  For instance, one of Helmholtz’ most well-known innovations is the study of perception in his monumental Handbook of Physiological Optics (1856–1867) (Helmholtz 1924–5). This work was the source of his so-called ‘sign-theory’, based on the idea that perceptions are signs or place-holders for what they signify, but do not resemble or copy them in any way. In his early writings, Helmholtz thought sensations are signs of their external causes, so the associations among sensations represent corresponding regularities among their sources. Consequently, it is the regular changes of signs, their serial structure, which reflects the order of their underlying causes. But this latter consequence implies a causal theory of perception which is fundamentally antithetical to Kant’s understanding of causality. After all, Kant had restricted the operations of causality to the realm of appearances, thus excluding unobservable causes lying behind and causing observable phenomena. But this departure from a basic tenet of Kant’s thought was compounded by the fact that Helmholtz’ account of the knowledge of the location of objects in space is thoroughly empiricist, and also rests on the principle of causality, understood as a causal realist would. But in his 1881 notes to his memoir, Helmholtz corrected himself, recalling that Kant’s views on causality were limited to lawfulness among appearances (Friedman 2010, 631; Friedman 1997, 30–1). What Helmholtz then asserted, in his classic essay “On the Facts of Perception,” is that the inference to a hypostasized reality lying behind the appearances goes beyond what is warranted by the lawfulness obtaining among appearances. Indeed, all localizations of objects in space are really nothing more than the discovery of the lawfulness of the connections obtaining among our motions and our perceptions. The difference between what is genuinely perceived and its realistic interpretation is just the difference between the regularities in our perceptions and the hypothesis of enduring, substantial sources of the perceived regularities (Helmholtz 1977, 138–140). 