Hermann von Helmholtz

“Our sensations are effects brought forth in our organs by means of exterior causes, and how such an effect manifests itself depends of course quite essentially on the nature of the apparatus on which the cause operates. Insofar as the quality of our sensations gives us information about the peculiarities of the exterior process that excites it, it can count as a sign of that process, but not as a picture. For one expects of a picture some sort of similarity with the pictured object . . . But a sign need have no similarity of any sort whatever with that of which it is the sign. The relation between them is only that the same object, working its effects in the same way, produces the same sign, and that unequal signs always correspond to unequal causes.
To the popular view, which naively and complacently assumes the full truth of the pictures that our senses give us of things, this remainder of similarity that we recognise may seem rather paltry. In truth it is not; with its aid something of the greatest significance can be achieved: the representation of the regularities in the processes of the real world . . . So even if our sense impressions in their qualities are only signs, whose special nature depends wholly on our internal organisation, they are nonetheless not to be dismissed as empty appearance, but are in fact a sign of something, whether this is something existing or something occurring; and what is most important, they can picture the law of this occurring.”
― Hermann Helmholtz