Ontological argument

The ontological argument uses the definition of God to prove His existence. It was originally proposed by Anselm and remained influential for centuries, and was later used by Descartes. Anselm's proof is as follows:

God is that for which no greater being is conceivable.

That which exists is greater than that which does not exist.

That which does not exist fails to satisfy the definition of God.

Therefore God exists.

In a sense this proof of the existence of God is similar to the mathematical proofs based on deriving a contradiction of something is not true. Accepting the definition of God, as most would, while then denying that God exists leads to a contradiction as existence is a necessary element of perfection in this context.

Criticism

Atheists tend to agree with the first premise defining God, and from that follows in a strong, apparently deductive fashion. Anselm's proof bypasses Hume's Fork.

But Guanilo, a contemporary of Anselm, used reductio ad absurdum and envisaged a 'lost, yet most perfect island'. Guanilo argued that just because we can imagine this island does not mean it actually exists.

Kant weighed in against Descartes' version, challenging the premise that existence is a predicate of perfection. Kant said he could imagine a hundred thalers (coins) in such great detail that whether or not they actually existed was arbitrary, as there was no difference other than this between the imagined and the existent thalers.

Davies insisted that Anselm's argument begs the question- it assumes what it sets out to prove. In other words, if a God existed, he would be supremely perfect, and would therefore exist. In explicit form, the first might be construed to mean 'The concept of God is the most supremely perfect concept'. It has been suggested that the ontological argument is therefore flawed in that it uses equivocation to progress from a concept of God to an existent of God to an existent God, and flawed in that the entire argument collapses irreparably once this is exposed.