If the eyes are open and there is light, seeing occurs; the stimulus is but a condition of the fulfillment of the proper function of the organ, not an outside interruption. To some extent, then, all direction or control is a guiding of activity to its own end; it is an assistance in doing fully what some organ is already tending to do.

The Middle Works, 1899-1924 (ed. SIU Press, 1980) - ISBN: 9780809309337