Anselm

...the crucial step in Anselm’s argument is this: if (b) is true, and no perfect being exists, then (a) must be false—the Fool [of Psalm 14] is not thinking of a perfect being, because a perfect being has, among its other perfect-making properties or features, existence. Put the other way round: if (a) is true—if the Fool is genuinely thinking of a perfect being—then (b) must be false, and so God, the perfect being, exists. --Alex Byrne, God: Philosophers weigh in, Boston Review, January/February 2009 (via Arts & Letters Daily)