We have said that we must choose the mean, and that the mean is that which the 'right reason' points out. But how is the 'right reason' determined? What makes it the 'right reason'? What is it that 'the right reason' has in view in fixing on this point rather than on that as the mean? The musician, for example, in tuning his instrument, must have some standard of tension before his mind. It is true that the strings must not be too tight or too loose, but just the right tightness. Again, it is true that the patient must get just what a skilful physician would prescribe - neither more nor less: - all this is true, but it is not definite enough. So in morals it is true that we must choose the mean as the right reason directs: but not definite enough. We must know definitely what the right reason is, that is, why, or in relation to what, it is 'right.'
We have distinguished the virtues of the soul as virtues of the moral character and virtues of the intellect. We have discussed the moral virtues: let us now discuss the intellectual - but first a few words about the soul generally. We have seen that there are two parts of the soul, the part which has reason, and the irrational part. Now let us divide the part which has reason into (I) that part by which we perceive necessary truth, and (2) that by which we perceive contingent truth: for, as the objects are generically distinct, there must be generically distinct faculties of the soul naturally corresponding to each class of objects, knowledge implying a certain similarity and kinship between faculty and object. Let us call (I) the Scientific Faculty, and (2) the Calculative Faculty, for to deliberate and to calculate is the same thing, and no one deliberates about necessary truths. The best state of each of these faculties will be the virtue of each. We have to discover, then, what is the 'virtue' of each, or the state which enables it to perform its proper function.