Assuming with Euler that colours are isochronous vibrations pulsus) of the aether, as tones are of the air set in vibration by sound, and, what is most important, that the mind not only perceives by the senses their effect in stimulating the organs, but also, by reﬂection, the regular play of the impressions (and consequently the form in which different representations are united) which I, still, in no way doubt then colour and tone would not be mere sensations.