The Complete Aristotle (eng.)

Because
that which was the essence of a house is present. And why is this
individual thing, or this body having this form, a man? Therefore
what we seek is the cause, i.e. the form, by reason of which the
matter is some definite thing; and this is the substance of the
thing. Evidently, then, in the case of simple terms no inquiry nor
teaching is possible; our attitude towards such things is other
than that of inquiry.
    Since that which is compounded out of something so that the
whole is one, not like a heap but like a syllable-now the syllable
is not its elements, ba is not the same as b and a, nor is flesh
fire and earth (for when these are separated the wholes, i.e. the
flesh and the syllable, no longer exist, but the elements of the
syllable exist, and so do fire and earth); the syllable, then, is
something-not only its elements (the vowel and the consonant) but
also something else, and the flesh is not only fire and earth or
the hot and the cold, but also something else:-if, then, that
something must itself be either an element or composed of elements,
(1) if it is an element the same argument will again apply; for
flesh will consist of this and fire and earth and something still
further, so that the process will go on to infinity. But (2) if it
is a compound, clearly it will be a compound not of one but of more
than one (or else that one will be the thing itself), so that again
in this case we can use the same argument as in the case of flesh
or of the syllable. But it would seem that this ‘other’ is
something, and not an element, and that it is the cause which makes
this thing flesh and that a syllable. And similarly in all other
cases. And this is the substance of each thing (for this is the
primary cause of its being); and since, while some things are not
substances, as many as are substances are formed in accordance with
a nature of their own and by a process of nature, their substance
would seem to be this kind of ‘nature’, which is not an element but
a principle. An element, on the other hand, is that into which a
thing is divided and which is present in it as matter; e.g. a and b
are the elements of the syllable.

Book VIII
    Translated by W. D. Ross
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    div id="section93" class="section" title="1">
1
    We must reckon up the results arising from what has been said,
and compute the sum of them, and put the finishing touch to our
inquiry. We have said that the causes, principles, and elements of
substances are the object of our search. And some substances are
recognized by every one, but some have been advocated by particular
schools. Those generally recognized are the natural substances,
i.e. fire, earth, water, air, &c., the simple bodies; second
plants and their parts, and animals and the parts of animals; and
finally the physical universe and its parts; while some particular
schools say that Forms and the objects of mathematics are
substances. But there are arguments which lead to the conclusion
that there are other substances, the essence and the substratum.
Again, in another way the genus seems more substantial than the
various spccies, and the universal than the particulars. And with
the universal and the genus the Ideas are connected; it is in
virtue of the same argument that they are thought to be substances.
And since the essence is substance, and the definition is a formula
of the essence, for this reason we have discussed definition and
essential predication. Since the definition is a formula, and a
formula has parts, we had to consider also with respect to the
notion of ‘part’, what are parts of the substance and what are not,
and whether the parts of the substance are also parts of the
definition. Further, too, neither the universal nor the genus is a
substance; we must inquire later into the Ideas and the objects of
mathematics; for some say these are substances as well as the
sensible substances.
    But now let us resume the discussion of the generally recognized
substances. These are the sensible substances, and sensible
substances all have matter. The substratum is substance, and this
is in one sense the matter (and by matter I mean that which, not
being a ‘this’ actually, is potentially a ‘this’), and in another
sense the formula or shape (that which being a ‘this’ can be
separately formulated), and thirdly the complex of these two, which
alone is generated and destroyed, and is, without qualification,
capable of separate existence; for of substances