Gaunilo's criticism does not explicitly demonstrate a flaw in Anselm's argument; rather, it argues that if Anselm's argument is sound, so are many other arguments of the same logical form, which cannot be accepted. He offered a further criticism of Anselm's ontological argument, suggesting that the notion of God cannot be conceived, as Anselm had asserted. He argued that many theists would accept that God, by nature, cannot be fully comprehended. Therefore, if humans cannot fully conceive of God, the ontological argument cannot work. Anselm responded to Gaunilo's criticism by arguing that the argument applied only to concepts with necessary existence. He suggested that only a being with necessary existence can fulfill the remit of "that than which nothing greater can be conceived". Furthermore, a contingent object, such as an island, could always be improved and thus could never reach a state of perfection. For that reason, Anselm dismissed any argument that did not relate to a being with necessary existence. Other parodies have been presented, including the devil corollary, the no devil corollary and the extreme no devil corollary. The devil corollary proposes that a being than which nothing worse can be conceived exists in the understanding (sometimes the term lesser is used in place of worse). Using Anselm's logical form, the parody argues that if it exists in the understanding, a worse being would be one that exists in reality; thus, such a being exists. The no devil corollary is similar, but argues that a worse being would be one that does not exist in reality, so does not exist. The extreme no devil corollary advances on this, proposing that a worse being would be that which does not exist in the understanding, so such a being exists neither in reality nor in the understanding. Timothy Chambers argued that the devil corollary is more powerful than Gaunilo's challenge because it withstands the challenges that may defeat Gaunilo's parody. He also claimed that the no devil corollary is a strong challenge, as it "underwrites" the no devil corollary, which "threatens Anselm's argument at its very foundations". Thomas Aquinas, while proposing five proofs of God's existence in his "Summa Theologica", objected to Anselm's argument. He suggested that people cannot know the nature of God and, therefore, cannot conceive of God in the way Anselm proposed. The ontological argument would be meaningful only to someone who understands the essence of God completely.