What is the ontological argument?

The ontological argument tries to reach God by thought alone. Around 1078, Anselm defined God as 'that than which nothing greater can be conceived.' If such a being existed only in the mind, we could conceive something greater — the same being existing in reality too. So the greatest conceivable being must exist. Its boldness has divided thinkers ever since; the young Bertrand Russell briefly cried 'the ontological argument is sound' before changing his mind.

Kant lodged the most famous objection: existence is not a real predicate, not a property you add to a concept to make it greater. A hundred real coins contain no more, as a concept, than a hundred imagined ones. Against Anselm's and Descartes' versions, this has real force, and the honest defender concedes it.

That is why the modern version, Alvin Plantinga's, shifts ground. Using modal logic, it argues that if a maximally great being — one that exists necessarily, in every possible world — is even possible, then it exists. The whole weight rests on that possibility premise. And here is the honest limit: 'possibly it exists' and 'possibly it does not' seem equally conceivable, yet both cannot be true, and mere conceivability is a feeble guide.

So Plantinga claims no more than the argument warrants: not a proof that compels the doubter, but a demonstration that theistic belief can be held rationally. It still forces a real result — the atheist must claim not merely that God happens not to exist, but that God is impossible, a far heavier burden. Anselm offered his 'faith seeking understanding' as a prayer, not a knockout; the believer may find that the God she worships cannot, in the end, be coherently thought away.