  In the same context, Descartes also characterizes the ontological argument as a proof from the “essence” or “nature” of God, arguing that necessary existence cannot be separated from the essence of a supremely perfect being without contradiction. In casting the argument in these terms, he is implicitly relying on a traditional medieval distinction between a thing’s essence and its existence. According to this tradition, one can determine what something is (i.e. its essence), independently of knowing whether it exists. This distinction appears useful to Descartes’ aims, some have thought, because it allows him to specify God’s essence without begging the question of his existence.