We may err in many ways (for evil, as the Pythagoreans said, is of the nature of the Infinite, good of the Finite), but we can do right only in one way; so much easier is the path of error.
Combining then this _differentia_ with the _genus_, as above established, the complete definition is given thus--'Virtue is an acquirement or fixed state, tending by deliberate purpose (genus), towards a mean relative to us (difference).' To which is added the following all-important qualification, 'determined by reason [Greek: logos], and as the _judicious man_ [Greek: ho Phronimos] would determine.' Such is the doctrine of the Mean, which combines the practical matter-of-fact quality of moderation, recognized by all sages, with a high and abstract conception, starting from the Pythagorean remark quoted by Aristotle, 'the Infinite, or Indefinite, is evil, the Finite or the Definite is good,' and re-appearing in Plato as 'conformity to measure' [Greek: metriotaes], by which he (Plato) proposes to discriminate between good and evil.

The concluding qualification of virtue--'a rational determination, according to the ideal judicious man'-- is an attempt to assign a standard or authority for what is the proper 'Mean;' an authority purely ideal or imaginary; the actual authority being always, rightly or wrongly, the society of the time.
Aristotle admits that his doctrine of Virtue being a mean, cannot have an application quite universal; because there are some acts that in their very name connote badness, which are wrong therefore, not from excess or defect, but in themselves (VI.).

He next proceeds to resolve his general doctrine into particulars; enumerating the different virtues stated, each as a mean, between two extremes--Courage, Temperance, Liberality, Magnanimity, Magnificence, Meekness, Amiability or Friendliness, Truthfulness, Justice (VII.).

They are described in detail in the two following books.

In chap.VIII., he qualifies his doctrine of Mean and Extremes, by the remark that one Extreme may be much farther removed from the Mean than the other.
Cowardice and Rashness are the extremes of Courage, but Cowardice is farthest removed from the Mean.
The concluding chapter (IX.) of the Book reflects on the great difficulty of hitting the mean in all things, and of correctly estimating all the requisite circumstances, in each particular case.
He gives as practical rules:--To avoid at all events the worst extreme; to keep farthest from our natural bent; to guard against the snare of pleasure.