 In his sign theory of perception as expressed in his early career (1848–1868), Helmholtz argues that the mind makes a series of mental adjustments, “unconscious inferences,” to construct a coherent picture of its experiences. Helmholtz argues that spatial position, often used as a criterion to individuate objects, is an interpretation of our sensations, and not their immediate result. Again, stereoscopic vision shows that what may appear, to us, as a single image is in fact two images resolved into one. Perspective can distort size, as when one puts a finger in front of the moon. Helmholtz believes that we learn how to interpret spatial concepts through experience, which means that he has what he calls an empirical theory of spatial perception. This theory coexists, in Helmholtz’s epistemology, with his above commitment to the sign theory, according to which spatial properties are only properties of representations. Helmholtz’s invention of the ophthalmoscope in 1850–1851 contributed to his understanding of the physiology of perception (Schett 1999; for images of the original Helmholtz ophthalmoscope see De Schweinitz and Randall 1899, 172ff). Philosophically, Helmholtz’s epistemology commits him to the view that representations arise in a physical process, but are signs and not copies of their objects. 