At our first encounter with this problem we remarked that in the supersensible no such sharp distinctions exist between different sense-spheres as are found in body-bound sense-perception. At the same time we remembered that even in physical perception we are inclined to attach acoustic attributes to colours and optical attributes to tones. In fact, it was precisely an instance of this kind of experience, namely, our conception of tone-colour, which gave us our lead in discussing the acoustic sphere in general. Our picture of the particular interaction of the two polar bodily systems in the acts of seeing and hearing now enables us to understand more clearly how these two spheres of perception overlap in man. For we have seen how the system which in seeing is the receiving organ, works in hearing as the responding one, and vice versa. As a result, optical impressions are accompanied by dim sensations of sound, and aural impressions by dim sensations of colour.