Ontological Argument

In theology and the philosophy of religion, an ontological argument for the existence of God is an argument that God's existence can be proved a priori, that is, by intuition and reason alone.

The argument works by examining the concept of God, and arguing that it implies the actual existence of God; that is, if we can conceive of God, then God exists — it is thus self-contradictory to state that God does not exist.

A modern description of the argument

Here's a short, and very general description of the ontological argument:

1) God is the greatest possible being and thus possesses all perfections.
2) Existence is a perfection.
3) God exists.

This is a shorter modern version of the argument.

1) God is the entity than which no greater entity can be conceived.
2) The concept of God exists in human understanding.
3) God does not exist in reality (assumed in order to refute).
4) The concept of God existing in reality exists in human understanding.
5) If an entity exists in reality and in human understanding, this entity is greater than it would have been if it existed only in human understanding (a statement of existence as a perfection).
6) from 1, 2, 3, 4, and 5 An entity can be conceived which is greater than God, the entity than which no greater entity can be conceived (logical self-contradiction).
7) Assumption 3 is wrong, therefore God exists in reality (assuming 1, 2, 4, and 5 are accepted as true).