 The influence of Hertz, and indirectly Helmholtz, on Wittgenstein is argued by many. Brian McGuinness, Wittgenstein’s biographer, and Peter Hacker, in Insight and Illusion, are perhaps the first to have so argued (see, e.g., Grasshoff 1993, 243, Hacker 1986, 2–4). Gerd Grasshoff traces the development in Grasshoff 1993, arguing that Hertz and Boltzmann were the first to develop a “picture theory,” according to which physical and linguistic theories give us pictures [Bilder in the original German] of the world. Hertz’s and Boltzmann’s picture (Bild) theories go well beyond Helmholtz’s view. Helmholtz argues that the constraints on using Bilder to construct scientific explanations are given by physical experience and natural science, and Hertz and Boltzmann argue that these constraints are determined mathematically. However, according to this interpretation, the historical roots of the picture theory can be found in Helmholtz’s substitution of the “sign theory” for nativistic explanations (see, e.g., Schiemann 1998, Patton 2009). According to Helmholtz’s “sign” theory, our physical perceptions are images or symbols that do not resemble their objects directly. Michael Heidelberger and Jean Leroux argue as well for the influence of Helmholtz’s sign theory on Hertz’s picture theory in Heidelberger 1998 and Leroux 2001. See section 2 for a detailed discussion of the sign theory.