There are some, e.g. the Megaric school, Footnote 1Founded 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.
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.
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, Footnote 2i.e. the form of house.
because it exists always), when the artist ceases to practice his art, he will not possess it;
and if he immediately starts building again, how will he have re-acquired the art?
The same is true of inanimate things. Neither the cold nor the hot nor the sweet nor in general any sensible thing will exist unless we are perceiving it (and so the result will be that they are affirming Protagoras’ theory Footnote 3Cf. IV. v., vi. ). Indeed, nothing will have the faculty of sensation unless it is perceiving, i.e. actually employing the faculty.
If, then, that is blind which has not sight, though it would naturally have it, and when it would naturally have it, and while it still exists, the same people will be blind many times a day; and deaf too.
Further, if that which is deprived of its potency is incapable, that which is not happening will be incapable of happening; and he who says that that which is incapable of happening is or will be, will be in error, for this is what incapable
meant. Footnote 4i.e., we have just said that that which is incapable is deprived of its potency—in this case, of its potency for happening.
Thus these theories do away with both motion and generation; for that which is standing will always stand, and that which is sitting will always sit; because if it is sitting it will not get up, since it is impossible that anything which is incapable of getting up should get up.
Since, then, we cannot maintain this, obviously potentiality and actuality are different. But these theories make potentiality and actuality identical; hence it is no small thing that they are trying to abolish.
Thus it is possible that a thing may be capable of being and yet not be, and capable of not being and yet be; and similarly in the other categories that which is capable of walking may not walk, and that which is capable of not walking may walk.
A thing is capable of doing something if there is nothing impossible in its having the actuality of that of which it is said to have the potentiality. I mean, e.g., that if a thing is capable of sitting and is not prevented from sitting, there is nothing impossible in its actually sitting; and similarly if it is capable of being moved or moving or standing or making to stand or being or becoming or not being or not becoming.
The term actuality,
with its implication of complete reality,
has been extended from motions, to which it properly belongs, to other things; for it is agreed that actuality is properly motion.
Hence people do not invest non-existent things with motion, although they do invest them with certain other predicates. E.g., they say that non-existent things are conceivable and desirable, but not that they are in motion. This is because, although these things do not exist actually, they will exist actually; for some non-existent things exist potentially; yet they do not exist, because they do not exist in complete reality.