St. Anselm of Canterbury (1033–1109 CE) was a Christian philosopher. He was Archbishop of Canterbury from the age of 60 until his death.
Anselm is perhaps most famous for developing the ontological argument for the existence of God.
Anselm felt that the claim that God could exist in intellect (we know what God is supposed to be) but not in reality (where he would actually exist) was absurd. To demonstrate this, he walks through an argument with "the fool" who does not believe in God. The fool's argument is that God exists in intellect alone, and not in reality. If the fool also agrees that God is that than which nothing greater can be conceived, his argument falls apart:
Ultimately, the argument says that than which nothing greater can be conceived must exist, because if it did not exist, we could always imagine something greater, which exists.
Anselm's claim says nothing about the nature of God — and certainly does not guarantee the existence of the Christian God. It only guarantees that if we can truly understand the definition of God, than God must exist.
Anselm's ontological argument has received criticism from a variety of sources, even within Christianity. The monk Gaunilo argues that the ontological argument proposed by Anselm could also be used to prove the existence of the perfect island, or the perfect anything. He argues that the jump from an intellectual concept to reality is not something within human capacity. Saint Thomas Aquinas argued against it on similar grounds, arguing that God's existence can not be known a priori.
Through his development of the ontological argument, Anselm is recognized as the founder of scholasticism, the medieval marriage of Christian theology and philosophy.
Birth Name: Anselmo d'Aosta
Born: 1033 in Aosta, Burgundy
Died: April 21, 1109
Office: Archbishop, Canterbury 1093-1109
Canonized: 1494 by Pope Alexander VI
Feast Day: April 21