The ontological argument is an a priori argument for the existence of God. It is generally considered a part of natural theology - the use of human reason to know things about God independently of any action on Her part (as in revealed theology). This puts the argument in the same category as other arguments for God's existence such as the teleological argument and the cosmological argument. It's generally agreed that the first ontological argument was formulated by Anselm, Archbishop of Canterbury, who lived in the 11th/12th century. It goes like this: God is the greatest possible being.God exists as an idea in one's mind.It is greater to exist both in the mind and in reality. If God exists only in the mind, then a greater being could be conceived to exist who existed both in the mind and in reality.Therefore, God exists both in the mind and in reality. The main thrust of the argument is that God's existence necessarily follows from the concept of 'God'. If we understand God as being the greatest possible being, we are necessarily led to the idea that he must exist in reality. The argument is not generally considered to be successful. Its main problem centres around its apparent assumption that existence can be considered a predicate. A predicate informs us about a subject’s attributes – what property it might be said to have. By making any kind of statement containing a subject and a predicate, Kant argued that to claim a thing had a predicate was to assume its existence in the first place, and to state that this thing had the added quality of existence actually added nothing to its concept at all. Existence was something one did to establish that a certain entity existed - and the way to determine what entity you're talking about was to determine the properties they had. So, existence claims were of a different logical order to property-ascriptions. Philosophers have attacked Kant on this claim, with arguably some success. The argument also runs into some theological concerns as well. Aquinas argued that we had no concept of God, since having a concept requires a full understanding of what it would be to talk about that thing and to point it out in the world. But, if God can be understood by creatures, he is a creature - and hence would be an idol. Hence, we cannot say that we have a concept of God, which is required by the ontological argument.