to make an appeal to my will and to put, so to speak, an elementary question to my motor activity. Every such question is what is termed a perception. Thus perception is diminished by one of its elements each time one of the threads termed sensory is cut, because some part of the external object then becomes unable to appeal to activity; and it is also diminished whenever a stable habit has been formed, because this time the ready-made response renders the question unnecessary. What disappears in either case is the apparent reflexion of the stimulus upon itself, the return of the light on the image whence it comes; or rather that dissociation, that discernment, whereby the perception is disengaged from the image. We may therefore say that while the detail of perception is moulded exactly upon that of the nerves termed sensory, perception as a whole has its true and final explanation in the tendency of the body to movement.
The cause of the general illusion on this point lies in the apparent indifference of our movements to the stimulation which excites them. It seems that the movement of my body in order to reach and to modify an object is the same, whether I have been told of its existence by the ear or whether it has been revealed to me by sight or touch. My motor activity thus appears as a separate entity, a sort of reservoir whence movements issue at will, always the same for the same action, whatever the kind of image which has called it into being.
