The Complete Aristotle (eng.)

ought
themselves also to be relatives, as is the case with ‘double’: for
multiple’, which is the genus of ‘double’, is itself also a
relative. To speak generally, the genus ought to fall under the
same division as the species: for if the species be a substance, so
too should be the genus, and if the species be a quality, so too
the genus should be a quality; e.g. if white be a quality, so too
should colour be. Likewise, also, in other cases.
    Again, see whether it be necessary or possible for the genus to
partake of the object which has been placed in the genus. ‘To
partake’ is defined as ‘to admit the definition of that which is
partaken. Clearly, therefore, the species partake of the genera,
but not the genera of the species: for the species admits the
definition of the genus, whereas the genus does not admit that of
the species. You must look, therefore, and see whether the genus
rendered partakes or can possibly partake of the species, e.g. if
any one were to render anything as genus of ‘being’ or of ‘unity’:
for then the result will be that the genus partakes of the species:
for of everything that is, ‘being’ and ‘unity’ are predicated, and
therefore their definition as well.
    Moreover, see if there be anything of which the species rendered
is true, while the genus is not so, e.g. supposing ‘being’ or
‘object of knowledge’ were stated to be the genus of ‘object of
opinion’. For ‘object of opinion’ will be a predicate of what does
not exist; for many things which do not exist are objects of
opinion; whereas that ‘being’ or ‘object of knowledge’ is not
predicated of what does not exist is clear. So that neither ‘being’
nor ‘object of knowledge’ is the genus of ‘object of opinion’: for
of the objects of which the species is predicated, the genus ought
to be predicated as well.
    Again, see whether the object placed in the genus be quite
unable to partake of any of its species: for it is impossible that
it should partake of the genus if it do not partake of any of its
species, except it be one of the species reached by the first
division: these do partake of the genus alone. If, therefore,
‘Motion’ be stated as the genus of pleasure, you should look and
see if pleasure be neither locomotion nor alteration, nor any of
the rest of the given modes of motion: for clearly you may then
take it that it does not partake of any of the species, and
therefore not of the genus either, since what partakes of the genus
must necessarily partake of one of the species as well: so that
pleasure could not be a species of Motion, nor yet be one of the
individual phenomena comprised under the term ‘motion’. For
individuals as well partake in the genus and the species, as (e.g.)
an individual man partakes of both ‘man’ and ‘animal’.
    Moreover, see if the term placed in the genus has a wider
denotation than the genus, as (e.g.) ‘object of opinion’ has, as
compared with ‘being’: for both what is and what is not are objects
of opinion, so that ‘object of opinion’ could not be a species of
being: for the genus is always of wider denotation than the
species. Again, see if the species and its genus have an equal
denotation; suppose, for instance, that of the attributes which go
with everything, one were to be stated as a species and the other
as its genus, as for example Being and Unity: for everything has
being and unity, so that neither is the genus of the other, since
their denotation is equal. Likewise, also, if the ‘first’ of a
series and the ‘beginning’ were to be placed one under the other:
for the beginning is first and the first is the 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’