The Complete Aristotle (eng.)

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substance of that which is one is one, and things whose substance
is numerically one are numerically one, evidently neither unity nor
being can be the substance of things, just as being an element or a
principle cannot be the substance, but we ask what, then, the
principle is, that we may reduce the thing to something more
knowable. Now of these concepts ‘being’ and ‘unity’ are more
substantial than ‘principle’ or ‘element’ or ‘cause’, but not even
the former are substance, since in general nothing that is common
is substance; for substance does not belong to anything but to
itself and to that which has it, of which it is the substance.
Further, that which is one cannot be in many places at the same
time, but that which is common is present in many places at the
same time; so that clearly no universal exists apart from its
individuals.
    But those who say the Forms exist, in one respect are right, in
giving the Forms separate existence, if they are substances; but in
another respect they are not right, because they say the one over
many is a Form. The reason for their doing this is that they cannot
declare what are the substances of this sort, the imperishable
substances which exist apart from the individual and sensible
substances. They make them, then, the same in kind as the
perishable things (for this kind of substance we
know)—’man-himself’ and ‘horse-itself’, adding to the sensible
things the word ‘itself’. Yet even if we had not seen the stars,
none the less, I suppose, would they have been eternal substances
apart from those which we knew; so that now also if we do not know
what non-sensible substances there are, yet it is doubtless
necessary that there should he some.-Clearly, then, no universal
term is the name of a substance, and no substance is composed of
substances.
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17
    Let us state what, i.e. what kind of thing, substance should be
said to be, taking once more another starting-point; for perhaps
from this we shall get a clear view also of that substance which
exists apart from sensible substances. Since, then, substance is a
principle and a cause, let us pursue it from this starting-point.
The ‘why’ is always sought in this form—’why does one thing attach
to some other?’ For to inquire why the musical man is a musical
man, is either to inquire—as we have said why the man is musical,
or it is something else. Now ‘why a thing is itself’ is a
meaningless inquiry (for (to give meaning to the question ‘why’)
the fact or the existence of the thing must already be evident-e.g.
that the moon is eclipsed-but the fact that a thing is itself is
the single reason and the single cause to be given in answer to all
such questions as why the man is man, or the musician musical’,
unless one were to answer ‘because each thing is inseparable from
itself, and its being one just meant this’; this, however, is
common to all things and is a short and easy way with the
question). But we can inquire why man is an animal of such and such
a nature. This, then, is plain, that we are not inquiring why he
who is a man is a man. We are inquiring, then, why something is
predicable of something (that it is predicable must be clear; for
if not, the inquiry is an inquiry into nothing). E.g. why does it
thunder? This is the same as ‘why is sound produced in the clouds?’
Thus the inquiry is about the predication of one thing of another.
And why are these things, i.e. bricks and stones, a house? Plainly
we are seeking the cause. And this is the essence (to speak
abstractly), which in some cases is the end, e.g. perhaps in the
case of a house or a bed, and in some cases is the first mover; for
this also is a cause. But while the efficient cause is sought in
the case of genesis and destruction, the final cause is sought in
the case of being also.
    The object of the inquiry is most easily overlooked where one
term is not expressly predicated of another (e.g. when we inquire
‘what man is’), because we do not distinguish and do not say
definitely that certain elements make up a certain whole. But we
must articulate our meaning before we begin to inquire; if not, the
inquiry is on the border-line between being a search for something
and a search for nothing. Since we must have the existence of the
thing as something given, clearly the question is why the matter is
some definite thing; e.g. why are these materials a house?