Reading Aquinas's Five Ways Rightly: Act, Potency and the God They Point To

Beyond the Summary

Most people meet St Thomas Aquinas’s Five Ways as a list: motion, causation, contingency, degrees of perfection and design. Each argument is then compressed into a sentence, and critics dismiss each sentence just as quickly. (For a clear walk through all five arguments, see What Are the Five Ways of St Thomas Aquinas?)

Aquinas did not write them as slogans. They sit at the start of the Summa Theologiae (I, q. 2, a. 3) on top of a whole philosophy of nature and being, inherited from Aristotle and developed by Aquinas himself. Without that background, the arguments are easy to misread. With it, they become far more interesting.

Act and Potency

The first key idea is the distinction between act and potency.

Anything that exists is actually some ways and potentially other ways. A cup of cold water is actually cold and potentially hot. A seed is actually a seed and potentially an oak. Change is the movement of something from potency to act.

Aquinas’s crucial principle is that a potency cannot actualise itself. The water does not heat itself up; something already actually hot, or actually capable of producing heat, must act on it. Potential is, by definition, not yet real; and what is not yet real cannot make anything happen.

The First Way builds directly on this. Whatever passes from potency to act does so through something already in act. If that cause is itself being actualised, it depends on a further cause, and so on. The argument concludes that there must be something that actualises without being actualised: something that is pure act, with no potency at all. That is what Aquinas means by an “unmoved mover.” It is not a being that happens to sit still. It is a being in whom there is nothing unrealised.

Two Kinds of Causal Series

The most common misreading of the Five Ways is to think they argue that the universe must have had a beginning in time: “the chain of causes cannot go back for ever, so there must have been a first event.”

Aquinas did not argue this. In fact, he held that reason alone cannot prove the world had a temporal beginning; he believed it did only because revelation says so (Summa Theologiae I, q. 46, a. 2). He even granted that an infinite series of causes stretching back in time is not impossible in principle (I, q. 46, a. 2, ad 7).

The key is a distinction later philosophers called accidentally ordered and essentially ordered series.

An accidentally ordered series is like a line of ancestors. Your great-grandfather begot your grandfather, who begot your father, who begot you. But your father can go on begetting children even after your grandfather has died. Each member, once caused, acts on its own. Aquinas allowed that such a series might, in principle, go back without end.

An essentially ordered series is different. Each member causes only while it is itself being caused, here and now. A stone is moved by a stick, which is moved by a hand, which is moved by the muscles, which are moved by the nerves. Take away the hand, and the stick and stone stop at once. In a series like this, the intermediate members have no causal power of their own; they only pass it on. Such a series cannot be infinite, because without a first member that has the causal power in itself, there would be nothing to pass along.

The Five Ways concern essentially ordered series. Their question is not “what started everything long ago?” but “what is holding everything in being and activity right now?”

Essence and Existence

The Third Way, from contingency, rests on another of Aquinas’s central ideas: the distinction between essence (what a thing is) and existence (that it is).

Knowing what a lion is does not tell you whether any lions exist. In every creature, essence and existence are distinct; nothing in a creature’s nature requires it to exist. So every creature receives its existence from outside itself.

Aquinas argues that this cannot go on without end in an essentially ordered way. There must be something whose very essence is to exist, which does not receive existence but simply is existence. He calls God ipsum esse subsistens: subsistent being itself.

This is why, in the Catholic tradition, God is not “a being” alongside other beings, even the greatest one. He is Being itself, the source in which everything else participates. When God revealed His name to Moses as “I AM WHO I AM” (Exodus 3:14), Christian thinkers saw a striking convergence between revelation and this philosophical conclusion.

The Classic Objections

”Then what caused God?”

The objection, popularised by Richard Dawkins in The God Delusion (2006), assumes that the arguments say “everything has a cause.” They do not. They say that whatever is changed, caused or contingent needs a cause. The conclusion is precisely something that is not changed, caused or contingent: pure act, subsistent being. Asking what caused it is like asking what makes a triangle have four sides. The question misunderstands what has been concluded.

Hume: causation is just habit

David Hume argued in the eighteenth century that we never observe causation itself, only one event regularly following another. If so, the principle that potencies need causes would be a mental habit, not a truth about reality.

Thomists reply that this scepticism proves too much. If causation is only habit, then science itself, which seeks the real causes of things, becomes impossible to justify. Most people, including most scientists, rightly assume that things do not happen for no reason at all.

Kant: the argument smuggles in the ontological argument

Immanuel Kant claimed that every cosmological argument secretly depends on the ontological argument, which tries to prove God’s existence from the mere idea of a perfect being. Aquinas himself rejected the ontological argument (I, q. 2, a. 1). His arguments begin from observed facts about the world, not from a definition of God. Many philosophers of religion, including some who are not believers, agree that Kant’s charge does not fit Aquinas’s reasoning.

What Comes Next in the Summa

The Five Ways take up only one article of the Summa. In the questions that follow (I, qq. 3–11), Aquinas asks what pure act and subsistent being must be like. He argues that such a being must be:

simple, without parts or composition;
perfect, lacking nothing;
good, indeed goodness itself;
infinite, unlimited by any essence;
present everywhere, as the cause of being in all things;
unchangeable, since it has no potency;
eternal, outside time;
one, since there cannot be two infinite, perfect beings.

He then explains in question 13 how human words can be applied to God: not in exactly the same sense as to creatures, nor in a completely different sense, but by analogy.

This is where the real payoff lies. The Five Ways do not just establish that “something” exists. Together with what follows, they arrive at the God of classical theism, the God that Jews, Christians and Muslims have recognised as the Creator.

Faith and the Five Ways

Aquinas called such truths the preambles of faith: truths about God that reason can reach, and which prepare the mind for faith. They do not make faith unnecessary, since reason cannot reach the Trinity or the Incarnation. But they show that faith is not a leap in the dark.

To follow the argument in Aquinas’s own words, see Glenn’s précis of Summa Theologiae, Question 2 on our theology site.