The Sign-Theory of Perception from Helmholtz to Schlick

Thomas Oberdan

Clemson University

The transformations of philosophical thought at the turn of the Twentieth Century, and their relations to the scientific developments which stimulated them, pose a daunting challenge to a synthetic understanding of the historical development of the philosophy of science. This challenge is epitomized by the development of the ‘sign theory of perception’, the signature doctrine of Hermann von Helmholtz, as it evolved into the early epistemology of Moritz Schlick. Michael Friedman has argued, in “Helmholtz’ Zeichentheorie and Schlick’s Allgemeine Erkenntnislehre” (Phil Topics 25, pp. 19-50) that, as the sign-theory evolved from Helmholz’ thought to Schlick’s, it was transformed from “a modified version of the Kantian conception of space as a subjective form of intuition” into an empiricist version of the causal theory of perception. The principal scientific development which influenced the understanding of spatial concepts featured in Schlick’s Allgemeine Erkenntnislehre was the new conception of geometry which emerged from the work of Moritz Pasch and David Hilbert. However, Friedman thinks that it is not the developments in geometry influencing Schlick’s views which impaired his understanding of Helmholtz’ sign-theory. Rather, it is Schlick’s radical separation of intuitive acquaintance and conceptual knowledge which impedes his grasp of the sign-theory as Helmholtz originally conceived it. Accordingly, Friedman concludes that Schlick’s misapprehension of Helmholtz’ sign-theory is due to his most fundamental epistemological commitments. And it is because of this misunderstanding that Schlick also fails to appreciate the significant role of spatial intuition in the foundations of his geometry. But the differences between Helmholtz’ sign-theory and its re-incarnation in Schlick’s early epistemology cannot be attributed to a simple misunderstanding. It will be argued that the contrast between Helmholtz’ original conception of the sign-theory and Schlick’s depends directly on philosophical developments in the interim which were incorporated into Schlick’s philosophical thinking. First of all, the sign-theory evolved, due to Max Planck and others, beyond its original limits as a theory of perception, into a generalized epistemological structuralism. As a consequence of this development, the sign-theory was separated from the independent neo-Kantian interpretation given it by Helmholtz, resulting in a variety of structural realism like that of Schlick’s Allgemeine Erkenntnislehre. Secondly, Helmholtz’ argument for grounding geometry in spatial intuition was challenged by Henri Poincaré, who established geometric conventionalism as an alternative to neo-Kantian and empiricist understandings of geometry. Combining Poincaré’s conventionalism with his own structural realism, Schlick then introduced his ‘method of coincidences’ to fashion a general understanding of the relations between concepts and intuitions. Applied to geometry, the resulting view was a precursor of the geometric conventionalism which distinguished early Logical Empiricist philosophy of science. And it was his account of intutions, concepts, and their relations in geometry which provided Schlick with reason for believing that Helmholtz’ attempt to ground geometry in spatial intuition could never succeed.