  The Republic contains a great deal of information that we can rely on in characterizing the three parts of the soul that Socrates introduces, information that can be found not only in Book 4 itself, but also (among other places) in the catalogue of corrupt forms of city and soul in Books 8 and 9.  Here is an outline of what emerges. Reason is the part of the soul that is, of its own nature, attached to knowledge and truth. It is also, however, concerned to guide and regulate the life that it is, or anyhow should be, in charge of, ideally in a way that is informed by wisdom and that takes into consideration the concerns both of each of the three parts separately and of the soul as a whole (442c); these concerns must be supposed to include a person's bodily needs, presumably via the concerns of appetite. The natural attachment of spirit is to honor and, more generally, to recognition and esteem by others (581a). As a motivating force it generally accounts for self-assertion and ambition. When its desires are frustrated, it gives rise to emotional responses such as anger and indignation, and to behavior that expresses and naturally flows from such responses.  Socrates takes spirit to be a natural ally of reason, at least part of its function being to support reason in such conflicts as may arise between it and appetite (440ef, 442ab). To assign it this function is neither to say nor to imply that spirit cannot, in the case of a corrupt and de-natured soul, turn against reason, even if well brought-up individuals like Glaucon are not familiar with such corruption either in their own case or in the case of others (440b). (Pace Robinson 1995, 45, who thinks Socrates is contradicting himself here.) Appetite is primarily concerned with food, drink and sex (439d, 580e). It gives rise to desires for these and other such things which in each case are based, simply and immediately, on the thought that obtaining the relevant object of desire is, or would be, pleasant. Socrates also calls appetite the money-loving part, because, in the case of mature human beings at least, appetite also tends to be strongly attached to money, given that it is most of all by means of money that its primary desires are fulfilled (580e-581a). The idea must be that given suitable habituation and acculturation in the context of a life lived in human society, appetite tends to become attached to money in such a way that it begins to give rise to desires for money which in each case are based, simply and immediately, on the thought that obtaining money is, or would be, pleasant; and this idea is natural and plausible enough. (Irwin 1995 &amp; Price 1995, 57-67, offer an alternative and incompatible interpretation.)