We have already said * Cf. Bk. 2.6 esp. 6.15. that it is right to choose the mean and to avoid excess and deficiency, and that the mean is prescribed by the right principle. Let us now analyze the latter notion. In the case of each of the moral qualities or dispositions that have been discussed, as with all the other virtues also, there is a certain mark to aim at, on which the man who knows the principle involved fixes his gaze, and increases or relaxes the tension * The words denote tightening and loosening a bowstring, and also tuning a lyre. The former image is suggested by the preceding words, but the latter perhaps is a better metaphor for that avoidance of the too much and the too little which, according to Aristotle, constitutes right conduct. accordingly; there is a certain standard determining those modes of observing the mean which we define as lying between excess and defect, being in conformity with the right principle.
This bare statement however, although true, is not at all enlightening. In all departments of human endeavor that have been reduced to a science, it is true to say that effort ought to be exerted and relaxed neither too much nor too little, but to the medium amount, and as the right principle decides. Yet a person knowing this truth will be no wiser than before: for example, he will not know what medicines to take merely from being told to take everything that medical science or a medical expert would prescribe.
Hence with respect to the qualities of the soul also, it is not enough merely to have established the truth of the above formula; we also have to define exactly what the right principle is, and what is the standard that determines it. * Book 6 thus purports to explain further the definition of Moral Virtue （2.615）, while at the same time （1.4） continuing the analysis of the definition of Happiness （1.7.15） by examining the Intellectual Virtues.
Now we have divided the Virtues of the Soul into two groups, the Virtues of the Character and the Virtues of the Intellect. The former, the Moral Virtues, we have already discussed. Our account of the latter must be prefaced by some remarks about psychology.
It has been said before * 1.13.9. that the soul has two parts, one rational and the other irrational. Let us now similarly divide the rational part, and let it be assumed that there are two rational faculties, one whereby we contemplate those things whose first principles are invariable, and one whereby we contemplate those things which admit of variation: since, on the assumption that knowledge is based on a likeness or affinity of some sort between subject and object, the parts of the soul adapted to the cognition of objects that are of different kinds must themselves differ in kind.
These two rational faculties may be designated the Scientific Faculty and the Calculative Faculty respectively; since calculation is the same as deliberation, and deliberation is never exercised about things that are invariable, so that the Calculative Faculty is a separate part of the rational half of the soul.
We have therefore to ascertain what disposition of each of these faculties is the best, for that will be the special virtue of each. But the virtue of a faculty is related to the special function which that faculty performs.