Let’s try looking at some of the main criticisms of why this argument is no longer considered valid and popular.Even in Anselm’s day, the argument had its critics.
For the readers who have not heard this argument, this argument is called an ontological argument, although that label is not Anselm’s - it is Immanuel Kant’s.
An ontological argument attempts to establish the existence of God by deductive reason alone without the inference of any empirical evidence.
In Kant's own words: Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing.
It is merely the positing of a thing, or of certain determinations in it. The proposition, God is omnipotent, contains two conceptions, which have a certain object or content; the word is, is no additional predicate-it merely indicates the relation of the predicate to the subject.
Here’s Gaunilo’s argument: Now if some one should tell me that there is …
an island [than which none greater can be conceived], I should easily understand his words, in which there is no difficulty.
Gaunilo of Marmoutier, a monk and contemporary of Anselm's, is responsible for one of the most important criticisms of Anselm's argument.
He pointed out that we could, by means of a similar line of reasoning, “prove” that a perfect island – an island as perfect as it is possible for any island to be – exists.
Armed with this concept of God, we can now argue for God’s existence as follows. That there exists a being greater than which cannot be conceived is at least a hypothesis we can entertain.
But, adds Anselm, as it is greater to exist in reality than merely in our imagination, this being must really exist.