Metaphysics — Substance
When we were investigating essence, we saw that our intellect holds universals, not individuals, meaning that, from the “standpoint of intelligibility” (ItP), being is essence. A second standpoint, however is that of being as things actually are. This is substance:
Substance is a thing or nature whose property is to exist by itself, or in virtue of itself (per se) and not in another thing. (ItP)
The “thing or nature” in this definition is the “subject of action itself (that which primarily exists) and to its nature considered precisely as a nature or essence” (ItP). The “subject of action” is also referred to as a suppositum, or person, “which is nothing but the substantial nature completed by a particular modality (subsistence or personality) which terminates it, as a point terminates a line, (without adding anything to it in its order of nature) and renders it absolutely incommunicable … But when we distinguish and contrast the nature (not terminated) and the subject of action, the term substance remains attached to the nature (not terminated) and is then contrasted with the subject of action taken as such.” (ItP) I would invite you to think about this hard definition before we proceed.
What it meant by existing “by itself” or “in itself” (per se) is that the being can exist on its own (ItP). For instance, a particular individual — Aristotle — can exist on his own and has no need to exist in another thing. On the other hand, a feeling, such as sorrow, cannot exist on its own — it has to exist in a human being, such as Aristotle.
Things that cannot exist on their own are called accidents:
An accident is a nature or essence whose property is to exist in something else. (ItP)
Notes on Substance
The substance of an object, so long as that object exists, is as such immutable. (ItP)
Since substance refers to the thing in itself, or the fundamental nature of the thing, its substance cannot change unless the thing changes, in which case it stops existing.
In the language of philosophy substance is intelligible in itself (per se) and sensible only accidentally (per accidens). (ItP)
This is because substance is “invisible, imperceptible by the senses. For the senses do not apprehend being as such” (ItP). Our senses perceive only the accidents of things, however, this does not mean that the accidents are unrelated to the substance. We can determine the substance of things in this manner. (ItP)