonged solely to God. Therefore, this causality was to be understood in terms of the facilitating of pre-existing absolute being into this or that particular being. Is this how we are to understand the cause of necessity in the third way? Certainly, if Aquinas suggested the presence of intermediate necessary agents in the causation of necessity, we would need to restrict our understanding to that of the second way. But Aquinas does not and cannot here suggest the role of intermediate causes. For by definition, a necessary being is one whose nature is not susceptible to substantial change, the existence or non-existence of which requires either creation ex nihilo or annihilation. Ex nihilo , by definition, refers to the absence of any necessary pre-existing thing 73 upon which the creative act depends. In other words, at the creation ex nihilo of a necessary being, that beings existence and essence is created. As we have seen, the role of intermediate agents in the cause of existence is necessarily limited to a facilitation of pre-existing absolute being into this or that particular being. Therefore, the act of the intermediate agent cannot be defined as a creative act. In the same way, there cannot possibly be an intermediate necessary agent in the act of creation ex nihilo . Therefore we may conclude that by cause of necessity in the second half of the third way, we may correctly understand cause of being . From this follows that if Aquinas is able to distinguish one necessary being whose necessity is of itself and who causes necessity in others, he will have also identified a necessary being whose being is of itself and who causes being in others. In De ente et essentia Aquinas writes:

Whatever belongs to something is either caused by the principles of its nature, ...or accrues to it from some extrinsic principle.... It is impossible that the act of existing itself be caused by the form or quiddity - and by "caused" I mean as by an efficient cause - for then something would be the cause of itself and produce itself into existence, which is impossible. It is therefore necessary that everything whose act of existing is other than its nature have its act of existing from another. And because everything which exists through another is reduced to that which exists through itself, as to a first cause, there must be something which causes all things to exist, inasmuch as it is subsistent existence alone. Otherwise we would proceed to infinity in causes, since everything which is not a subsistent act of existing has a cause for its act of existing, as we have just said. It is evident, therefore, that an intelligence is a form and an act of existing, and that it has its act of existing from the First Being, which is existence only; and this is the First Cause, God. 74

The infinite regress of causally dependent necessary beings mentioned in the above passage is explicitly argued against in the third way. This argument consists of the same reasoning found in the second way, to which Aquinas points his readers. In review, the argument against an infinite series is based upon a hierarchical perspective in which the existence and effects of intermediate causes are dependent upon the existence and end of a first cause. 75 The application of this argument to the third way results in the conclusion that an infinite series of necessary beings who derive their necessity from others is clearly impossible. Therefore, given the fact that the existence of necessary beings was already established by the first half, and given that an infinite series of them is impossible, Aquinas is able to conclude his third way with a Necessary Being whose necessity is of itself and who causes the necessity of other necessary things. The principle conclusion which Aquinas is able to make from this argument is that God is His own existence. This becomes a cornerstone of Aquinas' theology, from which he is able to expand his explication of the nature of God. The subsistence of God sets Him apart from all other beings.


Aquinas' Five Ways promise to prove the existence of God based on observable phenomena. The three ways which we have examined argue from motion, causal agents and transitory beings. The arguments have collectively arrived at the necessity for a Being who is First Mover, First Efficient Cause and First Being. This Being, Aquinas claims, is what we all understand to be God. Aquinas' all refers to both philosopher and peasant, cleric and laity. However confoundedly, all men are want to attribute these descriptions to what he or she believes to be God. Aquinas has used arguments implemented by Aristotle, Plato, Maimonides, Avicenna and Averroes, in order to claim for his own cause the conclusions they render. And he believes that the foundation he has built with these five building blocks is able to support his subsequent doctrine of God. Throughout the next 11 questions of the Summa Theologica 76 , Aquinas will repeatedly ground his arguments for the various attributes of God in the initial conclusions of the Five Ways.

As we have seen, some of the elements of Aquinas' arguments are brought into question by contemporary scholars. And indeed the Five Ways in great part rely upon the validity of several other arguments from antiquity. For example, Aquinas' proofs make frequent use of Aristotle's argument against an infinite series based on the hierarchical perspective of an essentially subordinated series of efficient causes. In the age of quantum physics and fractal theory, one may wonder whether this perspective is any more intuitive than the linear perspective, from which, apparently, an infinite series is possible. Aquinas' third argument made use of the linear perspective in order to demonstrate how all transitory beings will eventually cease, leaving nothing, but at the same time, he maintains that the celestial bodies will remain necessarily until annihilated by God 77 . If, as Aquinas allows, the celestial bodies remain forever in time, the linear causality would seem to be as accurately descript as the hierarchical. And if the linear, grounded in empirical observation, allows for an infinite series of causes or movements 78 , while the hierarchical causality, grounded in the inconclusive intuition that means exist for the end, how might the contemporary world be motivated to accept Aquinas' argument? And whereas Aquinas uses this hierarchical argument in all three of the ways examined, and yet also allows for the linear in the third, what degree of intuitive grip can Aquinas' conclusions maintain? Whereas the modern thinker may well agree with the notion of the planets always existing, it would be difficult to impress upon her the conviction that just as convincing is the fact that the physical order exists solely for teleological purposes. It is interesting to note, however, that Aristotle, Avicenna, Averroes and Aquinas were all of the view that the celestial bodies were eternal 79 , and yet they confidently employed the hierarchical schema in order to argue against the possibility of an infinite regress of efficient causes. Perhaps by identifying and reclaiming their intuition of the created order we might also reclaim the sense of telos which they attributed to their lives and their world.

This paper leaves other questions before the reader. For, as noted several times, the foundation of the Five Ways provides the foundation for Aquinas' doctrine of God. In this way they also provide the foundation for many, if not most, of the classical theological statements and arguments for God and His attributes. Therefore, having thus examined the foundation at length 80 , the next necessary task will be to examine the validity of Aquinas' reasoning from which he derives the various attributes. Alvin Plantinga, for example, takes great exception to Aquinas' doctrine of simplicity 81 , and argues briefly against the essence/existence equation in God. This equation stems directly from the third way, and rests on quite solid argumentation. If Aquinas' subsequent reasoning proves as plausible, critiques such as Plantinga will be required to argue against the Five Ways themselves. Plantinga, of course, does not tackle the third way, but rather restricts his argument to Aquinas' specific discussion on simplicity. But the point remains clear. The Five Ways were intended to support the entire schema of divine attributes. Therefore only a thorough knowledge of them and the implications they provide will allow for an intelligent and accurate response to those who argue against them.

1 ST , 1.q2.

2 ST , 1.q2.a1.

3 Human a priori knowledge does not grant us access to an adequate definition of the subject and predicate of this proposition. Whereas the equation of the subject and predicate are logically demonstrative, in that it may be proven that what we refer to as God is identical with absolute being, the proposition is self-evident in itself. But whereas we lack a priori knowledge of the terms of the proposition, it is not self-evident to us.

4 ST , 1.q2.a.2

5 Ibid.

6 Hence, the Five Ways and not the One Five-fold Way.

7 Summa Contra Gentiles (hereafter SCG ), 1.9. (additions mine)

8 ST , 1.q2.a2.

9 ST , 1.q3.

10 ST , 1.q4.

11 ST , 1.qq5-6.

12 ST , 1.q7.

13 ST , 1.q8.

14 ST , 1.q9.

15 ST , 1.q10.

16 ST , 1.q11.

17 Does God Have A Nature? (Milwaukee: Marquette Univ. Press; 1980) Plantinga argues that simplicity inevitably results in the proposition God is a property, which precludes the possibility of His personhood. He proposes the alternative understanding that God has a nature distinct from Himself which possesses properties likewise distinct from Himself and over which He has no control. The platonic realm of ideas and abstract objects exist eternally and outside God's control. Therefore, one property which he regards God as having is the property of affirming 7+5=12.

18 Mention will be made, however, of the other two Ways when necessary or relevant.

19 ST , 1.q2.a3.

20 This argument also appears in SCG , 1.13; De Potentia Dei, 3.5; De Veritate, 5.2; as well as in Aquinas' commentaries on Aristotle.

21 5.4.678-81. Reference found in William Lane Craig's The Cosmological Argument from Plato to Leibniz . (London: Macmillan Publ.; 1980) 162.n.20.

22 Aquinas writes: "When a substance comes into being or perishes, it does not move from one terminus of change to another - it simply ceases to exist." Commentary on Aristotle's 'Physics' . 5.3.662; 6.5.797. Q. in Craig, 162, n.21.

23 ST , 1.q2.a3.

24 SCG , 1.13.

25 This observation is made by Craig, 168.

26 Ibid.

27 ST , 1.q2.a3.

28 It would seem that of importance here to Aquinas is solely a refutation of a series receding infinitely into the past. For the conclusion of an unmoved mover does not provide content regarding the end to which movement is employed. We might therefore be able to posit a causal series beginning in the unmoved mover and continuing infinitely (aeviternally) into the future.

29 In ST , 1.q46.a2, Aquinas allows for the possibility of an infinite series of accidental efficient causes, but disallows the possibility of an infinite series of per se intermediate causes employed to one end. The former series is to be understood as linear, whereas the latter series is hierarchical. 1.q7.a4 also mentions the impossibility of a final end passing through an infinite medium.

30 A view which Aquinas attributes to Plato in SCG , 1.13.

31 Aquinas will much more frequently speak of this in terms of Pure Actuality in which there is no potentiality, or simply, God as Pure Act.

32 Although this conclusion is not explicitly established through the argument of the Summa Theologica , it is fully explicated in SCG , 1.13 and Physics , 7.8.

33 In addition, in SCG , 2.6.3 Aquinas concludes from the first way that God is the source of being in others.

34 ST , 1.q2.a3.

35 SCG , 1.13.

36 Metaphysics , B.2.994a.1-15. This reference and overview is borrowed from Craig, 176.

37 Ibid.

38 Etienne Gilson, The Christian Philosophy of St. Thomas Aquinas, trans. L. K. Shook (New York: Random House; 1956) 67. Cited in Ibid.

39 SCG , 2.6.2.

40 A claim which we will examine below. Also see the discussion on the same claim in the first way, above.

41 ST , 1.q45.

42 ST , 1.q45.a5.

43 Ibid. A similar line of reasoning is presented by Aquinas in SCG , 2.21.

44 Nothing, that is, in regard to the effect of the prior cause.

45 SCG , 2.21.10. Italics mine.

46 Ibid.

47 E.g., ST , 1.q45.a5; 1.q46.a2; 1.q115.aa3-4.

48 This conclusion is reached by Anthony Kenny in The Five Ways: St. Thomas Aquinas' Proofs of God's Existence , (New York: Schocken Books; 1969) 44. Cited in Craig, 177. Craig notes: "A study of Aquinas' historical predecessors makes it abundantly clear that the causal series contemplated in the second way is the Aristotelian system of the spheres, for al-Kindi, al-Farabi (Abunaser), ibn Sina (Avicenna), ibn Rushd (Averroes), and Maimonides all assigned to the spheres the function of causing things on earth." (200.n.94; additions mine.)

49 Aquinas implies this by equating the net result of several with that of one.

50 The supporting line of reasoning can be found in ST , 1.q3.a2.

51 This is contrasted to the essence of material beings which is composed of the matter/form composite. Aquinas' full argument for this can be found in ST , 1.q3.a3.

52 ST , 1.q3.a5.

53 ST , 1.q3.a6.

54 ST , 1.q3.a7.

55 ST , 1.q3.a8.

56 ST , 1.q4.

57 ST , 1.q5-6.

58 ST , 1.q7.

59 ST , 1.q8.

60 ST , 1.q9.

61 ST , 1.q10.

62 ST , 1.q11.

63 ST , 1.q2.a3.

64 This observation is borrowed from Craig, 182. An outline and discussion of Maimonides' argument can likewise be found in Craig, 142-9.

65 Craig, Op. cit. One example of the Summa Contra Gentiles' dependence upon Averroes is its demand for a cause which determines whether a merely possible being will acquire existence or not. (201.n.106)

66 These two halves correspond to points 1-2 & 3-4 of the outline presented here.

67 ST , 1.q10.a5; 1.q75.a6; 1.q115.a6.

68 De Caelo , 1.12.281a.28-30.

69 This observation is borrowed from Craig, 187.

70 However, as the first way points out, the first mover must be immaterial.

71 The summation here of Maimonides' position is borrowed from Craig, 190.

72 Ibid. Craig offers no citation from De Potentia .

73 That is, besides the Creator.

74 IV.

75 This shift back to a hierarchical perspective stands in contrast to the temporal perspective of the first half of the third way. This shift also parallels the shift from the physical argumentation regarding transitory and non-transitory being in the first half to the metaphysical argumentation based on essence/existence distinctions.

76 1.qq3-13.

77 ST , 1.q10.a5; 1.q75.a6; 1.q115.a6.

78 I fail to see how the celestial bodies could remain without their movement.

79 More accurately, Aristotle, Avicenna and Averroes believed the celestial bodies to be eternal; and Aquinas believed them to be necessary, a distinction which allows for their possible annihilation by God.

80 But certainly not exhaustively.

81 In Does God Have A Nature?