(19) Thus what we know as pleasure and pain may be identified: pain is our perception of a body despoiled, deprived of the image of the soul; pleasure our perception of the living frame in which the image of the soul is brought back to harmonious bodily operation. The painful experience takes place in that living frame; but the perception of it belongs to the sensitive phase of the soul, which, as neighbouring the living body, feels the change and makes it known to the principle, the imaging faculty, into which the sensations finally merge; then the body feels the pain, or at least the body is affected: thus in an amputation, when the flesh is cut the cutting is an event within the material mass; but the pain felt in that mass is there felt because it is not a mass pure and simple, but a mass under certain conditions; it is to that modified substance that the sting of the pain is present, and the soul feels it by an adoption due to what we think of as proximity.

And, itself unaffected, it feels the corporeal conditions at every point of its being, and is thereby enabled to assign every condition to the exact spot at which the wound or pain occurs. Being present as a whole at every point of the body, if it were itself affected the pain would take it at every point, and it would suffer as one entire being, so that it could not know, or make known, the spot affected; it could say only that at the place of its presence there existed pain- and the place of its presence is the entire human being. As things are, when the finger pains the man is in pain because one of his members is in pain; we class him as suffering, from his finger being painful, just as we class him as fair from his eyes being blue.

But the pain itself is in the part affected unless we include in the notion of pain the sensation following upon it, in which case we are saying only that distress implies the perception of distress. But we cannot describe the perception itself as distress; it is the knowledge of the distress and, being knowledge, is not itself affected, or it could not know and convey a true message: a messenger, affected, overwhelmed by the event, would either not convey the message or not convey it faithfully.

(5) But some doubt arises when we consider the phenomena of hearing.

Perhaps we are to understand the process thus: the air is modified by the first movement; layer by layer it is successively acted upon by the object causing the sound: it finally impinges in that modified form upon the sense, the entire progression being governed by the fact that all the air from starting point to hearing point is similarly affected.

Perhaps, on the other hand, the intervenient is modified only by the accident of its midway position, so that, failing any intervenient, whatsoever sound two bodies in clash might make would impinge without medium upon our sense?

Still air is necessary; there could be no sound in the absence of the air set vibrating in the first movement, however different be the case with the intervenient from that onwards to the perception point.

The air would thus appear to be the dominant in the production of sound: two bodies would clash without even an incipient sound, but that the air, struck in their rapid meeting and hurled outward, passes on the movement successively till it reaches the ears and the sense of hearing.

But if the determinant is the air, and the impression is simply of air-movements, what accounts for the differences among voices and other sounds? The sound of bronze against bronze is different from that of bronze against some other substance: and so on; the air and its vibration remain the one thing, yet the difference in sounds is much more than a matter of greater or less intensity.

If we decide that sound is caused by a percussion upon the air, then obviously nothing turning upon the distinctive nature of air is in question: it sounds at a moment in which it is simply a solid body, until it is sent pulsing outwards: thus air in itself is not essential to the production of sound; all is done by clashing solids as they meet and that percussion, reaching the sense, is the sound. This is shown also by the sounds formed within living beings not in air but by the friction of parts; for example, the grinding of teeth and the crunching of bones against each other in the bending of the body, cases in which the air does not intervene.

But all this may now be left over; we are brought to the same conclusion as in the case of sight; the phenomena of hearing arise similarly in a certain co-sensitiveness inherent in a living whole.