The Complete Aristotle (eng.)

comparatively large: thus (e.g.) ‘Being’
and ‘Unity’ are among the number of attributes that follow
everything. If, therefore, he has rendered ‘Being’ as a genus,
clearly it would be the genus of everything, seeing that it is
predicated of everything; for the genus is never predicated of
anything except of its species. Hence Unity, inter alia, will be a
species of Being. The result, therefore, is that of all things of
which the genus is predicated, the species is predicated as well,
seeing that Being and Unity are predicates of absolutely
everything, whereas the predication of the species ought to be of
narrower range. If, on the other hand, he has named as differentia
some attribute that follows everything, clearly the denotation of
the differentia will be equal to, or wider than, that of the genus.
For if the genus, too, be some attribute that follows everything,
the denotation of the differentia will be equal to its denotation,
while if the genus do not follow everything, it will be still
wider.
    Moreover, see if the description ‘inherent in S’ be used of the
genus rendered in relation to its species, as it is used of ‘white’
in the case of snow, thus showing clearly that it could not be the
genus: for ‘true of S’ is the only description used of the genus in
relation to its species. Look and see also if the genus fails to be
synonymous with its species. For the genus is always predicated of
its species synonymously.
    Moreover, beware, whenever both species and genus have a
contrary, and he places the better of the contraries inside the
worse genus: for the result will be that the remaining species will
be found in the remaining genus, seeing that contraries are found
in contrary genera, so that the better species will be found in the
worse genus and the worse in the better: whereas the usual view is
that of the better species the genus too is better. Also see if he
has placed the species inside the worse and not inside the better
genus, when it is at the same time related in like manner to both,
as (e.g.) if he has defined the ‘soul’ as a ‘form of motion’ or ‘a
form of moving thing’. For the same soul is usually thought to be a
principle alike of rest and of motion, so that, if rest is the
better of the two, this is the genus into which the soul should
have been put.
    Moreover, judge by means of greater and less degrees: if
overthrowing a view, see whether the genus admits of a greater
degree, whereas neither the species itself does so, nor any term
that is called after it: e.g. if virtue admits of a greater degree,
so too does justice and the just man: for one man is called ‘more
just than another’. If, therefore, the genus rendered admits of a
greater degree, whereas neither the species does so itself nor yet
any term called after it, then what has been rendered could not be
the genus.
    Again, if what is more generally, or as generally, thought to be
the genus be not so, clearly neither is the genus rendered. The
commonplace rule in question is useful especially in cases where
the species appears to have several predicates in the category of
essence, and where no distinction has been drawn between them, and
we cannot say which of them is genus; e.g. both ‘pain’ and the
‘conception of a slight’ are usually thought to be predicates of
‘anger in the category of essence: for the angry man is both in
pain and also conceives that he is slighted. The same mode of
inquiry may be applied also to the case of the species, by
comparing it with some other species: for if the one which is more
generally, or as generally, thought to be found in the genus
rendered be not found therein, then clearly neither could the
species rendered be found therein.
    In demolishing a view, therefore, you should follow the rule as
stated. In establishing one, on the other hand, the commonplace
rule that you should see if both the genus rendered and the species
admit of a greater degree will not serve: for even though both
admit it, it is still possible for one not to be the genus of the
other. For both ‘beautiful’ and ‘white’ admit of a greater degree,
and neither is the genus of the other. On the other hand, the
comparison of the genera and of the species one with another is of
use: e.g. supposing A and B to have a like claim to be genus, then
if one be a genus, so also is the other. Likewise, also, if what
has less claim be a genus, so also is what has more claim: e.g.