the ontological argument


ontological argument 

St Anselm and Descartes both famously presented an ontological argument for the existence of God. (The word ‘ontological’ comes from ‘ontology’, the study of (-ology) of what exists or ‘being’ (ont).)

Their versions of the argument are slightly different, but they both argue that we can deduce the existence of God from the idea of God. Just from thinking about what God is, we can conclude that God must exist. Because it doesn’t depend on experience in any way, the ontological argument is a priori.


Anselm’s argument relies on ‘conceivability’:

  1. By definition, God is a being greater than which cannot be conceived.

  2. I can conceive of such a being.

  3. It is greater to exist than not to exist.

  4. Therefore, God must exist.

The idea of God as the most perfect possible being has a long history. And perfection has also been connected to reality: what is perfect is more real than what is not. Anselm’s argument makes use of both these ideas.

Anselm starts from a definition of God – if we could think of something that was greater than the being we called God, then surely this greater thing would in fact be God. But this is nonsense – God being greater than God. The first being isn’t God at all. We cannot conceive of anything being greater than God – if we think we can, we’re not thinking of God.

The second premise says that this idea – a being greater than which we cannot conceive – is coherent. Now, if we think of two beings, one that exists and one that doesn’t, the one that actually exists is greater – being real is greater than being fictional! So if God didn’t exist, we could think of a greater being than God. But we’ve said that’s impossible; so God exists.


Gaunilo and the perfect island

Anselm received an immediate reply from a monk named Gaunilo: you could prove anything perfect must exist by this argument! I can conceive of the perfect island, greater than which cannot be conceived. And so such an island must exist, because it would be less great if it didn’t. But this is ridiculous, so the ontological argument must be flawed. You can’t infer the existence of something, Gaunilo argues, from the idea of its being perfect.

Anselm replied that the ontological argument works only for God, because the relation between God and greatness or perfection is unique. An island wouldn’t cease to be what


it is – an island – if it wasn’t perfect; of course, it wouldn’t then be a perfect island. But islands aren’t perfect by definition; perfection is something an island can have or not have. It is an ‘accidental’ not an ‘essential’ property of islands. It’s perfectly coherent to think of an island that isn’t perfect.

(An essential property is one that something must have to be the thing that it is. Islands must be areas of land surrounded


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