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Medieval Sourcebook:
Thomas Aquinas:
Reasons in Proof of the Existence of God, 1270

Summa Theologia

Article II. Whether the existence of God is demonstrable:

Let us proceed to the second point. It is objected (1) that the existence of God is not demonstratable: that God's existence is an article of faith, and that articles of faith are not demonstratable, because the office of demonstration is to prove, but faith pertains (only) to things that are not to be proven, as is evident from the Epistle to the Hebrews, 11. Hence that God's existence is not demonstratable. Again, (2) that the subject matter of demonstration is that something exists, but in the case of God we cannot know what exists, but only what does not, as Damascenus says (Of the Orthodox Faith, I., 4.) Hence that we cannot demonstrate God's existence. Again, (3) that if God's existence is to be proved it must be from what He causes, and that what He effects is not sufficient for His supposed nature, since He is infinite, but the effects finite, and the finite is not proportional to the infinite. Since, therefore, a cause cannot be proved through an effect not proportional to itself, it is said that God's exisence cannot be proved.

But against this argument the apostle says (Rom. I., 20), "The unseen things of God are visible through His manifest works." But this would not be so unless it were possible to demonstrate God's existence through His works. What ought to be understood concerning anything, is first of all, whether it exists. Conclusion. It is possible to demonstrate God's existence, atthough not a priori (by pure reason), yet a posteriori from some work of His more surely known to us.

In answer I must say that the proof is double. One is through the nature of a cause and is called propter quid: this is through the nature of preceding events sirnply. The other is through the nature of the effect, and is called quia, and is through the nature of preceding things as respects us. Since the effect is better known to us than the cause, we proceed from the effect to the knowledge of the cause. From any effect whatsoever it can be proved that a corresponding cause exists, if only the effects of it are sufficiently known to us, for since effects depend on causes, the effect being given, it is necessary that a preceding cause exists. Whence, that God exists, although this is not itself known to us, is provable through effects that are known to us.

To the first objection above, I reply, therefore, that God's existence, and those other things of this nature that can be known through natural reason concerning God, as is said in Rom. I., are not articles of faith, but preambles to these articles. So faith presupposes natural knowledge, so grace nature, and perfection a perfectible thing. Nothing prevents a thing that is in itself demonstratable and knowable, from being accepted as an article of faith by someone that does not accept the proof of it.

To the second objection, I reply that, since the cause is proven from the effect, one must use the effect in the place of a definition of the cause in demonstrating that the cause exists; and that this applies especially in the case of God, because for proving that anything exists, it is necessary to accept in this method what the name signifies, not however that anything exists, because the question what it is is secondary to the question whether it exists at all. The characteristics of God are drawn from His works as shall be shown hereafter, (Question XIII). Whence by proving that God exists through His works as shall be shown hereafter, (Question XIII). Whence by proving that God exists through His works, we are able by this very method to see what the name God signifies.

To the third objection, I reply that, although a perfect knowledge of the cause cannot be had from inadequate effects, yet that from any effect manifest to us it can be shown that a cause does exist, as has been said. And thus from the works of God His existence can be proved, although we cannot in this way know Him perfectly in accordance with His own essence.

Article III. Whether God exists.

Let us proceed to the third article. It is objected (1) that God does not exist, because if one of two contradictory things is infinite, the other will be totally destroyed; that it is implied in the name God that there is a certain infinite goodness: if then God existed, no evil would be found. But evil is found in the world; therefore it is objected that God does not exist. Again, that what can be accomplished through a less number of principles will not be accomplished through more. It is objected that all things that appear on the earth can be accounted for through other principles, without supposing that God exists, since what is natural can be traced to a natural principle, and what proceeds from a proposition can be traced to the human reason or will. Therefore that there is no necessity to suppose that God exists. But as against this note what is said of the person of God (Exod. III., 14) I am that I am. Conclusion. There must be found in the nature of things one first immovable Being, a primary cause, necessarily existing, not created; existing the most widely, good, even the best possible; the first ruler through the intellect, and the ultimate end of all things, which is God.

I answer that it can be proved in five ways that God exists.

The first and plainest is the method that proceeds from the point of view of motion. It is certain and in accord with experience, that things on earth undergo change. Now, everything that is moved is moved by something; nothing, indeed, is changed, except it is changed to something which it is in potentiality. Moreover, anything moves in accordance with something actually existing; change itself, is nothing else than to bring forth something from potentiality into actuality. Now, nothing can be brought from potentiality to actual existence except through something actually existing: thus heat in action, as fire, makes fire-wood, which is hot in potentiality, to be hot actually, and through this process, changes itself. The same thing cannot at the same time be actually and potentially the same thing, but only in regard to different things. What is actually hot cannot be at the same time potentially hot, but it is possible for it at the same time to be potentially cold. It is impossible, then, that anything should be both mover and the thing moved, in regard to the same thing and in the same way, or that it should move itself. Everything, therefore, is moved by something else. If, then, that by which it is moved, is also moved, this must be moved by something still different, and this, again, by something else. But this process cannot go on to infinity because there would not be any first mover, nor, because of this fact, anything else in motion, as the succeeding things would not move except because of what is moved by the first mover, just as a stick is not moved except through what is moved from the hand. Therefore it is necessary to go back to some first mover, which is itself moved by nothing---and this all men know as God.

The second proof is from the nature of the efficient cause. We find in our experience that there is a chain of causes: nor is it found possible for anything to be the efficient cause of itself, since it would have to exist before itself, which is impossible. Nor in the case of efficient causes can the chain go back indefinitely, because in all chains of efficient causes, the first is the cause of the middle, and these of the last, whether they be one or many. If the cause is removed, the effect is removed. Hence if there is not a first cause, there will not be a last, nor a middle. But if the chain were to go back infinitely, there would be no first cause, and thus no ultimate effect, nor middle causes, which is admittedly false. Hence we must presuppose some first efficient cause---which all call God.

The third proof is taken from the natures of the merely possible and necessary. We find that certain things either may or may not exist, since they are found to come into being and be destroyed, and in consequence potentially, either existent or non-existent. But it is impossible for all things that are of this character to exist eternally, because what may not exist, at length will not. If, then, all things were merely possible (mere accidents), eventually nothing among things would exist. If this is true, even now there would be nothing, because what does not exist, does not take its beginning except through something that does exist. If then nothing existed, it would be impossible for anything to begin, and there would now be nothing existing, which is admittedly false. Hence not all things are mere accidents, but there must be one necessarily existing being. Now every necessary thing either has a cause of its necessary existence, or has not. In the case of necessary things that have a cause for their necessary existence, the chain of causes cannot go back infinitely, just as not in the case of efficient causes, as proved. Hence there must be presupposed something necessarily existing through its own nature, not having a cause elsewhere but being itself the cause of the necessary existence of other things---which all call God.

The fourth proof arises from the degrees that are found in things. For there is found a greater and a less degree of goodness, truth, nobility, and the like. But more or less are terms spoken of various things as they approach in diverse ways toward something that is the greatest, just as in the case of hotter (more hot) which approaches nearer the greatest heat. There exists therefore something that is the truest, and best, and most noble, and in consequence, the greatest being. For what are the greatest truths are the greatest beings, as is said in the Metaphysics Bk. II. 2. What moreover is the greatest in its way, in another way is the cause of all things of its own kind (or genus); thus fire, which is the greatest heat, is the cause of all heat, as is said in the same book (cf. Plato and Aristotle). Therefore there exists something that is the cause of the existence of all things and of the goodness and of every perfection whatsoever---and this we call God.

The fifth proof arises from the ordering of things for we see that some things which lack reason, such as natural bodies, are operated in accordance with a plan. It appears from this that they are operated always or the more frequently in this same way the closer they follow what is the Highest; whence it is clear that they do not arrive at the result by chance but because of a purpose. The things, moreover, that do not have intelligence do not tend toward a result unless directed by some one knowing and intelligent; just as an arrow is sent by an archer. Therefore there is something intelligent by which all natural things are arranged in accordance with a plan---and this we call God.

In response to the first objection, then, I reply what Augustine says; that since God is entirely good, He would permit evil to exist in His works only if He were so good and omnipotent that He might bring forth good even from the evil. It therefore pertains to the infinite goodness of God that he permits evil to exist and from this brings forth good.

My reply to the second objection is that since nature is ordered in accordance with some defined purpose by the direction of some superior agent, those things that spring from nature must be dependent upon God, just as upon a first cause. Likewise, what springs from a proposition must be traceable to some higher cause which is not the human reason or will, because this is changeable and defective and everything changeable and liable to non-existence is dependent upon some unchangeable first principle that is necessarily self-existent as has been shown.


From: Oliver J. Thatcher, ed., The Library of Original Sources (Milwaukee: University Research Extension Co., 1907), Vol. V: The Early Medieval World, pp. 359-363.

Scanned by Jerome S. Arkenberg, Dept. of History, Cal. State Fullerton

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© Paul Halsall May 998