The Ontological Argument for the existence of God was first propounded by Anselm (c. 1033–1109), abbot of Bee and later archbishop of Canterbury, in his Proslogion (Chs. 2–4) and in his Reply to a contemporary critic.

He begins (Proslogion 2) with the concept of God as "something than which nothing greater can be conceived" (aliquid quo nihil maius cogitari possit, and other equivalent formulations). It is clear that by "greater" Anselm means "more perfect." (Sometimes he uses melius, "better," instead of maius, "greater": for instance, Proslogion 14 and 18.) Since we have this idea, it follows that "Something than which nothing greater can be conceived" at least exists in our minds (in intellectu) as an object of thought. The question is whether it also exists in extramental reality (in re). Anselm argues that it must so exist, since otherwise we should...