In 1078, a monk named Anselm astonished the world by arguing that if it is possible that God exists, then it follows logically that God does exist. Anselm defined God as the greatest conceivable being. If it is possible that God exists, then God exists in at least one possible world. But since God is the greatest conceivable being, if he exists in one possible world, then he exists in every possible world, including the actual world. The Ontological Argument thus shows that if God possibly exists, then God actually exists.