On The Soul
the most general possible definition of it.
We are in the habit of recognizing, as one determinate kind of
what is, substance, and that in several senses, (a) in the sense of
matter or that which in itself is not 'a this', and (b) in the sense
of form or essence, which is that precisely in virtue of which a thing
is called 'a this', and thirdly (c) in the sense of that which is
compounded of both (a) and (b). Now matter is potentiality, form
actuality; of the latter there are two grades related to one another
as e.g. knowledge to the exercise of knowledge.
Among substances are by general consent reckoned bodies and
especially natural bodies; for they are the principles of all other
bodies. Of natural bodies some have life in them, others not; by
life we mean self-nutrition and growth (with its correlative decay).
It follows that every natural body which has life in it is a substance
in the sense of a composite.
But since it is also a body of such and such a kind, viz. having
life, the body cannot be soul; the body is the subject or matter,
not what is attributed to it. Hence the soul must be a substance in
the sense of the form of a natural body having life potentially within
it. But substance is actuality, and thus soul is the actuality of a
body as above characterized. Now the word actuality has two senses
corresponding respectively to the possession of knowledge and the
actual exercise of knowledge. It is obvious that the soul is actuality
in the first sense, viz. that of knowledge as possessed, for both
sleeping and waking presuppose the existence of soul, and of these
waking corresponds to actual knowing, sleeping to knowledge
possessed but not employed, and, in the history of the individual,
knowledge comes before its employment or exercise.
That is why the soul is the first grade of actuality of a natural
body having life potentially in it. The body so described is a body
which is organized. The parts of plants in spite of their extreme
simplicity are 'organs'; e.g. the leaf serves to shelter the pericarp,