The Complete Aristotle (eng.)

If, then, you are
overthrowing a view, it is useful to see whether the genus fails to
be predicated in the category of essence of those things of which
the species too is predicated. If establishing a view, it is useful
to see whether it is predicated in the category of essence: for if
so, the result will be that the genus and the species will be
predicated of the same object in the category of essence, so that
the same object falls under two genera: the genera must therefore
of necessity be subordinate one to the other, and therefore if it
be shown that the one we wish to establish as genus is not
subordinate to the species, clearly the species would be
subordinate to it, so that you may take it as shown that it is the
genus.
    Look, also, at the definitions of the genera, and see whether
they apply both to the given species and to the objects which
partake of the species. For of necessity the definitions of its
genera must be predicated of the species and of the objects which
partake of the species: if, then, there be anywhere a discrepancy,
clearly what has been rendered is not the genus.
    Again, see if he has rendered the differentia as the genus, e.g.
‘immortal’ as the genus of ‘God’. For ‘immortal’ is a differentia
of ‘living being’, seeing that of living beings some are mortal and
others immortal. Clearly, then, a bad mistake has been made; for
the differentia of a thing is never its genus. And that this is
true is clear: for a thing’s differentia never signifies its
essence, but rather some quality, as do ‘walking’ and ‘biped’.
    Also, see whether he has placed the differentia inside the
genus, e.g. by taking ‘odd’ as a number’. For ‘odd’ is a
differentia of number, not a species. Nor is the differentia
generally thought to partake of the genus: for what partakes of the
genus is always either a species or an individual, whereas the
differentia is neither a species nor an individual. Clearly,
therefore, the differentia does not partake of the genus, so that
‘odd’ too is no species but a differentia, seeing that it does not
partake of the genus.
    Moreover, see whether he has placed the genus inside the
species, e.g. by taking ‘contact’ to be a ‘juncture’, or ‘mixture’
a ‘fusion’, or, as in Plato’s definition,’ ‘locomotion’ to be the
same as ‘carriage’. For there is no necessity that contact should
be juncture: rather, conversely, juncture must be contact: for what
is in contact is not always joined, though what is joined is always
in contact. Likewise, also, in the remaining instances: for mixture
is not always a ‘fusion’ (for to mix dry things does not fuse
them), nor is locomotion always ‘carriage’. For walking is not
generally thought to be carriage: for ‘carriage’ is mostly used of
things that change one place for another involuntarily, as happens
in the case of inanimate things. Clearly, also, the species, in the
instances given, has a wider denotation than the genus, whereas it
ought to be vice versa.
    Again, see whether he has placed the differentia inside the
species, by taking (e.g.) ‘immortal’ to be ‘a god’. For the result
will be that the species has an equal or wider denotation: and this
cannot be, for always the differentia has an equal or a wider
denotation than the species. Moreover, see whether he has placed
the genus inside the differentia, by making ‘colour’ (e.g.) to be a
thing that ‘pierces’, or ‘number’ a thing that is ‘odd’. Also, see
if he has mentioned the genus as differentia: for it is possible
for a man to bring forward a statement of this kind as well, e.g.
that ‘mixture’ is the differentia of ‘fusion’, or that change of
place’ is the differentia of ‘carriage’. All such cases should be
examined by means of the same principles: for they depend upon
common rules: for the genus should have a wider denotation that its
differentia, and also should not partake of its differentia;
whereas, if it be rendered in this manner, neither of the aforesaid
requirements can be satisfied: for the genus will both have a
narrower denotation than its differentia, and will partake of
it.
    Again, if no differentia belonging to the genus be predicated of
the given species, neither will the genus be predicated of it; e.g.
of ‘soul’ neither ‘odd’ nor ‘even’ is predicated: neither therefore
is ‘number’. Moreover, see whether the species is naturally prior
and abolishes the genus