On the Vital Principle/Book 2/Prelude to Chapter 11
Commentators have differed widely in their interpretation of Aristotle's meaning in the opening passage upon the Touch. But it may, with some confidence, be assumed that, from being unacquainted with the nervous system, and observing the wide-spread and varying delicacy of the sense, he was led to suppose that it might either be diffused, so to speak, as several organs, over the body, or be somehow identified with or included in the flesh which covers the body[1]. The flesh is the muscular substance, and as it envelopes, so to say, the body, it was probably supposed to be the seat or cause of the sense, as every part is sensible to Touch; and the analogue of flesh is the colourless substance of the Insanguinea insects, &c [2] And there is a close analogy between the two substances, “as the muscles of the highest class of animals, during their development pass through the soft, colourless, homogeneous and gelatinous condition of the Invertebrata, before assuming the red colour." As the Touch, besides, was regarded as the first in order of the senses and characteristic of animals, so the flesh was said to be the origin of all other parts of animal bodies, bone and skin, sinews, veins, hair and nails; and this hypothesis may have confirmed the opinion that it is either the sense or the seat of the sense of Touch.