In Book Theta of the Metaphysics, Aristotle informs us of a group of ancient thinkers, known as the Megarians, who were notorious for their denial of the existence of potentialities or capacities apart from their actual exercise. He first presents their view and then its refutation, which is crucial for his intentions as a whole in that work. Here I’m going to leave aside his refutation and its overall purpose for what he has to say later on in the text and instead concentrate only on what the Magerian position, as he presents it, amounts to. As counter-intuitive as their claim may be, it’s deeply interesting.
At the beginning Book Theta, chap.III, 1046b30-2, the First Teacher lays out the Magerian doctrine as follows:
There are some, […], who say that a thing has a potency only when it is active, but that when it is not active it has no potency for that activity; for example, he who is not building is not able to build, but he can build only when he is building, and similarly with the [other cases].
Let me here try and state, following the helpful suggestions of J. Beere, the reasoning involved in the Megarian view.
Basically, the Megarian position amounts to holding that to be able i.e., to have the potency or power, to do x is to actually do x. And if you’re not actually doing x, then you cannot do it i.e., you do not have the potency or power do it. For example, to be able to build i.e., to have a potency for that activity, is nothing but to actually engage in the activity of building. Otherwise, apart from actually building, a builder does not have the power to build. And the point can be extended across the board to all things, whether rational or non-rational, which have causal efficacy of some sort i.e., which can either act or be acted on. Thus, a thing can move only when it is moving; a thing can be burned only when it is being burned; a thing is breakable only when it is being broken, and so on. Hence, to have a potency to act on is nothing apart from actually acting on, and the potency to be acted on is nothing apart from actually being acted on. However, it is not immediately obvious that this view precludes the possibility of the existence powers. It seems it does not. For the Megarians, as Beere says, it is not the case that they deny the existence of potencies pure and simple; rather, they just aren’t willing to grant the existence of any sorts of potencies that are not currently being exercised. It seems reasonable, say the Megarians, to say that if a thing is not doing x, it does not have the power to do x; but, if it is doing x now, it obviously means it has the power to do x. And the relevant conclusion would then be drawn; so they would then seem to be committed to the proposition that ‘there exist potencies that are actualized’ (or, again, ‘potencies exist when actualized’) but not the proposition that ‘there exist unactualized potencies’. What exactly do these bizarre claims taken together mean Beere brings out nicely with his discussion of the inflammability (or not) of a piece of wood and the Megarian recourse to the circumstances relevant in determining whether or not a thing can act a given way. Sticking to the example Aristotle uses in his text, the general idea here is the following. Suppose the Megarian is asked if the builder, who is not building at the moment, can in fact build. As should be clear by now, the response from the Megarian will be in the negative; as things currently are, he will say, no, the builder cannot build. And it isn’t the case that the builder can i.e., has the potency to, build now, but just happens not to be building; for, the Megarian will insist, there must be a reason why he is not presently building. This reason, according to the Megarian, is that certain external conditions have not yet been fulfilled in order for the builder to be able to build. For example, it may be the case that he does not have his tools or his material work on, etc. Otherwise, i.e., if the sufficient conditions were met, the builder would not in fact be not building at the moment. For what else, the Megarian will ask, can, at the present moment, explain the fact that the builder is not actually building? Therefore, unless these various conditions are fulfilled, at present the builder does not have the potency to build. But suppose all those conditions are met i.e., he has the tools and the material. And suppose he is still is not building. Why is that? It is, again, because some other condition has not yet been fulfilled. And so one so forth once this other condition is met as well. The crucial point the Megarian seeks to drive home is that even if all these various conditions are present, unless the builder is actually building, he will not, obviously enough, in fact be building and so will not be able to build. It turns out that even the fact that the tools and materials are there is no reason for saying he can therefore build; for he clearly is not yet building. It seem the only reasonable conclusion to infer, therefore, is to say that only when the builder is actually building is it the case that he possess the power to build. Therefore, there is no potentiality for building before actually building. This then constitutes the Megarian position.
 Beere (2009), p. 93-108
 Ibid., p. 94
 Ibid., p. 95
 Ibid., at pp. 95-6 Beer writes: “For until that moment [i.e., of the stick actually burning], some condition or other that would have been sufficient for the burning of the stick was not fulfilled – we know that from the simple fact that the stick did not burn. If [it didn’t burn], then obviously no condition sufficient for the burning of the stick was fulfilled. The core idea is that, [for the stick to burn], conditions that are sufficient for the burning have to be fulfilled. [If not], then, the stick does not and cannot burn.”
Beere, Jonathan. Doing and Being an Interpretation of Aristotle’s Metaphysics Theta. Oxford: Oxford UP, 2009.