The ontological argument for the existence of God relies on logic as a basis to substantiate its claims. This type of argument is known as ‘a proiri’, as it is a claim based on knowledge independent of experience.

  There are two classical ontological arguments for the existence of God. The first to put forward a theory was St Anselm of Canterbury. He began by defining God as ‘That than which nothing greater can be conceived’ and this is what he used to base his theories on. He developed this in ‘Prosologion 2’. He used his definition to prove God’s existence in this way: Something that exists is always greater than something only imagined in the mind. Therefore, God must exist, because His existence in reality would be greater than His existence as a mere concept.

 In ‘Prosologion 3’, Anselm went on to not only prove God’s existence, but His necessity. He did this using the following arguments: We can conceive of something that can be conceived not to exist. For God to be ‘That than which nothing greater can be conceived’, He must be this. Therefore, if God cannot be conceived not to exist (which would be superior to something which can be conceived not to exist), He must be necessary.

  Gaunilo of Marmoutier was in opposition to Anselm. He responded to his arguments by describing ‘the most perfect island’: even if there was a most perfect island in the world, one could always imagine one even more perfect than this. He is attempting to criticise Anselm’s jump from the conceived God to the real one. Unfortunately, Anselm never compared two particular objects, of which of course there would be limitless possibilities for perfect. He was describing something greater than any other thing imaginable. Plantinga pointed out, correctly, that islands have no ‘intrinsic maximum’. We also have to remember that the basis for Anselm’s argument is that even if there were an intrinsic maximum for everything, God is further than that because He is greater, not merely as great, as the greatest thing.

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  There are other flaws in Anselm’s argument. The fact is, even if we could imagine something which could not be imagined not to exist, we can easily imagine God not existing, demonstrated by the many examples of people trying to disprove his existence.

  The next classical ontological argument was presented by Descartes. He argued, quite simply, that existence is a predicate of a perfect being: God must exist, because without existence, He would not be supremely perfect. Descartes used the example of a triangle – if we were to imagine a triangle without three sides, the idea ...

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