[How is it that sensations are accompanied by pleasure and pain? Sensations arise thus. An object comes into contact with an organ of sense. This, if composed of fine particles, like the eye or ear, readily transmits the motion to the soul; if of larger, like the bones, less readily. The result is sensation.—As regards pleasure and pain—an organ consisting of large particles is more liable to them than an organ of the opposite kind. Pain arises when the particles are suddenly disturbed, pleasure when they are suddenly restored to their natural state.] The most important of the affections which concern the whole body remains to be considered,—that is, the cause of pleasure and pain in the perceptions of which I have been speaking, and in all other things which are perceived by sense through the parts of the body, and have both pains and pleasures attendant on them.