A thousand years ago, Anselm of Canterbury offered his now-famous Ontological argument for the existence of God. He was trying to produce the simplest, most powerful argument he could for God’s existence. While it is simple in construct, it is challenging to grasp. For a thousand years, philosophers and theologians have argued if it is a valid argument. After wrestling with it for a while, I believe (with Alvin Palntinga) that it is a valid argument. Read about the argument here in the Internet Encyclopedia of Philosophy along along with criticism of the argument.

There are actually many variations of the argument. Here is the classic version of the argument:

1. It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (that is, the greatest possible being that can be imagined).
2. God exists as an idea in the mind.
3. A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
4. Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (that is, a greatest possible being that does exist).
5. But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.)
6. Therefore, God exists.

Here is the complete text of his argument below in his Proslogion.