The Complete Aristotle (eng.)

terms
also, like ‘odd number’, will not be definable (but this escapes
our notice because our formulae are not accurate.). But if these
also are definable, either it is in some other way or, as we
definition and essence must be said to have more than one sense.
Therefore in one sense nothing will have a definition and nothing
will have an essence, except substances, but in another sense other
things will have them. Clearly, then, definition is the formula of
the essence, and essence belongs to substances either alone or
chiefly and primarily and in the unqualified sense.
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6
    We must inquire whether each thing and its essence are the same
or different. This is of some use for the inquiry concerning
substance; for each thing is thought to be not different from its
substance, and the essence is said to be the substance of each
thing.
    Now in the case of accidental unities the two would be generally
thought to be different, e.g. white man would be thought to be
different from the essence of white man. For if they are the same,
the essence of man and that of white man are also the same; for a
man and a white man are the same thing, as people say, so that the
essence of white man and that of man would be also the same. But
perhaps it does not follow that the essence of accidental unities
should be the same as that of the simple terms. For the extreme
terms are not in the same way identical with the middle term. But
perhaps this might be thought to follow, that the extreme terms,
the accidents, should turn out to be the same, e.g. the essence of
white and that of musical; but this is not actually thought to be
the case.
    But in the case of so-called self-subsistent things, is a thing
necessarily the same as its essence? E.g. if there are some
substances which have no other substances nor entities prior to
them-substances such as some assert the Ideas to be?-If the essence
of good is to be different from good-itself, and the essence of
animal from animal-itself, and the essence of being from
being-itself, there will, firstly, be other substances and entities
and Ideas besides those which are asserted, and, secondly, these
others will be prior substances, if essence is substance. And if
the posterior substances and the prior are severed from each other,
(a) there will be no knowledge of the former, and (b) the latter
will have no being. (By ‘severed’ I mean, if the good-itself has
not the essence of good, and the latter has not the property of
being good.) For (a) there is knowledge of each thing only when we
know its essence. And (b) the case is the same for other things as
for the good; so that if the essence of good is not good, neither
is the essence of reality real, nor the essence of unity one. And
all essences alike exist or none of them does; so that if the
essence of reality is not real, neither is any of the others.
Again, that to which the essence of good does not belong is not
good.-The good, then, must be one with the essence of good, and the
beautiful with the essence of beauty, and so with all things which
do not depend on something else but are self-subsistent and
primary. For it is enough if they are this, even if they are not
Forms; or rather, perhaps, even if they are Forms. (At the same
time it is clear that if there are Ideas such as some people say
there are, it will not be substratum that is substance; for these
must be substances, but not predicable of a substratum; for if they
were they would exist only by being participated in.)
    Each thing itself, then, and its essence are one and the same in
no merely accidental way, as is evident both from the preceding
arguments and because to know each thing, at least, is just to know
its essence, so that even by the exhibition of instances it becomes
clear that both must be one.
    (But of an accidental term, e.g.’the musical’ or ‘the white’,
since it has two meanings, it is not true to say that it itself is
identical with its essence; for both that to which the accidental
quality belongs, and the accidental quality, are white, so that in
a sense the accident and its essence are the same, and in a sense
they are not; for the essence of white is not the same as the man
or the white man, but it is the same as the attribute white.)
    The absurdity of the separation would appear also if one were to
assign a name to each of the essences; for there would be yet
another essence besides the original one, e.g.