The Complete Aristotle (eng.)

‘what’ in some class
of things and tries to prove the other truths, with more or less
precision. Some get the ‘what’ through perception, others by
hypothesis; so that it is clear from an induction of this sort that
there is no demonstration. of the substance or ‘what’.
    There is a science of nature, and evidently it must be different
both from practical and from productive science. For in the case of
productive science the principle of movement is in the producer and
not in the product, and is either an art or some other faculty. 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. And if
there is such a kind of thing in the world, here must surely be the
divine, and this must be the first and most dominant principle.
Evidently, then, there are three kinds of theoretical
sciences-physics, mathematics, theology. The class of theoretical
sciences is the best, and of these themselves the last named is
best; for it deals with the highest of existing things, and each
science is called better or worse in virtue of its proper
object.
    One might raise the question whether the science of being qua
being is to be regarded as universal or not. Each of the
mathematical sciences deals with some one determinate class of
things, but universal mathematics applies alike to all. Now if
natural substances are the first of existing things, physics must
be the first of sciences; but if there is another entity and
substance, separable and unmovable, the knowledge of it must be
different and prior to physics and universal because it is
prior.
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8
    Since ‘being’ in general has several senses, of which one is
‘being by accident’, we must consider first that which ‘is’ in this
sense. Evidently none of the traditional sciences busies itself
about the accidental. For neither does architecture consider what
will happen to those who are to use the house (e.g. whether they
have a painful life in it or not), nor does weaving, or shoemaking,
or the confectioner’s art, do the like; but each of these sciences
considers only what is peculiar to it, i.e. its proper end. And as
for the argument that ‘when he who is musical becomes lettered
he’ll be both at once, not having been both before; and that which
is, not always having been, must have come to be; therefore he must
have at once become musical and lettered’,-this none of the
recognized sciences considers, but only sophistic; for this alone
busies itself about the accidental, so that Plato is not far wrong
when he says that the sophist spends his time on non-being.
    That a science of the accidental is not even possible will be
evident if we try to see what the accidental really is. We say that
everything either is always and of necessity (necessity not in the
sense of violence, but that which we appeal to in demonstrations),
or is for the most part, or