Book 10
1
Since contraries are other in form,It appears that in this chapter (apart from 5, which may be a later addition) the terms εἶδος and γένος are used in a non-technical sense. Cf. Ross on Aristot. Met. 1058b 28. and the perishable and imperishable are contraries (for privation is a definite incapacity), the perishable must be other in kind than the imperishable. But so far we have spoken only of the universal terms; and so it might appear to be unnecessary that anything perishable and imperishable should be other in form, just as in the case of white and black.
2
For the same thing may be both at the same time, if it is a universal (e.g, man may be both white and black); and it may still be both if it is a particular, for the same person may be white and black, although not at the same time. Yet white is contrary to black. But although some contraries
3
(e.g. those which we have just mentioned, and many others) can belong to certain things accidentally, others cannot; and this applies to the perishable and the imperishable. Nothing is accidentally perishable; for that which is accidental may not be applicable; but perishability is an attribute which applies necessarily when it is applicable at all. Otherwise one and the same thing will be imperishable as well as perishable, if it is possible for perishability not to apply to it.
4
Thus perishability must be either the substance or in the substance of every perishable thing. The same argument also applies to the imperishable; for both perishability and imperishability are attributes which are necessarily applicable. Hence the characteristics in respect of which and in direct consequence of which one thing is perishable and another imperishable are opposed; and therefore they must be other in kind.
