Thomas Aquinas holds that the existence of God is self-evident in itself (because God’s essence is his existence) but not to us (since we do not know the divine essence). Giles of Rome agrees with the first part of Thomas’s claim, but he parts company with Aquinas by maintaining that God’s existence is self-evident to the wise. Since the wise can know that God is his existence, they cannot think of him as not existing. This paper reexamines Thomas’s teaching in the light of Giles’s criticisms. By examining closely what is involved in the claim that God’s essence is his existence, and how one’s knowledge of this claim is related to the knowledge that God exists, it argues that Thomas’s position has the resources to withstand Giles’s objections.