Chapter 2 covers elements in Aquinas’s metaphysics and his views on causation as part of the philosophical background for understanding the Five Ways. Attention is given to Aristotle’s explanations of change and his four causes: material, formal, efficient, and final. The chapter then examines Aquinas’s metaphysics of existence and his distinction between essence and existence, which features a contrast between caused and uncaused existence prominent in the Five Ways. There is a brief look at some more recent views of existence influenced by Immanuel Kant and others, which call Aquinas’s views into question. Finally, the chapter explains Aquinas’s model of explanation, that is, his views on what needs accounting for and how it is to be done. (There is some contrast with Gottfried Wilhelm Leibniz’s views.) Aquinas seeks a complete account for why contingently existing individual objects exist at all and undergo changes. He thinks that what has caused existence must ultimately be accounted for by what has uncaused existence.