The soul is the guardian of the body, inseparably united with it, as the odour is with frankincense; nor can the soul be disconnected from the body without its own destruction. This intimate union of soul and body is proved by many facts. They are born, they grow, and they decay together. The mind is liable to disease, like the body. Its affections are often dependent on bodily conditions. The difficulties of imagining the state of the soul as existing independently of the body are next urged; and the book concludes with a long passage of sustained elevation of feeling, in which the folly and the weakness of fearing death are passionately insisted upon. The fourth book, which treats of the images which all objects cast off from themselves, and, in connexion with that subject, of the senses generally, and of the passion of love, is intimately connected with the preceding book. If there is no life after death, what is the origin of the universal belief in the existence of the souls of the departed? Images cast off from the surface of bodies, and borne incessantly through space without force or feeling, appearing to the living sometimes in sleep and sometimes in waking visions, have suggested the belief in the ghosts of the dead, and in many of the portents of ancient mythology. The rapid formation and motion of these images and their great number are explained by various analogies. Some apparent deceptions of the senses are next mentioned and explained. These deceptions are shown to be not in the senses, but in our minds not rightly interpreting their intimations. There is no error in the action of the senses. They are our 'prima fides'--the foundation of all knowledge and of all conduct-- Non modo enim ratio ruat omnis, vita quoque ipsa Concidat extemplo, nisi credere sensibus ausis[11]. Images that are too fine to act on the senses sometimes directly affect the soul itself. Discordant images unite together in the air, and present the appearance of Centaurs, Scyllas, and the like. In sleep, images of the dead-- Morte obita quorum tellus amplectitur ossa[12],-- appear, and give rise to the belief in the existence of ghosts. The mind sees in dreams the objects in which it is most interested, because, although all kinds of images are present, it can discern only those of which it is expectant. Several other questions are discussed in connexion with the doctrine of the 'simulacra.' The final cause of the senses and the appetites is denied, and, by implication, the argument from design founded on the belief in final causes. The use of everything is discovered through experience. We do not receive the sense of sight in order that we may see, but having got the sense of sight, we use it-- Nil ideo quoniam natumst in corpore ut uti Possemus, sed quod natumst id procreat usum[13].