outline the ontological argument

The ontological argument is an a priori argument for the existence of God. This means it is a proposition based on definition and use of logic alone; no experience is needed. It is also deductive, meaning its conclusion implied directly by the premises. The argument is a 'reduction ad absurdum' which attempts to prove God's existence by showing that to deny God's existence is absurd. Anselm thought it was illogical to deny God's existence and compared atheists to the fool in psalm 14: 'the fool hath said in his heart, there is no God'. 
The first distinctive feature of the ontological argument is the argument by Anslem, who argued that God is 'that than which nothing greater can be conceived'.  His first form is the idea of existence v reality he argued that God must exist in reality because what exists in re (in reality) is far greater than what exists only in intellectu (in the mind). If we assume that God is the greatest conceivable being, then it wouldn't be possible for him to go out of existence because a being which could not cease to exist would be greater. Therefore, Anselm shows that the greatest conceivable being (God) cannot not exist. Everything but God exists contingently, but only God exists necessarily. Anselm places existence in the same category as he would place goodness, love wisdom or justice. By doing so he treats existence as a predicate (a defining characteristic)