According to Aristotle, all of nature develops, changes, comes into being, and passes out of being through the operations of the four causes. However—and here Aristotle’s metaphysics and philosophy of science take on a theological tone, not unlike Plato’s—causal chains cannot be infinite, so there must be a first cause, something that is not itself caused, an “unmoved mover.” The unmoved mover that is the cause of everything cannot be an efficient, material, or even a formal cause, because all of those are contained in things that exist. The unmoved mover is the ultimate final cause, that to which everything is aiming. It is the greatest good and the purpose of life, and Aristotle tells us that it is “noûs”—or mind—and its essence is thought, which is always active. It thinks about itself: nous contemplating nous.