  It is a law that unsupported objects close to the earth will fall. What this means, Reid thinks, is that the physical cause of the fall of an object regularly causes objects to fall when they are unsupported. Physical causation, for him, is parasitic upon the more basic kind of causation, namely efficient causation. But what is efficient causation? Reid writes, “In the strict and proper sense, I take an efficient cause to be a being who had power to produce the effect, and exerted that power for that purpose” (Letter to James Gregory, in Correspondence 174). Efficient causation is necessitation of the sort that Hume thought not to exist. To efficiently cause an event is to be capable of seeing to it that the event occurs, and to make an effort to see to it that it does. Thus, at the bottom of Reid's theory of causation are his notions of power and exertion.