Book 9
5
For the rational account includes both, though not in the same way; and it is in the soul, which contains a principle of motion, and will therefore, by means of the same principle, set both processes in motion, by linking them with the same rational account. Hence things which have a rational potency produce results contrary to those of things whose potency is irrationalThe meaning of this awkward sentence is clearly shown in the latter part of 4.; for the results of the former are included under one principle, the rational account.
6
It is evident also that whereas the power of merely producing (or suffering) a given effect is implied in the power of producing that effect well , the contrary is not always true; for that which produces an effect well must also produce it, but that which merely produces a given effect does not necessarily produce it well.
1
There are some, e.g. the Megaric school,Founded by Euclides of Megara, an enthusiastic admirer of Socrates. The Megarics adopted the Eleatic system and developed it along dialectical lines. who say that a thing only has potency when it functions, and that when it is not functioning it has no potency. E.g., they say that a man who is not building cannot build, but only the man who is building, and at the moment when he is building; and similarly in the other cases.
2
It is not difficult to see the absurd consequences of this theory. Obviously a man will not be a builder unless he is building, because to be a builder is to be capable of building; and the same will be true of the other arts.
3
If, therefore, it is impossible to possess these arts without learning them at some time and having grasped them, and impossible not to possess them without having lost them at some time (through forgetfulness or some affection or the lapse of time; not, of course, through the destruction of the object of the art,i.e. the form of house. because it exists always), when the artist ceases to practice his art, he will not possess it;
