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'.
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