Book 10
3
Therefore this difference must be otherness of genus (I say otherness of genus because by difference of genus I mean an otherness which makes the genus itself other); this, then, will be a form of contrariety. This is obvious by induction.Aristotle does not use induction to prove his point; indeed he does not prove it at all. For all differentiation is by opposites, and we have shownIn ch. 4. that contraries are in the same genus, because contrariety was shown to be complete difference. But difference in species is always difference from something in respect of something; therefore this is the same thing, i.e. the genus, for both.
4
(Hence too all contraries which differ in species but not in genus are in the same line of predication,Or category. and are other than each other in the highest degree; for their difference is complete, and they cannot come into existence simultaneously.) Hence the difference is a form of contrariety.
4
To be other in species, then, means this: to be in the same genus and involve contrariety, while being indivisible
5
(and the same in species applies to all things which do not involve contrariety, while being indivisible); for it is in the course of differentiation and in the intermediate terms that contrariety appears, before we come to the indivisibles.i.e., indivisible species and individuals.
