What Are Thomas Aquinas’ Five Ways?

Rational argument was of importance to Medieval theologian and philosopher Thomas Aquinas (1225-1274) because he thought it necessary for engaging with unbelievers. This is best expressed in Aquinas’ Five Ways or five arguments for the existence of God.

What is interesting about the Five Ways is that they seek to prove the existence of God through facts that all people can accept. One need not accept God’s existence because the Church or the priest says so; rather, one ought to, on Aquinas’ view, accept God’s existence because reason and fact demand it. We will observe the five arguments (from movement, causality, dependency, gradation, and teleology) from the Summa Theologica and express them in simple syllogisms that break down their logic into digestible parts.

The First Way is Aquinas’ argument from motion. He writes in full,

“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.”

Then comes the important part to the argument,

“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 need 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.”

We can represent the argument as follows (1):

P1: Things in this world change or are in motion,
P2: Things that are in motion are put in motion by something else,
P3: This entails a regress of motion,
P4: This regress cannot be infinite,
C: Therefore, there must be a first mover (God).

Here Aquinas is arguing that because a moving thing cannot move itself it must have been moved by something else. This movement, however, cannot go on for eternity and this means that there must be a first mover that, writes Aquinas, “everyone understands to be God.” We will notice that Aquinas uses a similar pattern of logic that things cannot go back infinitely in several of his other Ways.

The Second Way is based on efficient causation; as per Aquinas,

“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.”

We can represent the argument as follows (2):

P1: We see effects in the world,
P2: Effects are caused by something else,
P3: That entails a regress of causation (cause of the cause, cause of the cause of the cause, etc.),
P4: That regress cannot be infinite,
C: There must be a first cause.

Aquinas argues that in the physical world everything has a cause, but nothing causes itself. Further, causal chains cannot go backward infinitely and must at some point stop in a first cause. Importantly, the first cause, which Aquinas identities as God, is also beyond or outside of the physical world otherwise it would also require a cause. Because the first cause is the progenitor of the chain of causes it itself cannot be within the world. If this follows, then it proves that there is a supernatural first cause.

A critic might argue that if everything requires a cause then so must the first cause. But Aquinas is careful to place the first cause outside of the physical universe so it seems to escape this objection. This seems similar to some contemporary formulations of the cosmological argument for the existence of God that posits that only those things that begin to exist have a cause, thus not everything (e.g. the first cause) has a cause.

The Third Way argues from the dependency of some things on others and runs as follows,

“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.”

Let’s represent the argument as follows: (3)

P1: There are contingent things,
P2: If something is contingent it is necessarily contingent,
P3: There cannot be an infinite number of contingent things,
C: There is a necessary being.

This logic posits that there are things that are contingent and as opposed to being necessary. Aquinas argues that although one thing is contingent on something else this cannot go on into infinity. There must be something that is the ground of contingency that itself must be metaphysically necessary. Again, this is similar to the First and Second ways. Contingent things are within this universe (like effects and things in motion) and have to terminate at some point to avoid an infinite regress.

The Fourth Way contains the argument from gradation,

“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.”

One can interpret the Fourth Way as follows (4),

P1: Some things are more F—true, good, noble, etc.— than others,
P2: One thing is more F than another if it more closely resembles what is maximally F,
P3: So, there must be something that is maximally F, for each perfection of F,
P4: Anything maximally F for every perfection F is also maximally real,
P5: What is maximally F is the cause of all the Fs,
C: Something—God—is the cause of everything else and their perfections.

This argument is likely the most problematic of the Five Ways. P1 seems uncontroversial: certain actions certainly are better than others, for example, philanthropical and altruistic acts are better than behaviors based on avarice or hatred. P2, however, seems to assume that there is something that is maximally F in that because some things are good or true there must be something that is maximally good or true. But why assume that this is the case? What justifies this? Aquinas attempts to use the example of fire, but what does he mean when he says that something is of maximum heat? Is it possible to heat anything to a maximum heat? Perhaps the analogy of numbers would cause us some pause regarding this argument: does it make sense to say that one number (perhaps the number 3) is greater than another number (say the number 2) because it more closely resembles the maximum number? The same question can be asked of this as one can ask of the fire analogy: what is meant here by maximum? Is there such a thing as a maximum number? What Aquinas’ second premise seems to be missing is a scale of a sort. It makes sense to say that one act is more morally good than another if there is a scale to represent goodness. But it does not follow that one needs to have a maximum point on the scale. P2 is thus problematic.

The Fifth Way is an early version of a teleological argument; Aquinas writes,

“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 awakes, 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 was 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.”

This argument can be represented as follows (5):

P1: Things lacking intelligence act toward an end
P2: What lacks intelligence and acts toward an end is directed by a being with knowledge and intelligence
C: Therefore, some intelligent being, God, directs all things toward an end.

There are a few problems with this argument. Is it reasonable to suppose in light of P1 that all things lacking intelligence, like neutrons and stars, act toward an end? This logic seems alien to our modern thinking because ever since Isaac Newton we no longer find the need to use teleological explanations to explain the behavior of objects in the world: if I throw a tennis ball and it falls back to the Earth, this is not because it is returning to the Earth for some teleological purpose or goal, but rather because gravity pulls it down.

It is important to remember that Aquinas was theorizing centuries prior to the Scientific Revolution and therefore before this was accepted knowledge. But there seems some wiggle room inviting further reflection. For example, if we consider that gravity is an object finding the shortest path in curved space-time, this can strike one as teleological. In thermodynamics, entropy increases which seems to imply that the universe wants to become less orderly. This requires further discussion.

In response to P2, one might wonder if everything that lacks intelligence acting toward an end is directed by a being with knowledge and intelligence. We might grant that this is sometimes the case, such as a human being operating a vehicle. But what about gravity? Is a boulder that slides down the side of a hill necessarily being directed to an end (the ground) by a being with knowledge and intelligence? It would not seem so and neither does it seem so for neutrons, stars, galaxies, and so on.

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