Saint Anselm’s Ontological Argument for the existence of God is famous because it offers a way to prove God’s existence based entirely on reason. There is no need to look for physical evidence, instead, we can use logic. With the use of a reductio ad absurdum, or reductio proof—’something proved to be true by showing that the opposite is untrue,” a proof by contradiction—Anselm is able to create contradiction and therefore prove the existence of God. In order to do this, Anselm created a deductive argument for the existence of God based on his understand of God’s nature, or the definition of God—“something-than-which-nothing-greater-can-be-thought.”
Thus making God the best thing we could imagine. Anselm expressed that there are two ways something can exist; only in our minds or in our minds and reality. Things that exists in reality, accord to Anselm, are always better than things that exists just in our minds. Although, if God exists only in our minds then, by definition, he wouldn’t be that that which nothing greater can exists— contradiction! Therefore, God must exist in reality. Through this argument, Anselm believed he had deductively proved God’s existence in a way that was immune to error.
Gaunilo of Marmoutiers, a Benedictine monk living during the same time as Anselm had a response to the ontological argument. Gaunilo figured that we could run the same line of reasoning to prove the existence of literally anything imaginable. For example, an island. Not just any island, but the best possible island imaginable. This mythical lost island was an island “blessed with all manner of priceless riches and delights in abundance, much more than even the Happy Isles, and, having no owner or inhabitant, it is superior everywhere in abundance of riches to all those other lands that men inhabit.”
