“Book 12 is usually considered the culmination of Aristotle’s work in metaphysics, and in it he offers his teleological system. Before he draws any grand conclusions, he begins with the idea of substance, of which there are three kinds: changeable and perishable (eg plants and animals), changeable and eternal (eg heavenly bodies) and immutable.”
From “Metaphysics” in the Aristotle section of SparkNotes
- Read the rest of the SparkNotes article here (search for “Book XII”).
- Read Aquinas’ commentary on Metaphysics 12.
Aristotle’s Metaphysics is a difficult book, partly because of the language used, and the different translations offered. It is recommended that you start by reading this introduction from the Internet Encyclopedia of Philosophy.