God then exists in the understanding, since we understand this concept. But if He only existed in the understanding, something greater could be conceived, for a being that exists in reality is greater than one that exists only in the understanding. But then we can conceive of something greater than that than which nothing greater can be conceived, which is contradictory. Hence, God cannot exist only in the understanding, but exists in reality.
In Anselm's own time the argument was criticized by a monk called Gaunilo, who urged that the same pattern of reasoning would prove the existence of a perfect island (for a perfect island existing only in the imagination is obviously not as good as one that really exists). The argument was not accepted by Aquinas, but was resurrected by Descartes, who made plain the requirement that existence be thought of as part of the definition or essence of a supremely perfect being. This, in turn, opened the way to criticism by Hume and especially Kant, that existence is not a property or predicate. Kant's criticism has been generally sustained by modern logic.
