The Complete Aristotle (eng.)

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.
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    div id="section37" class="section" title="2">
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 addition that walking does
not partake of any of the species of motion produced by the same
division except locomotion. For of necessity what partakes of the
genus partakes also of one of the species produced by the first
division of the genus. If, therefore, walking does not partake
either of increase or decrease or of the other kinds of motion,
clearly it would partake of locomotion, so that locomotion would be
the genus of walking.
    Again, look among the things of which the given species is
predicated as genus, and see if what is rendered as its genus be
also predicated in the category of essence of the very things of
which the species is so predicated, and likewise if all the genera
higher than this genus are so predicated as well. For if there be
anywhere a discrepancy, clearly what has been rendered is not the
true genus: for had it been the genus, then both the genera higher
than it, and it itself, would all have been predicated in the
category of essence of those objects of which the species too is
predicated in the category of essence.