The Complete Aristotle (eng.)

it is, so that quality also is
a ‘what a thing is’,-not in the simple sense, however, but just as,
in the case of that which is not, some say, emphasizing the
linguistic form, that that is which is not is-not is simply, but is
non-existent; so too with quality.
    We must no doubt inquire how we should express ourselves on each
point, but certainly not more than how the facts actually stand.
And so now also, since it is evident what language we use, essence
will belong, just as ‘what a thing is’ does, primarily and in the
simple sense to substance, and in a secondary way to the other
categories also,-not essence in the simple sense, but the essence
of a quality or of a quantity. For it must be either by an
equivocation that we say these are, or by adding to and taking from
the meaning of ‘are’ (in the way in which that which is not known
may be said to be known),-the truth being that we use the word
neither ambiguously nor in the same sense, but just as we apply the
word ‘medical’ by virtue of a reference to one and the same thing,
not meaning one and the same thing, nor yet speaking ambiguously;
for a patient and an operation and an instrument are called medical
neither by an ambiguity nor with a single meaning, but with
reference to a common end. But it does not matter at all in which
of the two ways one likes to describe the facts; this is evident,
that definition and essence in the primary and simple sense belong
to substances. Still they belong to other things as well, only not
in the primary sense. For if we suppose this it does not follow
that there is a definition of every word which means the same as
any formula; it must mean the same as a particular kind of formula;
and this condition is satisfied if it is a formula of something
which is one, not by continuity like the Iliad or the things that
are one by being bound together, but in one of the main senses of
‘one’, which answer to the senses of ‘is’; now ‘that which is’ in
one sense denotes a ‘this’, in another a quantity, in another a
quality. And so there can be a formula or definition even of white
man, but not in the sense in which there is a definition either of
white or of a substance.
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5
    It is a difficult question, if one denies that a formula with an
added determinant is a definition, whether any of the terms that
are not simple but coupled will be definable. For we must explain
them by adding a determinant. E.g. there is the nose, and
concavity, and snubness, which is compounded out of the two by the
presence of the one in the other, and it is not by accident that
the nose has the attribute either of concavity or of snubness, but
in virtue of its nature; nor do they attach to it as whiteness does
to Callias, or to man (because Callias, who happens to be a man, is
white), but as ‘male’ attaches to animal and ‘equal’ to quantity,
and as all so-called ‘attributes propter se’ attach to their
subjects. And such attributes are those in which is involved either
the formula or the name of the subject of the particular attribute,
and which cannot be explained without this; e.g. white can be
explained apart from man, but not female apart from animal.
Therefore there is either no essence and definition of any of these
things, or if there is, it is in another sense, as we have
said.
    But there is also a second difficulty about them. For if snub
nose and concave nose are the same thing, snub and concave will be
the thing; but if snub and concave are not the same (because it is
impossible to speak of snubness apart from the thing of which it is
an attribute propter se, for snubness is concavity-in-a-nose),
either it is impossible to say ‘snub nose’ or the same thing will
have been said twice, concave-nose nose; for snub nose will be
concave-nose nose. And so it is absurd that such things should have
an essence; if they have, there will be an infinite regress; for in
snub-nose nose yet another ‘nose’ will be involved.
    Clearly, then, only substance is definable. For if the other
categories also are definable, it must be by addition of a
determinant, e.g. the qualitative is defined thus, and so is the
odd, for it cannot be defined apart from number; nor can female be
defined apart from animal. (When I say ‘by addition’ I mean the
expressions in which it turns out that we are saying the same thing
twice, as in these instances.) And if this is true, coupled