In a relatively well-known passage of his little metaphysical treatise, Diancia de Sencia,

on being in essence, St. Thomas Aquinas explains the meanings of several basic philosophical terms.

All of them, he tells us, refer to the same reality, but they do so from different angles or under different conceptions.

Two of the terms are essence and nature.

Something is called an essence, he says, quote, insofar as through it and in it, an entity ends, has being, estate.

But that which is an essence is also called a nature,

quote, insofar as it has an order toward a thing's proper activity. Now, maybe a little bit less well known,

at least judging from how my own students react to it,

is that St. Thomas, following Aristotle,

regards this meaning of nature as only an extended meaning,

a secondary meaning.

He thinks the principle meaning is the one laid out in book two of Aristotle's physics,

a thing's primary intrinsic principle of motion and rest.

In this sense, not every essence is a nature, for not everything is mobile.

Incorporial substances have activities, they have intellectual activities,

but they don't have any motions in the proper sense.

Only bodies are moved.

Later on, I'll talk about why that's the case.

But speaking strictly then, nature's only belong to, if you'll pardon the redundancy, physical substances. How did it get extended, the term nature, how did it get extended to essences generally?

Aristotle says it's by the fact that the natures of physical substances do turn out to

coincide with their essences.

The essence of a physical substance consists of its form and its matter,

...