g. 'endowed with feet' is a differentia of
'animal'; again the differentia of 'animal endowed with feet' must
be of it qua endowed with feet. Therefore we must not say, if we are
to speak rightly, that of that which is endowed with feet one part has
feathers and one is featherless (if we do this we do it through
incapacity); we must divide it only into cloven-footed and not cloven;
for these are differentiae in the foot; cloven-footedness is a form of
footedness. And the process wants always to go on so till it reaches
the species that contain no differences. And then there will be as
many kinds of foot as there are differentiae, and the kinds of animals
endowed with feet will be equal in number to the differentiae. If then
this is so, clearly the last differentia will be the substance of
the thing and its definition, since it is not right to state the
same things more than once in our definitions; for it is
superfluous. And this does happen; for when we say 'animal endowed
with feet and two-footed' we have said nothing other than 'animal
having feet, having two feet'; and if we divide this by the proper
division, we shall be saying the same thing more than once-as many
times as there are differentiae.
If then a differentia of a differentia be taken at each step,
one differentia-the last-will be the form and the substance; but if we
divide according to accidental qualities, e.g. if we were to divide
that which is endowed with feet into the white and the black, there
will be as many differentiae as there are cuts.
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