1. The fool understands the phrase “something than which nothing greater can be thought.”
2. Hence, “something than which nothing greater can be thought” exists at least in the fool’s mind.
3. It is greater to exist in reality than to exist in the mind alone.
4. So if “that than which nothing greater can be thought” existed only in the fool’s mind, it would be possible to think of something greater (the same thing existing in reality).
5. But this would be a contradiction, since it is impossible to think of something greater than “that than which nothing greater can be thought.”
6. Hence “something than which nothing greater can be thought” must exist both in the fool’s mind and in reality. God exists!
~ St. Anselm of Canterbury