No one of these terms, in and by itself, involves an affirmation; it
is by the combination of such terms that positive or negative
statements arise. For every assertion must, as is admitted, be
either true or false, whereas expressions which are not in any way
composite such as 'man', 'white', 'runs', 'wins', cannot be either
true or false.

                                 5

  Substance, in the truest and primary and most definite sense of
the word, is that which is neither predicable of a subject nor present
in a subject; for instance, the individual man or horse. But in a
secondary sense those things are called substances within which, as
species, the primary substances are included; also those which, as
genera, include the species. For instance, the individual man is
included in the species 'man', and the genus to which the species
belongs is 'animal'; these, therefore-that is to say, the species
'man' and the genus 'animal,-are termed secondary substances.

  It is plain from what has been said that both the name and the
definition of the predicate must be predicable of the subject. For
instance, 'man' is predicted of the individual man. Now in this case
the name of the species man' is applied to the individual, for we
use the term 'man' in describing the individual; and the definition of
'man' will also be predicated of the individual man, for the
individual man is both man and animal. Thus, both the name and the
definition of the species are predicable of the individual.

  With regard, on the other hand, to those things which are present in
a subject, it is generally the case that neither their name nor
their definition is predicable of that in which they are present.
Though, however, the definition is never predicable, there is
nothing in certain cases to prevent the name being used. For instance,
'white' being present in a body is predicated of that in which it is
present, for a body is called white: the definition, however, of the
colour white' is never predicable of the body.

  Everything except primary substances is either predicable of a
primary substance or present in a primary substance. This becomes
evident by reference to particular instances which occur. 'Animal'
is predicated of the species 'man', therefore of the individual man,
for if there were no individual man of whom it could be predicated, it
could not be predicated of the species 'man' at all. Again, colour
is present in body, therefore in 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.

  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