We are conscious of making an exertion, sometimes with difficulty, in order to produce certain effects. And exertion made deliberately and voluntarily, in order to produce an effect, implies a conviction that the effect is in our power. No man can deliberately attempt what he does not believe to be in his power. The language of all mankind, and their ordinary conduct in life, demonstrate that they have a conviction of some active power in themselves to produce certain motions in their own and in other bodies, and to regulate and direct their own thoughts. This conviction we have so early in life, that we have no remembrance when, or in what way, we acquired it.
Thomas Reid, "Of the Liberty of Moral Agents," Essays on the Active Powers.