Natural consciousness, which is entirely absorbed in the interest of observing the external world, and has little inducement to direct its attention to the Ego that appears always the same amid the multi-coloured variations of outside objects, is not in the habit of noticing that the properties of the objects that are seen and touched are their effects, partly on other natural bodies, but mainly on our senses. Now as our nervous system and our sensation-faculty, as being the constant reagent on which the effect is exerted, is thus left out of account entirely, and as the difference of the effect is regarded as being simply a difference in the object from which it proceeds ... and so comes to be considered objectively as being a property of the body and merely as belonging to it. [O]ur impression, consequently, seems to us to be a pure image of the external state of affairs reflecting only that external condition and depending solely on it.

Hermann von Helmhotz, Treatise on Physiological Optics, Vol. III, p. 32

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