Selection from Ralph McInerny, A First Glance at St. Thomas Aquinas: A Handbook for Peeping Thomists, (Notre Dame, IN: University of Notre Dame Press, 1990), 75-76.

A natural thing is made up of matter and form and each of these components is a source of changes we attribute to it. Any natural or physical thing will, if let go of, drop. We attribute this to its matter because a man, his dog, and a geranium will all fall when dropped so we do not think this a peculiarity of humans, dogs, or geraniums. A person's laughter, on the other hand, is thought to belong to her because of the specific kind of thing she is.

Form sets a thing off from other things. This is true of form in its primary sense of shape. You can sort out the square ones and the round ones from the rectangular ones. It is because form enables us to sort things that the word for external shape was extended to mean what sets one thing off from another in terms of more than its contours. The fact that things can be distinguished by color and place led to calling these features forms too and finally what makes a thing to be a substance of this type rather than that is called its form. Natural kinds or sorts are read from form rather than matter. Nonetheless, change due to the thing's matter as well as those due to its form will be said to be natural to it.

Its nature is that in the thing which is at the bottom of the changes it undergoes and the activities it engages in. It is intrinsic to the thing. Artificial changes, by contrast, come from outside in the sense that they require the intervention of a human being.

When a tree is felled, the wood obtained is a product of nature. Many years ago, an acorn germinated, rain fell, the sun shone, the acorn became the mighty oak that now lies fallen in the forest. Logs, insofar as they are sections of the felled tree made with saw or axe, are artificial things. Lumber is even more so. And the house built from the boards is an even more complicated artifact.

An artifact comes into being when a shape or form is imposed by a human agent on a natural material. So the artifact is composed of matter and form. In the case of logs, the matter is obviously produced by nature. When the logs are the matter from which lumber is made, we are twice removed from nature, the house yet further removed.

An artificial change presupposes a natural matter whether proximately or remotely. No nature, no art.