We said before that the soul consists of two parts, the rational and the irrational part. We will now make a similar division of the former, and will assume that there are two rational faculties: (1) that by which we know those things that depend on invariable principles, (2) that by which we know those things that are variable. For to generically different objects must correspond generically different faculties, if, as we hold, it is in virtue of some kind of likeness or kinship with their objects that our faculties are able to know them.

Let us call the former the scientific or demonstrative, the latter the calculative or deliberative faculty, For to deliberate is the same as to calculate, and no one deliberates about things that are invariable. One division then of the rational faculty may be fairly called the calculative faculty.

Our problem, then, is to find what each of these faculties becomes in its full development, or in its best state; for that will be its excellence or virtue.

But its excellence will bear direct reference to its proper function.