The ontological argument for God's existence

The final blow to the property view of existence is dealt by the so-called ontological argument for the existence of God. The argument, whose original version is due to St Anselm, can be presented as follows. Let us define God as a being than which nothing greater can be conceived. As you should recall, underlying the property view is the assumption that to each description there corresponds an object which can be either existent or non-existent. Suppose that the object which corresponds to the above description is a non-existent entity (i.e. that God does not exist). In that case it can be argued that a greater being can be conceived, namely God that possesses the property of existence. Hence we have a contradiction, since the object which was to satisfy the description ("the greatest conceivable being") in fact does not satisfy it. The cause of the contradiction was the assumption that God does not exist, thus we conclude that God has to exist.

Analogously we could argue that there exists the greatest imaginable island, or any other object for that matter. It is even possible to eliminate the controversial assumption that existence is greater than non-existence. We could simply define the following concept: God who possesses the property of existence. By the same reasoning we can show that the assumption that the referent of this description is a non-existent object leads to a contradiction. But it cannot be so easy to prove the existence of God or any other object you could think of. The culprit was the assumption that each description has a referent - existent or non-existent. If we abandon it, the argument can't even get off the ground, since we cannot argue that it is possible to conceive a greater being than non-existent God. There is no non-existent God, hence we cannot compare it with anything else.

The quantifier view of existence deals with the problem easily. If God existed, it would be the greatest being conceivable, but if God does not exist, there is simply no object in the whole universe than which nothing greater can be conceived (it is not true that for some x, x is God). In a similar way, Kant stressed that existence is not a property whose absence can be detrimental to the greatness of an object. Kant's view should not be interpreted as implying that an existing coin does not differ from a non-existent one. Rather, a non-existent coin is not an object at all, so "it" cannot differ from anything. If we formulate a description of an object, adding to its definitional characteristic that it should exist does not make the description more informative. 

Required reading: 
B. Garrett, "The Ontological Argument", in What is this thing called metaphysics?, pp.  4-7