Greatest Conceivable Being

St. Anselm of Canterbury defined God as “that than which nothing greater can be conceived” (or more simply, greatest conceivable being - GCB).

This definition is important because from it Anselm will be able to derive a number of necessary properties of God that would otherwise be difficult to establish.

For example, not only must God be omnipotent, omniscient, and omnibenevolent, but God must also be eternal, simple, actual, necessary, impassible, and immutable. The reason God must have these properties is because if It did not, than we could imagine a being greater than God, which is exactly what the definition of God as GCB asserts is impossible.