The Third Way — Contingency and Necessity
From things that can fail to be, to a necessary being.
We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt.
The earlier ways traced change and causal dependence. The Third Way looks at the most basic fact of all: that things exist at all, when they need not have. A tree, a star, you — each is contingent: able either to be or not to be. It came to be, and it will pass away. Its existence is not guaranteed by what it is; it is received.
Aquinas asks what such fragile, borrowed existence ultimately depends on.
The argument, step by step:
- Contingent things exist: things that come into being and later pass away, that can be or not be.
- What can not be at some time is not; so if everything were contingent, then at some point nothing would have existed.
- But from nothing, nothing comes — so if there had ever been nothing, nothing would exist now. Yet things plainly exist.
- Therefore not everything is contingent; there must exist something whose existence is necessary.
- A necessary being either has its necessity from another or not. This cannot regress infinitely,
- so there must exist a being necessary of itself, not receiving existence from another — this all men speak of as God.
A clarification. Step 2 is the famously debated one, so it is worth stating fairly. Aquinas is not making a simple logical slip like "if each thing can fail, then the whole must fail." His real point is that things which merely might exist cannot be the whole story: a world made up entirely of things that borrow their being, with nothing to borrow it from, has no ultimate source of existence. Whether you put it in terms of time (as the medieval text does) or in terms of a dependence happening right now, the conclusion is the same: things that could have failed to exist point beyond themselves to something that cannot.
See it today: the flame of a candle
A candle flame is one of the clearest pictures of contingency you can watch.
The flame exists right now — but notice how it exists. It is not a fixed object; it is a continuous event, sustained moment by moment:
- It burns only while wax is drawn up the wick and vaporized now.
- The vapor ignites only while oxygen reaches it now.
- The whole flame persists only while heat feeds back to melt the next wax now.
Cut off any of these — pinch the wick, cover the flame, remove the air — and it does not linger. It simply stops being. The flame never owns its existence; it holds it on lease, renewed instant by instant, and depends entirely on conditions it does not supply.
Now widen the lens. The candle is contingent in exactly the same way, only slower: it was made, it will be consumed. The intuition resists here: a candle flame obviously borrows its being, but a rock or a planet seems simply to sit there, self-standing. Yet duration is not the same as self-sufficiency. The boulder endures only while its atoms hold together now; the planet coheres only while gravity binds it now. They are not exempt from the lease — they simply hold a longer one. "Lasting a billion years" is still borrowing existence a moment at a time; it is just a very patient flame. So is the table, the room, the mountain, you. Each holds existence on lease. And here is the pivot: a lease presupposes an owner. Borrowed existence, however long the chain of borrowers, must finally rest on something that does not borrow — whose existence is not leased, received, or sustained from outside, because it simply is existence.
That is why the common objection "well, what caused God?" misfires. It assumes God is one more candle-flame: one more contingent item needing to be lit and kept lit. But the whole argument arrives at something of a different kind: not a being that has existence and might lose it, but the being whose very essence is to exist. You do not ask who the owner borrows from.
Where this leads. The First and Second Ways ascend through chains of causes; the Third ascends through the fragility of existence itself. Every contingent thing is a rung that could have not-been, and no such rung explains its own being. The ladder of borrowers cannot hang from nothing; it must be held by One whose existence is unborrowed and unfailing — this all men speak of as God.