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