Xenophon, Memorabilia of Socrates 10

It may be objected: before giving his companions lessons in politics Socrates had better have taught them sobriety. 

Without disputing the principle, I would point out that a teacher cannot fail to discover to his pupils his method of carrying out his own precepts, and this along with argumentative encouragement. 

Now I know that Socrates disclosed himself to his companions as a beautiful and noble being, who would reason and debate with them concerning virtue and other human interests in the noblest manner. 

And of Critias and Alcibiades I know that as long as they were companions of Socrates even they were temperate, not assuredly from fear of being fined or beaten by Socrates, but because they were persuaded for the moment of the excellence of such conduct. 

Perhaps some self-styled philosophers may here answer: "No, the man truly just can never become unjust, the temperate man can never become intemperate, the man who has learnt any subject of knowledge can never be as though he had learnt it not." 

That, however, is not my own conclusion. It is with the workings of the soul as with those of the body; want of exercise of the organ leads to inability of function, here bodily, there spiritual, so that we can neither do the things that we should nor abstain from the things we should not. 

And that is why fathers keep their sons, however temperate they may be, out of the reach of wicked men, considering that if the society of the good is a training in virtue so also is the society of the bad its dissolution. 

—from Xenophon, Memorabilia 1.2