 Significant questions in the empirism – nativism debate include how to explain perception of depth and relief (Turner 1993, 156). The “projection” theory explained relief perception by claiming that the mind interprets visual cues to construct a combined, relief picture out of two flat pictures provided by the eyes (Turner 1993, 158). By 1867, when Helmholtz published his Handbook of Physiological Optics, Helmholtz had developed his “sign” theory, according to which the brain’s construction of phenomena such as depth and separation in space is learned. This account brought Helmholtz into conflict with Hering. 