Thomas Aquinas, The Five Ways

Note: Last Updated 6/13/2025.

Thomas Aquinas:

     I answer that, The existence of God can be proved in five ways. 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 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. 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 in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion 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 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, 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. 

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

     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 “Summa Theologica” of St. Thomas Aquinas: Part I. QQ. I.—XXVI., trans. Fathers of the English Dominican Province, [London: Burns, Oates & Washbourne, LTD., 1920], Q. 1, Art. 3, Answer, pp. 24-27.)

R. C. Sproul:

     The first proof Thomas offers is the proof from motion. He begins with the evidence for motion in the world (Zeno notwithstanding). Borrowing heavily from Aristotle, Thomas argues that whatever is moved is moved by another (based on what we call the law of inertia). Thomas defines motion as the reduction of something from potentiality to actuality. An object at rest may have the potential to move, but it does not move until or unless this potential is actualized. But, Thomas argues, nothing can be reduced from potentiality to actuality except by something that is already in a state of actuality. For example, fire can make a piece of wood, which is only potentially hot, actually hot. Something cannot be at the same time both actual and potential. What is actually hot may be at the same time potentially cold, but it cannot be potentially hot while it is actually hot. It could be potentially hotter than it actually is, but to become hotter it must be moved to that state. Whatever is moved must be moved by some prior actuality. But this change cannot regress to infinity, because in that case the motion could never begin. Therefore, Thomas concludes, there must be a first mover, and everyone understands this to be God.

     Thomas’s second proof is the proof from efficient cause. The law of causality asserts that every effect must have an antecedent cause. This is not the same thing as saying that every thing must have a cause (as John Stuart Mill and Bertrand Russell claim). If every thing must have a cause, then God himself would require a cause. The law of causality refers only to effects and is an extension of the law of noncontradiction. The law is formally true, because it is true by definition. An effect is defined as that which is produced by a cause. An effect cannot be an effect unless it has a cause. Likewise, a cause (properly speaking) is by definition that which produces an effect. A cause cannot be a cause unless it causes or produces something. An uncaused (self-existent) being violates no rule of reason; an uncaused effect, however, is irrational and absurd.

     In Aristotle’s scheme the efficient cause is that which produces the effect. In the case of the statue, it is the sculptor. Without the sculptor there will be no statue because there is nothing to cause it. No event can be its own cause. Every event requires a prior cause. Any change in anything is an event. Every prior cause must have its own cause (if the prior event is itself an effect). At some point the series must end. It is impossible to regress to infinity, as the idea of an infinite regress involves the notion of a causeless effect, an absurdity infinitely compounded.

     Thomas Aquinas’s third proof of God’s existence is the proof from necessary being (ens necessarium). Although this is usually thought of as part of the cosmological argument, it is more properly called “ontological,” because it is an argument from being. In nature we find things that are contingent, things that can be or not be (a possibility Hamlet fully understood about himself). Such things or “beings” do not always exist. They also experience the changes involved in generation and decay. There was a time when I was not. To say that it is possible for something not to exist can mean that once in the past it did not exist, that it can go out of existence in the future (at least as an individuated entity), or both. Possible being then refers to beings that possibly can not be.

     No merely possible being is self-existent; it does not have the power of being in itself. If all things in reality were only possible, then at one time there would have been nothing in existence. If there was a time when nothing existed, then nothing could ever start to exist and nothing would exist now. But if something does exist now, there must have always been something in existence; something must exist that possesses necessary existence—its existence is not merely possible but necessary. It cannot not be. It does not receive its existence from something else. There never was a time when it was not. In other words, if anything exists now, then something must have the power of being within itself, that is, something must have necessary being. This being, whose being is both logically and ontologically necessary, is God.

     Thomas’s fourth proof is the proof from degrees of perfection, in which he borrows heavily from Augustine. This is an argument from the comparative. We are aware of degrees of the good, the true, and the noble. But something can be deemed good or true only against some maximum norm or standard. Modern relativists posit truths with no truth, goods with no good, virtues with no virtue, and purposes with no purpose. But we cannot have a relative of anything unless the relative is measured against an absolute. Thomas argues that the maximum in any genus is the cause of everything in that genus. For example, fire, which is the maximum of heat, is the cause of all hot things. 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.

     It could be counter-argued that if this is true, then God would also have to be maximally or perfectly evil—to account for the relative degrees of evil in the world. This is why it was crucial to Thomas, following Augustine, to define evil by way of privation and negation. The ultimate standard by which we must judge evil is not maximum evil but maximum perfection.

     Thomas’s fifth and final proof of God’s existence proceeds from the evidence of order in the universe. This is a form of the so-called teleological argument. The term teleological comes from the Greek telos, which means “end, purpose, or goal.”

     In nature we observe things that lack intelligence but function in an orderly and purposive way. They act in predictable ways to achieve certain ends or functions. The seeds of a dandelion cast in the wind are designed for the plant’s reproduction. These things appear to act with a purpose. One cannot have purpose accidentally, nor can one have unintentional intentionality. In its simplest form the teleological argument rests on the evidence of design in the universe. Design demands a designer, an idea that deeply impressed both Immanuel Kant and David Hume despite their skepticism.

     Thomas argues that things lacking intelligence (note in this word the root telos) cannot act in a designed fashion unless they are first directed by something that does have intelligence. An arrow does not guide itself to the target unless it is first aimed there by the archer. “Smart bombs” are not really smart unless programmed by someone who is smart (and even then, they are not always so smart!).

     Aquinas concludes that there must be an intelligent being who directs all natural things to their end. This he calls God. It must be added that things cannot be directed to their ends by chance. Chance can direct nothing, because chance can do nothing. Chance can do nothing because chance is nothing. Chance is a perfectly meaningful term to describe mathematical possibilities, but the word becomes a sneaky bogeyman when used to describe something that has the power to influence anything. Chance has no being, and that which has no being has no power to do anything.

(R. C. Sproul, The Consequences of Ideas, [Wheaton: Crossway Books, 2000], pp. 71-75.)

καὶ αὐτός ἐστιν πρὸ πάντων καὶ τὰ πάντα ἐν αὐτῷ συνέστηκεν ~ Soli Deo Gloria