Book 10
3
But change from one genus into another is impossible except accidentally; e.g., from color to shape. Therefore intermediates must be in the same genus as one another and as the things between which they are intermediate.
3
But all intermediates are between certain opposites, for it is only from these per se that change is possible.
4
Hence there can be no intermediate between things which are not opposites; for then there would be change also between things which are not opposites. Of things which are opposites, contradiction has no intermediate term (for contradiction means this: an antithesis one term of which must apply to any given thing, and which contains no intermediate term); of the remaining types of opposites some are relative, others privative, and others contrary.
5
Those relative opposites which are not contrary have no intermediate. The reason for this is that they are not in the same genus— for what is intermediate between knowledge and the knowable?—but between great and small there is an intermediate. Now since intermediates are in the same genus, as has been shown, and are between contraries, they must be composed of those contraries. For the contraries must either belong to a genus or not. And if there is a genus in such a way
6
that it is something prior to the contraries, then the differentiae which constitute the contrary species (for species consist of genus and differentiae) will be contraries in a prior sense.
7
E.g., if white and black are contraries, and the one is a penetrativeThis is Plato’s definition. Cf. Plat. Tim. 67d, e. and the other a compressive color, these differentiae, penetrative and compressive, are prior, and so are opposed to each other in a prior sense.
