  Reid is clear that power is not the sort of thing that admits of a logical definition. We cannot reduce it to some set of simpler qualities. However, this doesn't mean that we can't say anything about what power is. On the contrary, Reid makes a variety of claims about power. Importantly, he claims that power is the quality that, when coupled with exertion, necessitates a particular effect. Taking a cue from ordinary language, he holds that it is a contradiction to say that an entity has the power to do something, and exerts that power, and yet the effect fails to come about. If the effect fails to come about when the entity exerts itself, we might say, what that shows is that the entity only seemed, but didn't actually, have the power to bring about the effect. Reid also claims that any agent who has the power to do something also has three other inter-related powers: the power not to do the thing, the power to try to do that thing (that is, the power to exert his power to do the thing), and the power not to try to do that thing. He claims further that any agent who has the power to do something must believe himself to have that power.