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The Cosmological Argument reasons from the existence of the universe — rather than from its apparent order — to a being on which that existence ultimately depends. It presses the question Leibniz made famous: why is there something rather than nothing? Like the design argument it is a posteriori, beginning from an observed feature of the world (here, the very fact that things exist and depend on other things), but its conclusion is a necessary being that does not itself depend on anything. For the AQA 7062 course the named focus is sharply defined: Aquinas' Third Way — the argument from contingency and necessity — together with its two principal critics, Hume (the fallacy of composition and the questioning of the causal principle) and Russell ("the universe is just there, and that's all"). This lesson sets out the Third Way precisely, develops those criticisms, and reaches a judgement about whether the existence of a contingent universe really requires a necessary God. The Kalam argument and Leibniz are treated as enrichment that sharpens the debate, not as the spec-named core.
St Thomas Aquinas (1225–1274) presents five "ways" to God in the Summa Theologica; the Third Way is the cosmological argument the specification names. It is not the argument from motion (the First Way) nor from efficient causation (the Second Way), and it is important not to muddle them. The First Way reasons from change (the reduction of potentiality to actuality) to an Unmoved Mover; the Second from a chain of efficient causes to an Uncaused Cause; the Third — the one we focus on — reasons from the contingency of things to a necessary being. Although all three share the structure "observed feature → no infinite regress of the relevant kind → a terminus identified with God," they start from different features and should never be merged into a single "first cause argument." The Third Way turns specifically on the distinction between contingent and necessary being.
Key term: Contingent being — a being that might or might not have existed, and which can come into existence and pass out of existence. Its existence is not self-explanatory; it depends on something else.
Key term: Necessary being — a being that cannot fail to exist, whose existence is grounded in its own nature rather than in anything else. Aquinas identifies this with God.
Aquinas' reasoning runs as follows. We find in nature things that are possible to be and possible not to be — they are generated and they decay; that is what it is to be contingent. Now, Aquinas argues, what is possible-not-to-be at some time is not. So if everything were contingent, then — given enough time — there would have been a moment at which nothing existed at all (since each contingent thing, taken over the whole of time, has some moment of non-existence). But if at some point nothing existed, then nothing could have begun to exist thereafter, because nothing comes from nothing (ex nihilo nihil fit) — there would have been no cause to bring anything into being. In that case, nothing would exist now. Yet plainly things do exist now. Therefore it is false that everything is merely contingent: there must exist at least one being that is necessary — that does not have the possibility of not-being.
Aquinas then adds a second step that is often forgotten. A necessary being might owe its necessity to another (its necessity could be caused). But a chain of necessary-beings-whose-necessity-is-caused-by-another cannot, Aquinas argues (on the same principle that rejects an infinite regress of per se causes), go back forever. So there must be a being that is necessary of itself (per se), deriving its necessity from no other — "and this all men speak of as God."
| Step | Claim |
|---|---|
| 1. Observation | Some things are contingent — they come to be and pass away |
| 2. Inference | If all things were contingent, at some time nothing would have existed |
| 3. Consequence | From nothing, nothing comes — so nothing would exist now |
| 4. But | Things do exist now |
| 5. Therefore | At least one necessary being exists |
| 6. Further | Necessity cannot be borrowed infinitely; there is a being necessary of itself — God |
The Third Way is sometimes called an argument from contingency to a sufficient ground of contingent existence. Its driving intuition is that the totality of dependent things cannot be the whole story: dependence has to terminate in something that does not itself depend.
A common student error is to think Aquinas rejects all infinite series. He does not. The key distinction is between an accidentally ordered series and an essentially ordered series. In an accidentally ordered series the later members do not depend, here and now, on the continued operation of the earlier ones: a grandfather causes a father who causes a son, but the son keeps existing after the grandfather dies. Aquinas has no objection to such a series being infinite — he thought one could not prove by reason alone that the universe had a temporal beginning (he held that on faith, from Genesis). In an essentially ordered series, by contrast, every member depends simultaneously on the sustaining activity of a prior member: a hand moves a stick that moves a stone — remove the hand and the whole motion ceases at once. It is this kind of dependency chain that Aquinas argues cannot regress infinitely, because if there were no first member actually grounding the series now, there would be no series now. The Third Way concerns the dependence of contingent things on a ground of their being at every moment, not merely their origin in the past — which is why it does not need the universe to have had a beginning.
Key term: Essentially ordered series — a causal series in which each member depends, at the same time, on the present activity of a prior member (e.g. a hand–stick–stone); it is this that Aquinas says cannot be infinite, not a series spread out in time.
It is also important that Aquinas' "necessary being" is not (or not obviously) the logically necessary being that Anselm and the ontological argument require. For Aquinas a "necessary" thing is one that does not have, in its own nature, the possibility of corruption — it does not come to be and pass away as contingent things do. The angels and (on his view) heavenly bodies are "necessary" in a derived sense, their necessity caused by another; God alone is necessary per se, the underived ground of all else. This matters for evaluation because Hume and Russell will attack the very idea of a necessary being, and the defender must be clear which notion of necessity is in play: a being that cannot fail to exist because of what it is, rather than a mere truth of logic. The strongest answers keep Aquinas' metaphysical necessity distinct from the ontological argument's logical necessity.
Two later arguments sharpen the same intuition. Gottfried Wilhelm Leibniz (1646–1716) frames the issue through the Principle of Sufficient Reason (PSR): for everything that exists there must be a sufficient reason why it exists rather than not. Even an eternal universe, Leibniz argues, would still need a sufficient reason for why there is a universe at all; that reason cannot lie within the series of contingent things, so it must lie in a necessary being outside it.
Key term: Principle of Sufficient Reason (PSR) — the principle, associated with Leibniz, that for everything that is the case there is a sufficient reason why it is so and not otherwise.
Leibniz's version has a distinctive strength worth noting: it does not depend on the universe having a beginning, nor on rejecting infinite series. Even granting an eternal universe with an infinite past, Leibniz argues, the whole infinite series of contingent states is still contingent — there is no reason within the series why this series of states exists rather than another, or rather than none. He uses the image of a series of geometry books, each copied from the one before, stretching back infinitely: even an infinite regress of copies never explains why the book exists at all. The sufficient reason for the existence of the whole contingent series must therefore lie outside it, in a necessary being. This makes Leibniz's argument immune to the "but the past might be infinite" reply that troubles Kalam, and it presses precisely the question Russell will refuse: whether the totality of contingent things needs a reason.
The Kalam cosmological argument, rooted in medieval Islamic theology (notably al-Ghazali, 1058–1111) and revived by William Lane Craig (b. 1949), takes a different route: (1) whatever begins to exist has a cause; (2) the universe began to exist; (3) therefore the universe has a cause. Craig defends premise (2) by arguing against an actually infinite past and by appeal to Big Bang cosmology, and argues the cause must be timeless, immaterial and personal. Kalam differs from Aquinas in resting on the universe having a temporal beginning, whereas the Third Way does not require one — a difference worth flagging in evaluation.
David Hume (1711–1776), in the Dialogues Concerning Natural Religion and the Treatise, supplies the criticisms AQA names. They target the argument's two load-bearing assumptions: the causal principle and the move from parts to whole.
1. The fallacy of composition. Hume's most quoted objection (pressed later by Russell) is that the argument illegitimately infers a property of the whole from a property of its parts. Each thing in the universe may be contingent and require a cause; it does not follow that the universe as a whole is contingent or requires a cause. The whole may be a different kind of thing from its members. Hume's analogy: each member of a human family has a mother, but the family as a whole does not have a mother. So even if every constituent of the universe is contingent, the universe itself may need no external explanation.
Key term: Fallacy of composition — the error of inferring that what is true of each part of a whole must be true of the whole itself.
2. Questioning the causal principle. More radically, Hume challenges the premise that everything must have a cause. Our belief in universal causation, he argues, is not a logical truth but a habit of mind: we repeatedly observe one kind of event followed by another and come to expect the conjunction, but we never perceive a necessary connection between them. It is therefore not self-contradictory to conceive of something beginning to exist without a cause — "we can conceive the non-existence of any object," and what is conceivable is possible. If the causal principle is merely a psychological habit rather than a necessary truth, the cosmological argument's foundation is uncertain.
3. Why not stop at the universe? Hume also presses a parsimony point: if we are permitted to terminate the search for explanation in a being that simply exists necessarily (God), why not terminate it one step earlier, in the universe itself? "Why may not the material universe be the necessarily existent being?" If a necessary terminus is acceptable, the universe is a candidate, and we save a hypothesis.
4. The whole is explained when the parts are. A fourth Humean point, developed in the Dialogues, is that the demand for an explanation of the collection may be confused. If I explain why each member of a group of twenty particles exists, Hume suggests, I have thereby explained the existence of the whole group; to ask for a further explanation of "the whole" over and above its parts is "unreasonable." The defender of the argument disputes exactly this: explaining each contingent thing by reference to a prior contingent thing never explains why there is a series at all, any more than explaining each carriage of a train by the one in front explains why there is a train. This disagreement — whether explaining the parts suffices to explain the whole — is one of the deepest fault-lines in the debate, and it recurs in the Russell exchange below.
It is worth seeing how Hume's objections interlock. Objections (1) and (4) attack the move from parts to whole — the claim that the universe-as-a-totality needs explaining at all. Objections (2) and (3) attack the argument's metaphysical machinery — the necessity of the causal principle and the privileging of God over the universe as the necessary terminus. Together they do not show the cosmological argument to be false; they aim to show that its key premises are unproven assumptions rather than self-evident truths, so that one may rationally decline the conclusion.
Because the AQA topic centres on the Third Way, the Kalam argument is enrichment — but it sharpens the debate about infinity and causation, so it repays attention. Recall its structure: (1) whatever begins to exist has a cause; (2) the universe began to exist; (3) therefore the universe has a cause. Unlike Aquinas' Third Way, Kalam requires the universe to have had a beginning in time, and this is where its most interesting arguments lie.
William Lane Craig (b. 1949) defends premise (2) with two philosophical arguments against an infinite past and two scientific considerations. The philosophical core is that an actual infinite — a completed, existing infinite totality — cannot exist in reality, because it generates absurdities. His favoured illustration, drawn from the mathematician David Hilbert, is Hilbert's Hotel: imagine a hotel with infinitely many rooms, all full. A new guest arrives; the manager moves the guest in room 1 to room 2, room 2 to room 3, and so on, freeing room 1 — so a "full" infinite hotel can always take more guests, and indeed infinitely more, which Craig argues is absurd for any real collection. If actual infinites cannot exist, then the series of past events cannot be actually infinite; the past must be finite; the universe began. He adds the scientific considerations: standard Big Bang cosmology, which describes the universe expanding from an initial state some 13.8 billion years ago, and the second law of thermodynamics, which (he argues) implies the universe has not existed for an infinite time, else it would already have reached maximum entropy.
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