Anselm's ontological argument claims that God must exist because God is defined as the greatest being that can be conceived. His first argument is that existence in reality is greater than just existing in the mind, so if God is the greatest being, God must exist in reality. His second argument is that a necessary being (one that cannot not exist) is greater than a contingent being, so God as the greatest being must be necessary and therefore must exist. Gaunilo criticized this argument using the example of a perfect island, saying you cannot define something into existence just because it is a perfect conception.