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beginning, so that either both expressions are identical or at any
rate neither is the genus of the other. The elementary principle in
regard to all such cases is that the genus has a wider denotation than
the species and its differentia: for the differentia as well has a
narrower denotation than the genus.
See also whether the genus mentioned fails, or might be generally
thought to fail, to apply to some object which is not specifically
different from the thing in question; or, if your argument be
constructive, whether it does so apply. For all things that are not
specifically different have the same genus. If, therefore, it be shown
to apply to one, then clearly it applies to all, and if it fails to
apply to one, clearly it fails to apply to any; e.g. if any one who
assumes 'indivisible lines' were to say that the 'indivisible' is
their genus. For the aforesaid term is not the genus of divisible
lines, and these do not differ as regards their species from
indivisible: for straight lines are never different from each other as
regards their species.
Part 2
Look and see, also, if there be any other genus of the given species
which neither embraces the genus rendered nor yet falls under it, e.g.
suppose any one were to lay down that 'knowledge' is the genus of
justice. For virtue is its genus as well, and neither of these genera
embraces the remaining one, so that knowledge could not be the genus
of justice: for it is generally accepted that whenever one species
falls under two genera, the one is embraced by the other. Yet a
principle of this kind gives rise to a difficulty in some cases. For
some people hold that prudence is both virtue and knowledge, and that
neither of its genera is embraced by the other: although certainly not
everybody admits that prudence is knowledge. If, however, any one were
to admit the truth of this assertion, yet it would still be generally
agreed to be necessary that the genera of the same object must at any
rate be subordinate either the one to the other or both to the same,
as actually is the case with virtue and knowledge. For both fall under
the same genus; for each of them is a state and a disposition. You
should look, therefore, and see whether neither of these things is
true of the genus rendered; for if the genera be subordinate neither
the one to the other nor both to the same, then what is rendered could
not be the true genus.
Look, also, at the genus of the genus rendered, and so continually at
the next higher genus, and see whether all are predicated of the
species, and predicated in the category of essence: for all the higher
genera should be predicated of the species in the category of essence.
If, then, there be anywhere a discrepancy, clearly what is rendered is
not the true genus. [Again, see whether either the genus itself, or
one of its higher genera, partakes of the species: for the higher
genus does not partake of any of the lower.] If, then, you are
overthrowing a view, follow the rule as given: if establishing one,
then-suppose that what has been named as genus be admitted to belong
to the species, only it be disputed whether it belongs as genus-it is
enough to show that one of its higher genera is predicated of the
species in the category of essence. For if one of them be predicated
in the category of essence, all of them, both higher and lower than
this one, if predicated at all of the species, will be predicated of
it in the category of essence: so that what has been rendered as genus
is also predicated in the category of essence. The premiss that when
one genus is predicated in the category of essence, all the rest, if
predicated at all, will be predicated in the category of essence,
should be secured by induction. Supposing, however, that it be
disputed whether what has been rendered as genus belongs at all, it is
not enough to show that one of the higher genera is predicated of the
species in the category of essence: e.g. if any one has rendered
'locomotion' as the genus of walking, it is not enough to show that
walking is 'motion' in order to show that it is 'locomotion', seeing
that there are other forms of motion as well; but one must show in