Aristotle’s view of the universe as eternal [ … ] leads him to question the cause of everlasting change. He attributes all the changes constantly occurring on earth to the motion of the heavenly bodies. But what keeps them everlasting in motion? It cannot be something that is itself in motion or changing in any way. If it were, it, too, would need a cause of its motion, a cause of its changing. Given infinite time, one might go back from effect to cause in an infinite series and never reach a first cause – mover in motion that is not itself moved by something else in motion.
A prime mover that moves everything that is in motion without moving and without being moved must cause motion by being attractive rather than propulsive [ … ] To move everything else without itself being moved or in motion, the prime mover, Aristotle argues, must function as an attractive or final cause. In thinking this, he did not have in mind the gravitational attraction that the earth exerts upon the bodies to its surface [ …] In his view, attractive or final causes operate on intelligences that can respond to them and adopt them as motives for action. When he says that a heavy body that falls to earth wishes to come to rest there, he is speaking metaphorically, not literally. That motion is only like the motion of the person that is attracted by the candy in the window to enter the store.
Thinking in this way, Aristotle found it necessary to endow the heavenly bodies with intelligences that function as their motors. As the engine of an automobile is its motor, so an intelligence is the motor that keeps a star in motion. But unlike the automobile engine, which must itself be set in motion, the celestial intelligences function as motors through being attracted by the prime mover of the universe.
To be an unmoved and eternal mover of a universe everlastingly in motion, the prime mover must be immutable. But to be immutable, in Aristotle’s view, it must also be immaterial. Anything that is material has potentialities: it is subject to change or motion. It is also imperfect, for at any time it is not actually all that it can be [ … ]
It is by such reasoning that Aristotle came to the conclusion that the prime mover is pure actuality – a being totally devoid of matter or potentiality. In addition, this immaterial being is a perfect being, a being lacking no perfection that remains for it to attain. This perfect being, which is the prime mover of the universe, Aristotle called God.
Mortimer J. Adler, Aristotle for Everybody (Simon and Schuster: 1978) pp. 185-187