Hume: No a priori proofs of matters of fact
Both Hume and Kant raised specific objections to the structure of the ontological argument. In the style='font-style:italic'>Dialogues Concerning Natural Religion Cleanthes makes the following objection:
I shall begin with observing that there is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by any argument a priori.
Nothing is demonstrable unless the contrary implies a contradiction. Nothing that is distinctly conceivable implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no being, therefore, whose nonexistence implies a contradiction. Consequently there is no being whose existence is demonstrable. I propose this argument as entirely decisive, and am willing to rest the whole controversy upon it.What Hume is saying here is that you can never establish the matter-of-fact existence of a particular thing by way of an a priori argument. (Remember that by “demonstration” Hume means proof, so “demonstrable” means provable.)
Hume's argument is a little more controversial than this rather breezy formulation suggests, because, as we have already seen, some people suppose we can establish the existence of certain things—a prime number between 17 and 23, for example—by way of proof. Numbers therefore seem to exist in every possible world. If that is the case, their existence refutes Hume's observation in the next paragraph that “the words necessary existence have no meaning.” But Hume actually didn't believe numbers existed—he thought mathematical truths were “relations of ideas,” not “matters of fact”—so that wouldn't impress him. And he is discussing not mathematical or abstract entities here but what he calls “matters of fact.” The existence of matter-of-fact entities—things such as people and planets— does seem to be something that cannot be decided a priori.
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Anselm or Descartes could reply that Hume was begging the question here, for if the ontological argument is correct, God, like the number 19, exists in every possible world and his existence is not just a “matter of fact.” If God is a necessary existence, then he's certainly unlike people and planets, so the fact that you can't prove the existence of planets and people a priori is neither here nor there. Still, Hume's objection captures something of our initial reluctance to accept Anselm's argument, I think: it just looks like you couldn't get an interesting conclusion from such a swift a priori argument.In any case, most religious people have a conception of God that is not just the rather arid conception of a “being greater than which none can be conceived.” So that even if there were such a being, it isn't clear that it would do as the object of religious faith. For, at least in the West, most people who have believed in God have thought of him as a person. In fact, the ontological argument doesn't seem to have moved many people from disbelief to belief. And St. Thomas Aquinas, who was the leading Christian thinker of the Middle Ages, rejected Anselm's arguments, even though, as we shall see soon, he thought that there were other sound arguments for the existence of God.
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