Clearly, then, intermediates are (1) all in the
same genus and (2) intermediate between contraries, and (3) all
compounded out of the contraries.
8
That which is other in species is other than something in
something, and this must belong to both; e.g. if it is an animal other
in species, both are animals. The things, then, which are other in
species must be in the same genus. For by genus I mean that one
identical thing which is predicated of both and is differentiated in
no merely accidental way, whether conceived as matter or otherwise.
For not only must the common nature attach to the different things,
e.g. not only must both be animals, but this very animality must
also be different for each (e.g. in the one case equinity, in the
other humanity), and so this common nature is specifically different
for each from what it is for the other. One, then, will be in virtue
of its own nature one sort of animal, and the other another, e.g.
one a horse and the other a man. This difference, then, must be an
otherness of the genus. For I give the name of 'difference in the
genus' an otherness which makes the genus itself other.
This, then, will be a contrariety (as can be shown also by
induction). For all things are divided by opposites, and it has been
proved that contraries are in the same genus. For contrariety was seen
to be complete difference; and all difference in species is a
difference from something in something; so that this is the same for
both and is their genus.
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