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For either there will be a genus including the contraries or there
will be none. And if (a) there is to be a genus in such a way that
it is something prior to the contraries, the differentiae which
constituted the contrary species-of-a-genus will be contraries prior
to the species; for species are composed of the genus and the
differentiae. (E.g. if white and black are contraries, and one is a
piercing colour and the other a compressing colour, these
differentiae-'piercing' and 'compressing'-are prior; so that these are
prior contraries of one another.) But, again, the species which differ
contrariwise are the more truly contrary species. And the
other.species, i.e. the intermediates, must be composed of their genus
and their differentiae. (E.g. all colours which are between white
and black must be said to be composed of the genus, i.e. colour, and
certain differentiae. But these differentiae will not be the primary
contraries; otherwise every colour would be either white or black.
They are different, then, from the primary contraries; and therefore
they will be between the primary contraries; the primary
differentiae are 'piercing' and 'compressing'.)
Therefore it is (b) with regard to these contraries which do not
fall within a genus that we must first ask of what their intermediates
are composed. (For things which are in the same genus must be composed
of terms in which the genus is not an element, or else be themselves
incomposite.) Now contraries do not involve one another in their
composition, and are therefore first principles; but the intermediates
are either all incomposite, or none of them. But there is something
compounded out of the contraries, so that there can be a change from a
contrary to it sooner than to the other contrary; for it will have
less of the quality in question than the one contrary and more than
the other. This also, then, will come between the contraries. All
the other intermediates also, therefore, are composite; for that which
has more of a quality than one thing and less than another is
compounded somehow out of the things than which it is said to have
more and less respectively of the quality. And since there are no
other things prior to the contraries and homogeneous with the
intermediates, all intermediates must be compounded out of the
contraries. Therefore also all the inferior classes, both the
contraries and their intermediates, will be compounded out of the
primary contraries. Clearly, then, intermediates are (1) all in the
same genus and (2) intermediate between contraries, and (3) all
compounded out of the contraries.

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. (Hence also all contraries which are
different in species and not in genus are in the same line of

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