A full picture of the human soul emerges only gradually from the Republic. In Book 4 we come first upon a conventional enough distinction between calculation and desire, which under pressure from the correspondence between the microcosm of the just individual and the macrocosm of the just society, with its three different classes, is complicated by the addition of a third element, the element of “high spirit” (thumos). At this stage the three elements in the soul are distinguished principally by their functions: calculation calculates, desire desires, spirit gets spirited. If the text is pressed to assign them an object or goal as well as an activity, the indications would be that calculation is concerned with the good (i.e., with the best course of action); desire is concerned with pleasure; while spirit reacts to perceived slights or wrongs.

When we revisit these three elements in Books 8 and 9, however, they have taken on a different look. In Book 4 they seemed most like faculties; now they seem more like drives. The desiring element is specified as the drive toward material satisfaction; spirit as the drive to win and to amount to something; calculation as the drive to discover truth. They have not shed their characteristic functions, but these have found a new context; and the biggest change is to the calculative element. Previously, it had been unclear whether this element even had a goal of its own or was merely a supervisor that placed limits on the interests of the other elements in the interest of the individual as a whole. Now it is assigned an object of desire all its own, and that object is not the good, whether the good of the individual or the good tout court, but wisdom. Wisdom is a good, of course, arguably the highest good.