The answers are: infinite being, immutability, omniscience.
By introspection, in this famous proof that Descartes provides for the existence of God, he comes to understand that God, being a perfect being, must be perfect in every which way, so that being (God being the infinite one or Being), immutability (the actual power to remain the same through eternity), and omniscience (or the actual capacity of knowing everything) are ideas that a human being can intuit but that are not of human nature, but, since a human being can somehow fathom them, they must have been, a fortiori, put there by a being capable of actualizing those ideas or else they would have no foundation, which would be absurd.