 It is important to note that, in Hartley’s account, the performance of a voluntary movement is not a dualistic, two-stage process, with an executive “faculty” of the Mind, the Will, first issuing an instruction that the body then carries out. Rather, a movement becomes voluntary through the living being’s interaction with the “innumerable … associated circumstances” of its environment, which interactions bring about a series of substitutions of the initiating stimulus. In this light, “will” does not name anything substantive: it is a word we use to describe an “idea, or state of mind.” And it is a word people often use to express bafflement as to what this “state of mind” amounts to. Hartley notes that adults start calling a child willful at the time he learns to walk, because “the child, in some cases, does not walk when desired, whilst yet the circumstances are apparently the same as when he does. For here the unapparent cause of walking, or not walking, is will” (OM 1, prop. 77). But, he adds, “a careful observation … will always shew … that when children do different things, the real circumstances, natural or associated, are proportionally different, and that the state of mind called will depends on the difference.”