individual bodies, for if there were no individual body in which
it was present, it could not be present in body at all. Thus
everything except primary substances is either predicated of
primary substances, or is present in them, and if these last did
not exist, it would be impossible for anything else to exist.

Of secondary substances, the 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.