Ultimately, it's all about "substance". - "Metaphysics" Book Lambda.1-5

Book Lambda of Aristotle's "Metaphysics" takes the concept of substance and uses it in a cosmological "Theory of Everything". Based on the idea that substance is most real, he then goes on to explain how substance makes the world go around, as in how it happens that planets go in circles and never seem to stop or slow down. At the same time, there are other material beings here on earth which almost never go in circles in space. Why is this? Aristotle's answer is pretty genius, and utterly different from anything you might have heard thus far.