The Complete Aristotle (eng.)

differentia belongs only by
accident to the object defined. For the differentia is never an
accidental attribute, any more than the genus is: for the
differentia of a thing cannot both belong and not belong to it.
    Moreover, if either the differentia or the species, or any of
the things which are under the species, is predicable of the genus,
then he could not have defined the term. For none of the aforesaid
can possibly be predicated of the genus, seeing that the genus is
the term with the widest range of all. Again, see if the genus be
predicated of the differentia; for the general view is that the
genus is predicated, not of the differentia, but of the objects of
which the differentia is predicated. Animal (e.g.) is predicated of
‘man’ or ‘ox’ or other walking animals, not of the actual
differentia itself which we predicate of the species. For if
‘animal’ is to be predicated of each of its differentiae, then
‘animal’ would be predicated of the species several times over; for
the differentiae are predicates of the species. Moreover, the
differentiae will be all either species or individuals, if they are
animals; for every animal is either a species or an individual.
    Likewise you must inquire also if the species or any of the
objects that come under it is predicated of the differentia: for
this is impossible, seeing that the differentia is a term with a
wider range than the various species. Moreover, if any of the
species be predicated of it, the result will be that the
differentia is a species: if, for instance, ‘man’ be predicated,
the differentia is clearly the human race. Again, see if the
differentia fails to be prior to the species: for the differentia
ought to be posterior to the genus, but prior to the species.
    Look and see also if the differentia mentioned belongs to a
different genus, neither contained in nor containing the genus in
question. For the general view is that the same differentia cannot
be used of two non-subaltern genera. Else the result will be that
the same species as well will be in two non-subaltern genera: for
each of the differentiae imports its own genus, e.g. ‘walking’ and
‘biped’ import with them the genus ‘animal’. If, then, each of the
genera as well is true of that of which the differentia is true, it
clearly follows that the species must be in two non-subaltern
genera. Or perhaps it is not impossible for the same differentia to
be used of two non-subaltern genera, and we ought to add the words
‘except they both be subordinate members of the same genus’. Thus
‘walking animal’ and ‘flying animal’ are non-subaltern genera, and
‘biped’ is the differentia of both. The words ‘except they both be
subordinate members of the same genus’ ought therefore to be added;
for both these are subordinate to ‘animal’. From this possibility,
that the same differentia may be used of two non-subaltern genera,
it is clear also that there is no necessity for the differentia to
carry with it the whole of the genus to which it belongs, but only
the one or the other of its limbs together with the genera that are
higher than this, as ‘biped’ carries with it either ‘flying’ or
‘walking animal’.
    See, too, if he has rendered ‘existence in’ something as the
differentia of a thing’s essence: for the general view is that
locality cannot differentiate between one essence and another.
Hence, too, people condemn those who divide animals by means of the
terms ‘walking’ and ‘aquatic’, on the ground that ‘walking’ and
‘aquatic’ indicate mere locality. Or possibly in this case the
censure is undeserved; for ‘aquatic’ does not mean ‘in’ anything;
nor does it denote a locality, but a certain quality: for even if
the thing be on the dry land, still it is aquatic: and likewise a
land-animal, even though it be in the water, will still be a and
not an aquatic-animal. But all the same, if ever the differentia
does denote existence in something, clearly he will have made a bad
mistake.
    Again, see if he has rendered an affection as the differentia:
for every affection, if intensified, subverts the essence of the
thing, while the differentia is not of that kind: for the
differentia is generally considered rather to preserve that which
it differentiates; and it is absolutely impossible for a thing to
exist without its own special differentia: for if there be no
‘walking’, there will be no ‘man’. In fact, we may lay