 An action is voluntary when its stimulus is an “idea, or state of mind … which we term the will” (OM 1, prop. 21). To illustrate how ideas of the form “the will to …” derive from “originally automatic” motions, Hartley describes how a child gains motor control over parts and functions of her body. “Originally automatic” motions are brought under control through a series of substitutions of the initiating stimulus. An infant grasps the finger placed in her palm, and then the toy she sees, and then grasps at “the sound of the words grasp, take hold, &amp;c., the sight of the nurse’s hand, in that state, the idea of a hand, and particularly the child’s own hand.” These and “innumerable other associated circumstances … will put the child upon grasping, till, at last, that idea, or state of mind which we may call the will to grasp, is generated and sufficiently associated with the action to produce it instantaneously.”