The Principles of the Philosophy of Nature
In order to understand just about anything that Aquinas says in philosophy or theology, one has to be familiar with the Aristotelian tradition in philosophy, especially as that tradition was handed on to the thinkers of the Middle Ages.
For Aristotle, the Philosophy of Nature is the study of what is in matter and motion, i.e. stuff that changes. Thus one should know the basic classifications of Aristotelian philosophy, what is meant by change, and the ways in which things are made to change.
Substance and Accident – First, as a kind of preliminary and as a tool for philosophical discourse, one should be familiar with the basic distinctions of Aristotle’s logic. The basic logical distinction for our purposes is between substance and accident. This distinction is the basis for Aristotle’s Ten Categories
The Problem of Change – Next, by analyzing change, Aristotle found that what is actual comes to be from what had been merely potential. In so doing, he identifies the three principles necessary for every change: matter, form and privation.
The Four Causes Aristotle identifies four causes, i.e. four positive principles, for every change. They are what changes, what it changes into, the source of the change and what the change is for.