Everything that exists has an explanation of its existence. Philosophers have used this principle in arguments for the existence of something ontologically fundamental, an ultimate ground of being, such as God. But if everything has an explanation of its existence, so, too, does whatever is fundamental. I analyze what five prominent historical figures—Anselm, Aquinas, Descartes, Spinoza, and Leibniz—have said about how a fundamental being, a being that exists a se, is explained. While none are satisfactory, several lessons are drawn from them to construct a novel proposal, which requires a new way of thinking about absolute and relative fundamentality. If what is fundamental is identified with God, certain Scholastic doctrines about the divine nature, such as absolute simplicity, will have to be abandoned—a consequence that I argue is a virtue rather than vice of the proposal.