*From: The Stanford Encyclopedia of Philosophy:
“Saint Anselm of Canterbury (1033–1109) was the outstanding Christian philosopher and theologian of the eleventh century. He is best known for the celebrated “ontological argument” for the existence of God in the Proslogion, but his contributions to philosophical theology (and indeed to philosophy more generally) go well beyond the ontological argument. In what follows I examine Anselm’s theistic proofs, his conception of the divine nature, and his account of human freedom, sin, and redemption…”
From: Think (1999), by Simon Blackburn:
“…the ontological argument of Anselm (1033-1109). Anselm defines God as a being ‘than which nothing greater can be conceived’. And he addresses himself to ‘the fool’ (from Psalm 14) who has said in his heart that there is no God:
But when this same fool hears me say ‘something than which nothing greater can be thought,’ he surely understands what he hears, and what he understands exists in his understanding: even if he does not understand that it exists (in reality)…So even the fool must admit that something than which nothing greater can be thought exists at least in his understanding, since he understands this when he hears it, and whatever is understood, exists in the understanding. And surely that than which a greater cannot be thought cannot exist only in the understanding. For if it exists only in the understanding, it can be thought to exist in reality as well, which is greater…[T]herefore, there is no doubt that something than which a greater cannot be thought exists both in the understanding and in reality.
The notable thing about this argument is that it is purely a priori…”