23. I explain: A living body is illuminated by soul: each organ and member participates in soul after some manner peculiar to itself; the organ is adapted to a certain function, and this fitness is the vehicle of the soul-faculty under which the function is performed; thus the seeing faculty acts through the eyes, the hearing faculty through the ears, the tasting faculty through the tongue, the faculty of smelling through the nostrils, and the faculty of sentient touch is present throughout, since in this particular form of perception the entire body is an instrument in the soul's service.
The vehicles of touch are mainly centred in the nerves- which moreover are vehicles of the faculty by which the movements of the living being are affected- in them the soul-faculty concerned makes itself present; the nerves start from the brain. The brain therefore has been considered as the centre and seat of the principle which determines feeling and impulse and the entire act of the organism as a living thing; where the instruments are found to be linked, there the operating faculty is assumed to be situated. But it would be wiser to say only that there is situated the first activity of the operating faculty: the power to be exercised by the operator- in keeping with the particular instrument- must be considered as concentrated at the point at which the instrument is to be first applied; or, since the soul's faculty is of universal scope the sounder statement is that the point of origin of the instrument is the point of origin of the act.
Now, the faculty presiding over sensation and impulse is vested in the sensitive and representative soul; it draws upon the Reason-Principle immediately above itself; downward, it is in contact with an inferior of its own: on this analogy the uppermost member of the living being was taken by the ancients to be obviously its seat; they lodged it in the brain, or not exactly in the brain but in that sensitive part which is the medium through which the Reason-Principle impinges upon the brain. They saw that something must be definitely allocated to body- at the point most receptive of the act of reason- while something, utterly isolated from body must be in contact with that superior thing which is a form of soul [and not merely of the vegetative or other quasi-corporeal forms but] of that soul apt to the appropriation of the perceptions originating in the Reason-Principle.
Such a linking there must be, since in perception there is some element of judging, in representation something intuitional, and since impulse and appetite derive from representation and reason. The reasoning faculty, therefore, is present where these experiences occur, present not as in a place but in the fact that what is there draws upon it. As regards perception we have already explained in what sense it is local.
But every living being includes the vegetal principle, that principle of growth and nourishment which maintains the organism by means of the blood; this nourishing medium is contained in the veins; the veins and blood have their origin in the liver: from observation of these facts the power concerned was assigned a place; the phase of the soul which has to do with desire was allocated to the liver. Certainly what brings to birth and nourishes and gives growth must have the desire of these functions. Blood- subtle, light, swift, pure- is the vehicle most apt to animal spirit: the heart, then, its well-spring, the place where such blood is sifted into being, is taken as the fixed centre of the ebullition of the passionate nature.