 The ground for this conclusion was supposed to be the fact, uncovered by physiology, that nerves are not faithful transmitters of external properties to consciousness (Helmholtz 1853, 13). Helmholtz inferred from this his famous sign theory, according to which “our sensations are for us only symbols of the objects of the external world, and correspond to them only in some such way as written characters or articulate words to the things they denote. They give us, it is true, information respecting the properties of things without us, but no better information than we give a blind man about colour by verbal descriptions” (Helmholtz 1853, 14). Thus, Helmholtz argues, “what the recent physiology of the senses has shown by the way of experience is what Kant had tried to show for the representations of the human mind in general when he laid out the participation of the particular, built-in rules of the mind, the organization of the mind as it were, in our representations” (Helmholtz 1855, 58). Helmholtz thinks the confirmation of Kant goes further. The only way to get from the “world of sensations” to the “world of reality”, the “external world”, is through an inference. We infer that there is an external world because there has to be a “cause of our nerve excitations because there can be no effect without cause.” But, Helmholtz asks, “How do we know that there is no effect without a cause?” This is not a principle we could learn from experience, since it must be in place before we can come to any conclusions about the world, including the conclusion that cause follows effect. Thus, “the investigation of sensory perception also leads us to what Kant had already recognized, namely that the principle, ‘No effect without cause’, is a law of our thought given before all experience” (Helmholtz 1855, 77). Thus, as he puts it later, what science shows us does not decide the question between idealism and realism: