And
similarly in practical science the movement is not in the thing
done, but rather in the doers. But the science of the natural
philosopher deals with the things that have in themselves a
principle of movement. It is clear from these facts, then, that
natural science must be neither practical nor productive, but
theoretical (for it must fall into some one of these classes). And
since each of the sciences must somehow know the 'what' and use this
as a principle, we must not fall to observe how the natural
philosopher should define things and how he should state the
definition of the essence-whether as akin to 'snub' or rather to
'concave'. For of these the definition of 'snub' includes the matter
of the thing, but that of 'concave' is independent of the matter;
for snubness is found in a nose, so that we look for its definition
without eliminating the nose, for what is snub is a concave nose.
Evidently then the definition of flesh also and of the eye and of
the other parts must always be stated without eliminating the matter.
Since there is a science of being qua being and capable of
existing apart, we must consider whether this is to be regarded as the
same as physics or rather as different. Physics deals with the
things that have a principle of movement in themselves; mathematics is
theoretical, and is a science that deals with things that are at rest,
but its subjects cannot exist apart. Therefore about that which can
exist apart and is unmovable there is a science different from both of
these, if there is a substance of this nature (I mean separable and
unmovable), as we shall try to prove there is.
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