20. As with bodily pain and pleasure so with the bodily desires; their origin, also, must be attributed to what thus stands midway, to that Nature we described as the corporeal.
Body undetermined cannot be imagined to give rise to appetite and purpose, nor can pure soul be occupied about sweet and bitter: all this must belong to what is specifically body but chooses to be something else as well, and so has acquired a restless movement unknown to the soul and by that acquisition is forced to aim at a variety of objects, to seek, as its changing states demand, sweet or bitter, water or warmth, with none of which it could have any concern if it remained untouched by life.
In the case of pleasure and pain we showed how upon distress follows the knowledge of it, and that the soul, seeking to alienate what is causing the condition, inspires a withdrawal which the member primarily affected has itself indicated, in its own mode, by its contraction. Similarly in the case of desire: there is the knowledge in the sensation [the sensitive phase of the soul] and in the next lower phase, that described as the "Nature" which carries the imprint of the soul to the body; that Nature knows the fully formed desire which is the culmination of the less formed desire in body; sensation knows the image thence imprinted upon the Nature; and from the moment of the sensation the soul, which alone is competent, acts upon it, sometimes procuring, sometimes on the contrary resisting, taking control and paying heed neither to that which originated the desire nor to that which subsequently entertained it.
But why, thus, two phases of desire; why should not the body as a determined entity [the living total] be the sole desirer?
Because there are [in man] two distinct things, this Nature and the body, which, through it, becomes a living being: the Nature precedes the determined body which is its creation, made and shaped by it; it cannot originate the desires; they must belong to the living body meeting the experiences of this life and seeking in its distress to alter its state, to substitute pleasure for pain, sufficiency for want: this Nature must be like a mother reading the wishes of a suffering child, and seeking to set it right and to bring it back to herself; in her search for the remedy she attaches herself by that very concern to the sufferer's desire and makes the child's experience her own.
In sum, the living body may be said to desire of its own motion in a fore-desiring with, perhaps, purpose as well; Nature desires for, and because of, that living body; granting or withholding belongs to another again, the higher soul.