[link to www.iep.utm.edu
St. Anselm's 2nd Ontological Argument for God (more elegant)
1.By definition, God is a being than which none greater can be imagined.
2. A being that necessarily exists in reality is greater than a being that does not necessarily exist.
3. Thus, by definition, if God exists as an idea in the mind but does not necessarily exist in reality, then we can imagine something that is greater than God.
4. But we cannot imagine something that is greater than God.
5. Thus, if God exists in the mind as an idea, then God necessarily exists in reality.
6. God exists in the mind as an idea.
7. Therefore, God necessarily exists in reality.
By the way, one need not believe in God or a Supreme Being at the beginning of this, only that it's possible and that one could imagine it i.e. define what God would be is God existed. From that point, the inevitable logic is that God exists.