The Unmoved Mover

Aristotle begins by describing substance, of which he says there are three types: the sensible, which is subdivided into the perishable, which belongs to physics, and the eternal, which belongs to "another science." He notes that sensible substance is changeable and that there are several types of change, including quality and quantity, generation and destruction, increase and decrease, alteration, and motion.
Aristotle's argument for the existence of the unmoved mover progresses as follows:

1. There exists movement in the world.
2. Things that move were set into motion by something else.
3. If everything that moves were caused to move by something else, there would be an infinite chain of causes. This can't happen.
4. Thus, there must have been something that caused the first movement.
5. From 3, this first cause cannot itself have been moved.
6. From 4, there must be an unmoved mover