The Complete Aristotle (eng.)

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.
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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. (Hence also all contraries which
are different in species and not in genus are in the same line of
predication, and other than one another in the highest degree-for
the difference is complete-, and cannot be present along with one
another.) The difference, then, is a contrariety.
    This, then, is what it is to be ‘other in species’-to have a
contrariety, being in the same genus and being indivisible (and
those things are the same in species which have no contrariety,
being indivisible); we say ‘being indivisible’, for in the process
of division contrarieties arise in the intermediate stages before
we come to the indivisibles. Evidently, therefore, with reference
to that which is called the genus, none of the species-of-a-genus
is either the same as it or other than it in species (and this is
fitting; for the matter is indicated by negation, and the genus is
the matter of that of which it is called the genus, not in the
sense in which we speak of the genus or family of the Heraclidae,
but in that in which