Nicomachean Ethics I.3

This section is one of the most important parts of the EN to grasp in order to understand the project. I've written about this short section many times in the past. This is where Aristotle grounds his ethical project in reality, in the strongest terms we will ever get until the American pragmatist movement of the 20th century. What makes something a virtue is that it works. 

He wants to be clear from the beginning that he means that a thing works for the most part. Luck and chance can interfere with anything in the real world. It is the mark of a wise man to understand that ethics doesn't admit of logical proofs -- poor Kant -- but of probabilistic arguments based on real-world empirical observation. 

If you don't get this part right you will be out to sea for the rest of the work. 

Our discussion will be adequate if it has as much clearness as the subject-matter admits of, for precision is not to be sought for alike in all discussions, any more than in all the products of the crafts. Now fine and just actions, which political science investigates, admit of much variety and fluctuation of opinion, so that they may be thought to exist only by convention, and not by nature. And goods also give rise to a similar fluctuation because they bring harm to many people; for before now men have been undone by reason of their wealth, and others by reason of their courage. We must be content, then, in speaking of such subjects and with such premisses to indicate the truth roughly and in outline, and in speaking about things which are only for the most part true and with premisses of the same kind to reach conclusions that are no better. In the same spirit, therefore, should each type of statement be received; for it is the mark of an educated man to look for precision in each class of things just so far as the nature of the subject admits; it is evidently equally foolish to accept probable reasoning from a mathematician and to demand from a rhetorician scientific proofs.

Emphasis added. This is not going to be a list of rules; it is not going to be a list of moral principles, even. It is certainly not going to try to be a deduction from logic. We are talking about developing a state of character that is fit for the world you live in. We judge whether a thing is a virtue by whether or not it works, making due allowances for the chance and fate that are also part of the world.

Who judges? Not every man equally.   

Now each man judges well the things he knows, and of these he is a good judge. And so the man who has been educated in a subject is a good judge of that subject, and the man who has received an all-round education is a good judge in general. Hence a young man is not a proper hearer of lectures on political science; for he is inexperienced in the actions that occur in life, but its discussions start from these and are about these; and, further, since he tends to follow his passions, his study will be vain and unprofitable, because the end aimed at is not knowledge but action. And it makes no difference whether he is young in years or youthful in character; the defect does not depend on time, but on his living, and pursuing each successive object, as passion directs. For to such persons, as to the incontinent, knowledge brings no profit; but to those who desire and act in accordance with a rational principle knowledge about such matters will be of great benefit.

These remarks about the student, the sort of treatment to be expected, and the purpose of the inquiry, may be taken as our preface.

It will turn out to be that the virtuous man is the best judge of virtue, for his education is complete. The man who is courageous is a good judge of courage; the man who is just in his treatment of others is a good judge of justice. Not to get too far ahead of ourselves, but both justice and the virtue he calls magnanimity have a claim to be 'complete virtue,' such that a truly just or magnanimous man can be said to have received an all-round education in virtue and to be a good judge in general. They differ in a key aspect, however, which we will discuss when we get there.