Dr. Thomas Brown observes, in his inquiry into Cause and Effect — “From the mere silence of any one, we cannot infer that he is dumb in consequence of organic imperfection. He may be silent only because he has no desire of speaking, not because speech would not have followed his desire: and it is not with the mere existence of any one, but with his desire of speaking, that we suppose utterance to be connected. A man who has no desire of speaking, has in truth, and in strictness of language, no power of speaking, when in that state of mind: since he has not a circumstance which, as immediately prior, is essential to speech. But since he has that power, as soon as the new circumstance of desire arises — and as the presence or absence of the desire cannot be perceived but in its effects — there is no inconvenience in the common language, which ascribes the power, as if it were possessed at all times, and in all circumstances of mind, though unquestionably, nothing more is meant than that the desire existing will be followed by utterance.” (Brown, Essay on the Relation of Cause and Effect, p. 200.)
This is the real sense of what Aristotle calls τὸ δὲ (λέγεται) δυνατόν, οἷον δυνατὸν εἶναι βαδίζειν ὅτι βαδισειεν ἂν, i.e. he will walk if he desires to do so (De Interpret. p. 23, a. 9-15).
4. According to the Megaric doctrine (says Aristotle) when you are now lying down, you have no power to rise: when you are standing up, you have no power to lie down: so that the present condition of affairs must continue for ever unchanged: nothing can come into existence which is not now in being. — Here again, the Megarics would have denied his inference. The man who is now standing up, has power to lie down, if he wills to do so — or he may be thrown down by a superior force: that is, he will lie down, if some new fact of a certain character shall supervene. The Megarics do not deny that he has power, if — so and so: they deny that he has power, without the if — that is, without the farther accompaniments essential to energy.
Potential as distinguished from the Actual — What it is.
On the whole, it seems to me that Aristotle’s refutation of the Megarics is unsuccessful. A given assemblage of conditions is requisite for the production of any act: while there are other circumstances, which, if present at the same time, would defeat its production. We often find it convenient to describe a state of things in which some of the antecedent conditions are present without the rest: in which therefore the act is not produced, yet would be produced, if the remaining circumstances were present, and if the opposing circumstances were absent.[58] The state of things thus described is the potential as distinguished from the actual: power, distinguished from act or energy: it represents an incomplete assemblage of the antecedent positive conditions — or perhaps a complete assemblage, but counteracted by some opposing circumstances. As soon as the assemblage becomes complete, and the opposing circumstances removed, the potential passes into the actual. The architect, when he is not building, possesses, not indeed the full or plenary power to build, but an important fraction of that power, which will become plenary when the other fractions supervene, but will then at the same time become operative, so as to produce the actual building.[59]
[58] Hobbes, in his Computation or Logic (chaps. ix. and x. Of Cause and Effect. Of Power and Act) expounds this subject with his usual perspicuity.
“A Cause simply, or an Entire Cause, is the aggregate of all the accidents, both of the agents, how many soever they be, and of the patient, put together; which, when they are all supposed to be present, it cannot be understood but that the effect is produced at the same instant: and if any one of them be wanting, it cannot be understood but that the effect is not produced” (ix. 3).
“Correspondent to Cause and Effect are Power and Act: nay, those and these are the same things, though for divers considerations they have divers names. For whensoever any agent has all those accidents which are necessarily requisite for the production of some effect in the patient, then we say that agent has power to produce that effect if it be applied to a patient. In like manner, whensoever any patient has all those accidents which it is requisite it should have for the production of some effect in it, we say it is in the power of that patient to produce that effect if it be applied to a fitting agent. Power, active and passive, are parts only of plenary and entire power: nor, except they be joined, can any effect proceed from them. And therefore these powers are but conditional: namely, the agent has power if it be applied to a patient, and the patient has power if it be applied to an agent. Otherwise neither of them have power, nor can the accidents which are in them severally be properly called powers: nor any action be said to be possible for the power of the agent alone or the patient alone.”
[59] Aristotle does in fact grant all that is here said, in the same book and in the page next subsequent to that which contains his arguments against the Megaric doctrine, Metaphys. Θ. 5, 1048, a. 1-24.