Anselm of Canterbury

Anselm was born in Aosta, in the Italian Alps, and made his name far to the north: first as a monk and abbot at Bec in Normandy, then as Archbishop of Canterbury, forever colliding with English kings over the freedom of the church. His motto was fides quaerens intellectum, faith seeking understanding. He believed first, and then asked reason to see why what he believed was true.

In the second chapter of the Proslogion he lays out an argument more audacious than any other in philosophy. Take the concept of God as "that than which nothing greater can be conceived." Even the fool who says in his heart "there is no God" understands the phrase – so such a being exists at least in his understanding. But if it existed in the understanding alone, a greater could be conceived: one that also existed in reality. And there is, by definition, nothing greater than "that than which nothing greater can be conceived." Therefore it must exist in reality. God's existence, derived from the concept of God alone, without a single glance at the world.

Nine centuries later, philosophers are still answering him. The monk Gaunilo objected in Anselm's own lifetime (imagine the most perfect island – does imagining make it real?). Aquinas rejected the argument, Descartes rebuilt it, Leibniz set out to repair it, Kant delivered the classic blow (existence is not a property of a thing), and in the twentieth century Gödel restated it in modal logic and proved it formally. In a separate work, Cur Deus Homo ("Why God Became Man"), Anselm also gave the West its theory of atonement: sin is a debt of honour, infinitely too large for a human to pay and payable only by a God who becomes human.