QuestionsDoes God exist?The third way: the contingent and the necessary
Does God exist?TheismCosmological

The third way: the contingent and the necessary

Thomas Aquinas 1225 · 1274Faith and reason cannot contradict each other.
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What it claims

There exists a being necessary of itself, which all call God.

Why

If everything could fail to exist, there would have been a time when nothing existed, and then nothing would exist now.

In short

"The third way: the contingent and the necessary" is an argument by Thomas Aquinas (Summa theologiae, I, q.2, a.3) on the question "Does God exist?" (answer: theism). Its claim: There exists a being necessary of itself, which all call God. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P7). AskPhilosophy presents 5 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Summa theologiae, I, q.2, a.3
Valid with an implicit premise

The first of the five ways (motion) takes up Aristotle's first mover, and the second (causes) is very close to it. The third, inspired by Avicenna and Maimonides, starts from a simple fact: things come to be and pass away. If everything could cease to exist, how is it that something still exists? (Leibniz would instead start from a general principle: nothing exists without a reason.)

P1Some things come to be and pass away: they can exist or not exist. They are called "contingent".
P2What can fail to exist, at some time does not exist. So if everything were contingent, there would have been a time when nothing existed.
P3Nothing begins to exist except through something that already exists. So if, at some time, nothing had existed, nothing would exist today.
P4Yet something exists today.
P5So not everything is contingent: at least one necessary being exists. And every necessary being has its necessity either from another or from itself.
P6We cannot go back to infinity among necessary beings that get their necessity from another. As with causes: without a first that holds the whole chain up, nothing holds.
P7 · implicit premiseA being necessary of itself, which causes the necessity of others, is what we call God (one, eternal, perfect).
Therefore there exists a being necessary of itself, which all call God.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P7, the computer finds a case where all the other premises are true and a being necessary of itself does exist, but it is not God: for instance matter, or the universe itself, as Hume suggests.
The structure is valid, but it rests on P7, which Aquinas does not justify in the article: he only writes "which all call God". He fills this gap later, in questions 3 to 11 of the Summa, by arguing that this being is simple, perfect, one (and that its essence is to exist). The most attacked premise remains P2: from "each thing can cease to exist" he moves to "everything could have ceased at once". His defenders answer that Aquinas is reasoning about an infinite past, in which every possibility is eventually realised, or that beings which all depend on one another can cease together. For P6, his defenders specify that it concerns a chain of dependence "here and now" (like a hand holding a stick that pushes a stone), not a series of events in time: Aquinas granted that reason alone cannot prove that the world had a beginning.
Analogy. Picture a line of people who each borrow a book from their neighbour. If nobody in the line owns the book in their own right, nobody could lend it, and yet the book is being passed along. So someone must have it without having borrowed it. Who that "someone" is remains open: that is the whole debate over P7.

Objections

5

Each objection is a conversation: click to see who replies and where the debate stands.

Objection anticipated by Aquinas (q.2, a.3, objection 2) attacks premise 5

No need for God: everything we observe is explained by other principles. What is natural is explained by nature, what is chosen is explained by human reason and will.

Thomas d'Aquin replies
Thomas d'Aquin
Nature and human will are themselves changeable and can fail. And everything that changes and can fail must be traced back to an unchanging first principle that is necessary of itself.
Where the debate stands (our summary)Aquinas answers that nature and human will, being changeable, point back to a first principle. The modern debate is less about this answer than about the argument as a whole, which assumes we cannot stop at a chain of explanations with no first term: the following objections dispute that.
M
Modern logicians (notably Anthony Kenny) 1969 · attacks premise 2

This is a logical error: that each thing can cease to exist does not imply there was a time when all had ceased at once. Each bulb in a chandelier can burn out, yet they can be replaced one by one without the room ever being dark.

no reply
summary: their text is not yet in our corpus
No reply possible: objection raised in 1969, after the author's death (1274).
Where the debate stands (our summary)Several commentators, including Kenny (1969), judge that the step from "each thing can fail to exist" to "at some time nothing existed" does not follow without an extra premise. Defenders of Aquinas debate the right reading of the third way: we have not retained a single widely accepted reply.
Hume (Cleanthes, in the Dialogues) 1779 · attacks premise 6

In an endless chain, each element is explained by the one before it. Asking further for a cause of "the whole" invents an object: gathering parts into a whole is only an act of the mind. Russell revived the point in 1948: every human has a mother, but that does not mean humanity has a mother.

Thomas d'Aquin replies
No reply possible: objection raised in 1779, after the author's death (1274).
William Rowe 1975
Summary: Rowe, himself a non-believer, nevertheless judges that the objection fails. When each member of a collection is explained by another member, it does not follow that the collection is explained: explaining each dependent being does not explain why there are dependent beings rather than nothing. The thread is followed in Leibniz's argument.
see the argument
Where the debate stands (our summary)Core disagreement: does explaining each part suffice to explain the whole (Hume, Edwards, Russell), or does it remain to explain why there are dependent beings rather than nothing (Rowe)? It depends on the principle of sufficient reason, which some find evident and others unproven. The thread is followed in Leibniz's argument.
Hume (Cleanthes, in the Dialogues) 1779 · attacks premise 7

Even granting a necessary being, why could it not be the material universe itself? We do not know all the properties of matter.

no reply
No reply possible: objection raised in 1779, after the author's death (1274).
Where the debate stands (our summary)Aquinas himself extends his proof to show that the first being is not a body (Summa theologiae, I, q. 3, a. 1). Hume's point is precisely that we do not know all the qualities of matter: the core disagreement is about what can be said of a "necessary" being.
Kant 1781 · attacks premise 7

To go from "a necessary being exists" to "it is God, the perfect being", one must assume that every necessary being is the supremely perfect being. That is exactly the idea of the ontological proof (deducing existence from a definition), which the argument claimed to avoid. The cosmological proof is just the ontological proof in disguise.

a defender replies
No reply possible: objection raised in 1781, after the author's death (1274).
B
Bruce Reichenbach defender of the argument · 1972
Summary: Reichenbach defends the argument from contingency by distinguishing logical necessity from factual necessity. The necessary being the argument needs is a being that has no cause and cannot cease to exist, without its existence being deduced from a mere definition. Kant's objection, which assumes logical necessity, would therefore not reach it.
Where the debate stands (our summary)Kant takes the cosmological proof to be a disguised ontological proof; defenders answer that it requires only a factual necessity, not a logical one. Still disputed: whether this necessary being is indeed God (perfect, unique, personal), which needs further steps (in Aquinas, the rest of the Summa).
What today's voices say
Defend it
Bishop Robert BarronHe says Aquinas's arguments are as strong as ever. His point that science studies contingent things matches this argument.
Criticize it
Richard DawkinsHe says Aquinas's Five Ways 'don't prove anything'.
In the same family · Cosmological
The unmoved first moverAristotle
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The necessary existent: the proof of the truthfulAvicenna · 1027
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Sufficient reason: why something rather than nothing?Gottfried Wilhelm Leibniz · debate over 4 levels
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The world began, so it has a causeAl-Ghazali · 1095 · debate over 4 levels
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Against the first cause: "the universe is just there"Bertrand Russell
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Whatever begins has a cause: the universe tooWilliam Lane Craig · 1979 · debate over 4 levels
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Time begins with the Big Bang: there is no 'before' in which a creator could have actedStephen Hawking · 2018
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