Now if I take the subject (God) with all its predicates (omnipotence being one), and say, God is, or There is a God, I add no new predicate to the conception of God, I merely posit or affirm the existence of the subject with all its predicates - I posit the object in relation to my conception.
Accordingly, what goes wrong with the ontological argument is that the notion of existence is being treated as the wrong logical type.
To be a little bit clearer, existence is not a property (in, say, the way that being red is a property of an apple).
Rather it is a precondition for the instantiation of properties in the following sense: it is not possible for a non-existent thing to instantiate any properties because there is nothing to which, so to speak, a property can stick. To say that x instantiates a property P is hence to presuppose that x exists.
On the seemingly safe assumption that there is no such island, it seems we have no choice but to accept that there is something wrong with this argument, including Anselm's similarly constructed argument.
The philosopher Immanuel Kant offers yet another well known criticisms of the ontological argument.However, Anselm's argument runs into another, if we, let's say, assume that existence is a great-making property.The difference between these two concepts is that we have added existence to the latter.Thus, on this line of reasoning, existence isn't a great-making property because it is not a property at all; it is rather a metaphysically necessary condition for the instantiation of any properties.Now Kant's point has been debated and it is not universally accepted, although I have not read any strong counter arguments.Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (i.e., a greatest possible being that does exist).But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.) Therefore, God exists.That God, if he exists, is such a being seems clear.If you conceive of a being, yet can also conceive of a still greater being, then the being you first thought of cannot be God.Concepts, as a logical matter, are defined entirely in terms of logical predicates.Since existence isn't a logical predicate, it doesn't belong to the concept of God; it rather affirms that the existence of something that satisfies the predicates defining the concept of God.