The Complete Aristotle (eng.)

species is more truly substance
than the genus, being more nearly related to primary substance. For
if any one should render an account of what a primary substance is,
he would render a more instructive account, and one more proper to
the subject, by stating the species than by stating the genus.
Thus, he would give a more instructive account of an individual man
by stating that he was man than by stating that he was animal, for
the former description is peculiar to the individual in a greater
degree, while the latter is too general. Again, the man who gives
an account of the nature of an individual tree will give a more
instructive account by mentioning the species ‘tree’ than by
mentioning the genus ‘plant’.
    Moreover, primary substances are most properly called substances
in virtue of the fact that they are the entities which underlie
every. else, and that everything else is either predicated of them
or present in them. Now the same relation which subsists between
primary substance and everything else subsists also between the
species and the genus: for the species is to the genus as subject
is to predicate, since the genus is predicated of the species,
whereas the species cannot be predicated of the genus. Thus we have
a second ground for asserting that the species is more truly
substance than the genus.
    Of species themselves, except in the case of such as are genera,
no one is more truly substance than another. We should not give a
more appropriate account of the individual man by stating the
species to which he belonged, than we should of an individual horse
by adopting the same method of definition. In the same way, of
primary substances, no one is more truly substance than another; an
individual man is not more truly substance than an individual
ox.
    It is, then, with good reason that of all that remains, when we
exclude primary substances, we concede to species and genera alone
the name ‘secondary substance’, for these alone of all the
predicates convey a knowledge of primary substance. For it is by
stating the species or the genus that we appropriately define any
individual man; and we shall make our definition more exact by
stating the former than by stating the latter. All other things
that we state, such as that he is white, that he runs, and so on,
are irrelevant to the definition. Thus it is just that these alone,
apart from primary substances, should be called substances.
    Further, primary substances are most properly so called, because
they underlie and are the subjects of everything else. Now the same
relation that subsists between primary substance and everything
else subsists also between the species and the genus to which the
primary substance belongs, on the one hand, and every attribute
which is not included within these, on the other. For these are the
subjects of all such. If we call an individual man ‘skilled in
grammar’, the predicate is applicable also to the species and to
the genus to which he belongs. This law holds good in all
cases.
    It is a common characteristic of all sub. stance that it is
never present in a subject. For primary substance is neither
present in a subject nor predicated of a subject; while, with
regard to secondary substances, it is clear from the following
arguments (apart from others) that they are not present in a
subject. For ‘man’ is predicated of the individual man, but is not
present in any subject: for manhood is not present in the
individual man. In the same way, ‘animal’ is also predicated of the
individual man, but is not present in him. Again, when a thing is
present in a subject, though the name may quite well be applied to
that in which it is present, the definition cannot be applied. Yet
of secondary substances, not only the name, but also the
definition, applies to the subject: we should use both the
definition of the species and that of the genus with reference to
the individual man. Thus substance cannot be present in a
subject.
    Yet this is not peculiar to substance, for it is also the case
that differentiae cannot be present in subjects. The
characteristics ‘terrestrial’ and ‘two-footed’ are predicated of
the species ‘man’, but not present in it. For they are not in man.
Moreover, the definition of the differentia may be predicated of
that of which the differentia itself is predicated. For instance,
if the characteristic ‘terrestrial’ is predicated of the species
‘man’, the definition also of that