There are many weakness of the Ontological argument starting with the strongest attack from Kant who argued that existence is not a predicate:
“Whatever, therefore and however much our concept of an object, may contain, we must go outside of it if we are to ascribe existence to the object.”
Both Anselm and Descartes rely on the fact that existence is a predicate that can be either posses or lacked. Kant observed that existence is not associated with the definition of something, since it did not add to our understanding of that thing. He claims we have to establish the existence of something before we can say what qualities it has and not the other way around. So, if there is a perfect being then he must exist, the same as if we say there is a triangle, and then it must have 3 sides. We cannot give a thing existence a priori to the definition of a perfect being. Aquinas also questioned this aspect of the argument. He was concerted about moving from definition of God to the existence of God. Aquinas also accused Anselm of making a definition of God which is not held by all believers which leads many believers not to accept the argument. For Aquinas, Gods existence in reality must be demonstrated in an a posterior way. This is what he did in the cosmological argument for the existence of God as he claims the empirical forms of data are the only reliable means of knowing anything about the universe and hence a move from a definition to reality if false.
Other criticisms of the argument include relate to the idea of God as ‘that than which nothing greater can be conceived’. The main criticism of this is that it is no coherent due to the idea of God being omniscient but yet he cant know a persons future choices. Not only this but the definition used for god is said to be mutually inconsistent. This is because it is argued that no being could be both omniscient and omnipotent because an omnipotent being could make a creature which had a secret unknown to anyone but itself, while an omniscient being must know every secret. Not only this but the definition used can be said to lead to a useless God due to the fact that even if we can make sense of the great making properties and show them to be mutully consistent, wont the concept of God at which we arrive be so distant from religion experience as to be useless.
Anselm’s argument is, by some, not classed as a deductive argument for three main reasons; existence is not a great making property, existence can never be an analytic proposition and it is impossible to define things into existence.
From the points above I conclude that the ontological argument does not amount to a proof. The fact that there are so many criticisms for the definition of God, and this being the main aspect of the argument, leads to the argument failing. Kant’s criticism of the argument is also a very strong and powerful one. For many theists I still believe the argument would fail but for some I think it would have more weight when seen along side the other argument for the existence of God.
