Concept
τὸ πρῶτον κινοῦν · prime mover
If whatever moves is moved by something, Aristotle reasoned, then the chain of movers cannot stretch back forever; there must be a first mover that is itself unmoved. This Unmoved Mover, argued in the Physics and in Book Lambda of the Metaphysics, is the eternal source of all motion and the supreme principle of his cosmos.
The puzzle is how something motionless can cause motion. Aristotle's answer is that it moves the heavens not by pushing but by being loved: it is the object of desire and of thought, drawing all things toward itself as a beloved draws a lover, while remaining perfectly at rest. The cosmos turns out of longing for its perfection.
Being pure actuality, with no matter and no unrealized potential, the Mover's sole activity is contemplation — and the only object worthy of it is itself. So Aristotle calls God 'thought thinking itself', an eternal Intellect eternally aware of its own activity. This austere divinity profoundly shaped later Platonist and medieval conceptions of God, even as the Platonists set the One above it.
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