Note 210 — Chapter III · THE RELATION OF BODY AND MIND.
Reading edition
“The retinal image is the last effect known of the action of objects on us; what happens beyond the retina we know not; our knowledge of the objective process has at present here its limit.”—Ewald Hering, Beiträge zur Physiologie, 1862, p. 166. That is to say, we have a definite translation of the process in geometric terms as far as the retina, and thence onwards Geometry fails us, and Neurology and Psychology are invoked.