[18]
Book 6
Ch. 1 [sect. 1] We have already said338 that it is right to choose the mean and to avoid excess and deficiency,
[20] 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 tension339 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.
Ch. 1 [sect. 2] 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.
Ch. 1 [sect. 3] 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.340
Ch. 1 [sect. 4]
Now we have divided the Virtues of the Soul into two groups,
[p. 1139a]
