I would not add to this fourth Part of the Principia Philosophiæ if, as I had previously intended, I were still going to write two other Parts; that is, a fifth Part concerning living things, or animals and plants, and a sixth concerning man … Up till now I have described the Earth and the whole of the visible universe as if it were a machine, and I have considered only the various shapes and motions of its parts. But our senses show us much else besides, namely colours, smells, sounds and similar things. And if I remained completely silent about these, I should seem to have omitted the principal part of the explanation of natural things.

I desire that you consider that all the functions that I have attributed to this machine, such as the digestion of food, the beating of the heart and the arteries, the nourishment and growth of the bodily parts, respiration, waking and sleeping; the reception of light, sounds, odours, smells, heat and other such qualities by the external sense organs; the impression of the ideas of them in the organ of common sense and the imagination, the retention or imprint of these ideas in the memory; the internal movements of the appetites and the passions;[…]