Capability and Potentiality and Actualityby Aristotle
The Megaric school says that a thing ‘can’ act only when it is acting. When it is not acting, it ‘cannot’ act.
- For example, he who is not building cannot build. Only one who is building can build when he is building.
A man will not be a builder unless he is building.
If, then, it is impossible to have such arts if one has not at some time learnt and acquired them, and it is then impossible not to have them if one has not sometime lost them (either by forgetfulness or by some accident or by time; for it cannot be by the destruction of the object, for that lasts for ever), a man will not have the art when he has ceased to use it, and yet he may immediately build again;
how then will he have got the art?
Similarly with regard to lifeless things= nothing will be either cold or hot or sweet or perceptible at all if people are not perceiving it; so that the upholders of this view will have to maintain the doctrine of Protagoras. But, indeed, nothing will even have perception if it is not perceiving, i.e. exercising its perception. If, then, that is blind which has not sight though it would naturally have it, when it would naturally have it and when it still exists, the same people will be blind many times in the day-and deaf too.
If that which is deprived of potentiality is incapable, that which is not happening will be incapable of happening. But he who says of that which is incapable of happening either that it is or that it will be will say what is untrue; for this is what incapacity meant.
Therefore these views do away with both movement and becoming.
For that which stands will always stand, and that which sits will always sit, since if it is sitting it will not get up; for that which, as we are told, cannot get up will be incapable of getting up. But we cannot say this, so that evidently potency and actuality are different (but these views make potency and actuality the same, and so it is no small thing they are seeking to annihilate), so that it is possible that a thing may be capable of being and not he, and capable of not being and yet he, and similarly with the other kinds of predicate; it may be capable of walking and yet not walk, or capable of not walking and yet walk.
A thing cam do something if there will be nothing impossible in its having the actuality of that of which it is said to have the capacity.
- If a thing can sit, there will be nothing impossible in its actually sitting.
- If it can move or be moved, or stand
We connect ‘actuality’ with ‘complete reality’. It has been extended from movements to other things.
Actuality in the strict sense is thought to be identical with movement. And so people do not assign movement to non-existent things, though they do assign some other predicates.
- E.g. they say that non-existent things are objects of thought and desire, but not that they are moved; and this because, while ex hypothesi they do not actually exist, they would have to exist actually if they were moved.
For of non-existent things some exist potentially; but they do not exist, because they do not exist in complete reality.