Helmholtz's works in physical geometry, the semiotics associated with his theory of perception and, in a more general vein, the Kantian influence of his epistemology have not failed to draw the attention of philosophers. However, Helmholtz's attitude towards scientific realism has scarcely been discussed.¹ While Helmholtz surely held a realist position toward laws of nature (thus espousing what could be called nomological realism), I want to underline significant aspects of his epistemology that indicate a rather sceptical stand towards the realist thesis, and put these aspects in a historical-philosophical perspective. With this in view, I will first indicate how Helmholtz, on the basis of his investigations in physiology, came to consider sensations as signs, this semiotic conception of sensations being at the center of his views on scientific realism. I will then discuss ensuing aspects of Helmholtz's theory of science that show strong anti-realist tendencies and appear to anticipate major themes of latter-day empiricism.