 According to the “projection” theory of the physiology of perception, a stimulus causes its response directly. That is, our nerves are malleable, like wax, and the objects “project” signals onto the nerves directly, like a key pressed into the wax. Based on experiments in his laboratory, and on the phenomena of binocular vision revealed by the stereoscope Charles Wheatstone invented in the 1830s, Müller observed that several phenomena of the physiology of perception contradict the projection theory. One such phenomenon is stereoscopic binocular vision, in which the two images on our retinas are resolved into one image, the image we see. If images are projected directly onto the sense nerves, Müller asked, then how is stereoscopic vision possible? Müller also cited the fact that images from objects are projected onto the retina upside down, but we see them as right side up. The projection theory has no explanation of these effects. 