The Five Ways
How does Aquinas actually argue that God exists?
The big question
Lesson 2.1 showed that the Church teaches God can be known by reason. But how? The most famous answer is a single page of St Thomas Aquinas’s Summa Theologiae (part I, question 2, article 3), where he gives five short arguments, the “Five Ways”. They are among the most discussed paragraphs ever written. This lesson sets them out.
What the Church teaches
The Church teaches that God’s existence can be proved by reason (Lesson 2.1). She has never defined which argument proves it.
How sure is this? The Five Ways themselves are not defined. They are an argument offered by one theologian, though one the Church has repeatedly commended. Leo XIII urged the study of St Thomas’s philosophy in Aeterni Patris (1879), and the Catechism lists, without naming Aquinas, several “ways” of approaching God from the world, such as motion, becoming, contingency, order and beauty (CCC 31–32).
So a Catholic may find one Way stronger than another, or prefer a different argument altogether, without departing from the faith.
Where it comes from
Scripture. The Five Ways put into philosophical form what Scripture says about knowing God from creation (Wisdom 13:1–5; Romans 1:20). St Paul told the Athenians that in God “we live and move and have our being” (Acts 17:28): the world depends on God at every moment, which is exactly what the Five Ways argue.
Before Aquinas. Aquinas did not invent these arguments from nothing. The argument from change goes back to Aristotle, who reasoned to a first unmoved mover. St John of Damascus (died about 749), a Doctor of the Church, gave arguments from change and from the order of the world (On the Orthodox Faith 1.3). Aquinas gathered and sharpened them.
How theologians explain it
Each Way starts from something anyone can observe, and reasons back to something that must exist to explain it.
| Way | Starts from | Concludes to |
|---|---|---|
| 1. Change | Things change | A first, unchanged cause of change |
| 2. Causes | Things are caused by other things | A first, uncaused cause |
| 3. Contingency | Things exist that need not exist | A being that exists necessarily, of itself |
| 4. Degrees | Things are more and less good, true and noble | A being that is goodness and being itself |
| 5. Direction | Things without minds act for ends | An intelligence that directs nature |
The First Way: from change. Things change: water heats, a seed grows. To change is to go from being able to be something (potency) to actually being it (act). Water is able to be hot; it becomes hot only through something already hot, such as a flame. So whatever changes is changed by something else. That something else may itself be changing, and so need a further cause. But this cannot go on for ever, or there would be no first source of the change and so no change at all. There must be a first cause of change that is not itself changed: pure act, with no unrealised potential.
The Second Way: from efficient causes. In the world we find causes arranged in order: one thing produces an effect through another. Nothing can be its own cause, because it would have to exist before itself. And the series of causes cannot go on for ever, or there would be no first cause, and so no intermediate causes and no effects. So there is a first efficient cause.
The Third Way: from contingency. Some things are able to exist or not exist: they come to be and pass away. Such a thing does not have existence of itself; it has to receive it. If everything were like that, Aquinas argues, there would be nothing to explain why anything exists. So there must be something that exists necessarily. And if its necessity comes from something else, we must eventually reach something necessary of itself, which gives existence to everything else.
The Fourth Way: from degrees. We judge things to be more or less good, true and noble. “More” and “less” are measured by closeness to a maximum. What is greatest in being is the cause of being in everything else, as Aquinas puts it, so there is something that is the cause of the being, goodness and every perfection of all things.
The Fifth Way: from direction. Things without minds, such as plants, chemical reactions and the planets in their orbits, act regularly toward ends: always, or nearly always, in the way that achieves a result. Something without knowledge can tend toward a goal only if it is directed by something with knowledge, as an arrow is aimed by an archer. So there is an intelligent being by whom all natural things are directed to their ends.
Each Way ends: “and this everyone understands to be God.” Aquinas knows this is only a beginning. The next questions of the Summa (questions 3 to 26) show that the first cause must be one, simple, perfect, infinite, unchanging, eternal, knowing and good. Lesson 2.3 covers those.
The point most people miss
When Aquinas says a series of causes “cannot go on to infinity”, he does not mean a chain stretching back in time, like a line of dominoes falling one after another since the Big Bang. Aquinas actually thought reason alone could not prove that the world had a beginning in time; he believed it had one because Scripture says so (Summa I, q. 46, a. 2).
He means a series of causes acting at the same moment, each depending on the one above it, like the links of a chain holding up a lamp. The lamp hangs from the bottom link, which hangs from the next, and so on. However many links there are, something has to hold the top one, or none of them holds anything. The Five Ways ask what holds the world in existence and in action now. That is why they are untouched by questions about the Big Bang.
Philosophy bridge
Every Way uses ideas from the philosophy course’s metaphysics: act and potency (the First Way), efficient causes (the Second), the difference between what a thing is and that it exists (the Third), participation in being (the Fourth), and final causes (the Fifth). Students who have studied these will find the arguments much clearer.
What the Church does not claim
- Not “everything has a cause, so who caused God?” None of the Ways says everything has a cause. They say that what changes, is caused, or need not exist depends on something else. God is the one reality that does not change, is not caused and exists of himself, so the question does not arise.
- Not a “God of the gaps”. The Ways do not point to something science hasn’t explained yet and say “God did it”. They start from facts science takes for granted, such as change, causation and natural regularity, and ask why there are such facts at all.
- Not that evolution is ruled out. The Fifth Way is about nature acting regularly toward ends, which evolution itself relies on. It is not the claim that each species was designed separately.
- Not that these are the only arguments. Other arguments, such as from the moral law, from conscience, or from the human desire for happiness, are also used by Catholic thinkers.
Check your understanding
- In the First Way, what do potency and act mean? Give an example.
Show answer
Potency is the ability to be something; act is actually being it. Cold water is potentially hot; it becomes actually hot only through something already actually hot, such as a flame.
- Why can’t the series of causes in the Second Way go on for ever?
Show answer
Because in a series where each cause depends on the one before it at this moment, if there were no first cause there would be no intermediate causes and no effect. Something has to be causing without being caused, or nothing in the series causes anything.
- A friend says, “Scientists now think the universe might have no beginning, so the First Cause argument fails.” How would Aquinas reply?
Show answer
The Five Ways do not depend on the universe having a beginning in time. Aquinas thought reason could not prove a beginning anyway. His argument concerns what keeps things existing and acting now, like a chain holding a lamp, not what started a series of events long ago.
- Does the Church require Catholics to accept the Five Ways?
Show answer
No. The Church has defined that God’s existence can be known with certainty by reason, but not which argument shows it. The Five Ways are highly commended but not defined.
- What does the Fifth Way argue from, and why is it not the same as saying God designed each species separately?
Show answer
It argues from the fact that things without minds act regularly toward ends, which requires something intelligent directing nature. It concerns the regular, goal-directed behaviour of nature as a whole, which evolution itself depends on, not the separate design of each species.