Every cause of a given effect itself demands a cause in order to account for its own existence; this cause in turn requires a cause, and so on. If we have an infinite series on our hands, in which each cause itself requires a cause, then nothing could ever have come into existence. Saint Thomas explains that there must, therefore, be an Uncaused Cause—a cause that is not itself in need of a cause. This first cause can therefore begin the sequence of causes. This first cause, Saint Thomas says, is God. God is the one self-existing being whose existence is part of His very essence. No human being must exist; there was a time before each one came into existence, and the world will continue to exist after each one perishes. Existence is not part of the essence of any human being. But God is different. He cannot not exist. And He depends on nothing prior to Himself in order to account for His existence.