3. Divisions of Substance
a) First substance, second substance. The most noted of
Aristotle's divisions is that of first and second substance.8
First substance is the concrete individual subject, as "Peter"
or "Socrates." This is not present in a subject, nor can it be
predicated of a subject. Second substance designates the
universal which expresses the essence of a subject, as "man,"
"horse," etc. Properly speaking this is not in a subject
either, but unlike first substance it can be predicated of a
subject, as in "Peter is a man," he has (or partakes of) the
essence of man.9 Based, as is evident, on the predicabilities
of substance, this division serves principally the logician.
For the metaphysician substance is simply the concrete
subject - first substance.
b) Material, immaterial substances. The basic division of predicamental substance is seen in the first dichotomy of the Tree of Porphyry, that of material (composite) and immaterial (simple) substances.
Distinctive of material substances is their composition of matter and form, complementary principles which, the human soul excepted, cannot exist apart. That material substances are ultimately composed of the distinct principles matter and form is evidenced, mainly, by the fact of their generation and corruption, the study of which falls under the philosophy of nature. From a logical standpoint they are subdivided according to the specific differences "living, non-living," "sentient, nonsentient," "rational, irrational." On a different basis was the ancient division of material substances by corruptible and incorruptible, a differentiation foreign to modern physics. The ground for the distinction was not the composition or noncomposition of matter and form; both admittedly bore this composition. Rather, the difference lay in the fact that corruptible substances (sublunary bodies) were subject to all manner of change, substantial included, whereas incorruptible substances (celestial bodies) were thought to be under no change but that of place.
Immaterial substances, on the other hand, are not composed of matter and form. They are known as pure forms, also spoken of as separate forms; yet this is only by analogy to material forms and, moreover, should not be taken in the Platonist sense of subsistent idea. The metaphysical and psychological (or noetic) investigation of these substances was not to meet with any real success until the advent of Christian philosophy, which could build its speculations in point on the firm foundation provided by the revealed doctrine of the angels. According to St. Thomas these substances, among other things, cannot be numerically multiplied; they lack, as will be seen below, what in Thomistic thought is the principle of such multiplicability, matter. Each angel, it follows, is the sole member of (is in fact) his species; and the totality of angelic species constitues a formal hierarchy, founded on the diversity of essence.