1. The creation of the world is the most marvellous achievement imaginable.
2. The merit of an achievement is the product of (a) its intrinisic quality, and (b) the ability of its creator.
3. The greater the disability (or handicap) of the creator, the more impressive the achievement.
4. The most formidable handicap for a creator would be nonexistence.
5. Therefor if we suppose that the universe is the product of an existent creator we can conceive a greater being - namely, one who created everything while not existing.
6. An existing God therefor would not be a being greater than which a greater cannot be conceived because an even more formidable and incredible creator would be a God which did not exist.
7. God does not exist.