Hermann von Helmholtz (1821–1894) was one of the most productive scientists to contribute to the understanding of perception. His treatises on vision and hearing continue to exert a notable influence on the sciences of perception.¹ For Helmholtz and his contemporaries, the science of perception was an independent enterprise, distinct from the philosophy of perception, just as the two are distinct today. However the two areas of inquiry were more closely in contact than they are today. In fact, Helmholtz developed a general framework for conceiving of perception that went beyond a purely naturalistic explanation of how humans perceive. The framework outlined a set of normative conditions for perceptual knowledge in general. His general theory has been called the empiricist theory of perception, and it includes as proper parts the sign-theory of perception (or Zeichentheorie) and the theory of unconscious inference. In this paper I explore an interpretation of the Zeichentheorie and argue that Helmholtz maintains a strong residue of a causal realist theory of perception throughout his life. The argument will include the discussion of a case study from Helmholtz's research on hearing, specifically, his influential resonance theory of hearing.