prime mover the original source and cause of motion (change) in the universe – an idea that was developed by Aristotle and became important in Judaic, Christian, and Islamic thought about God. According to Aristotle, something that is in motion (a process of change) is moving from a state of potentiality to a state of actuality. For example, water that is being heated is potentially hot and in the process of becoming actually hot. If a cause of change must itself actually be in the state that it is bringing about, then nothing can produce motion in itself; whatever is in motion is being moved by another. For otherwise something would be both potentially and actually in the same state. Thus, the water that is potentially hot can become hot only by being changed by something else (the fire) that is actually hot. The prime mover, the original cause of motion, must itself, therefore, not be in motion; it is an unmoved mover. Aquinas and other theologians viewed God as the prime mover, the ultimate cause of all motion. Indeed, for these theologians the argument to establish the existence of a first mover, itself unmoved, was a principal argument used in their efforts to prove the existence of God on the basis of reason. Many modern thinkers question the argument for a first mover on the ground that it does not seem to be logically impossible that the motion of one thing be caused by a second thing whose motion in turn is caused by a third thing, and so on without end. Defenders of the argument claim that it presupposes a distinction between two different causal series, one temporal and one simultaneous, and argue that the objection succeeds only against a temporal causal series. See also AGENT CAUSATION , AQUINAS , ARISTOTL. W.L.R.