The Discourses of Epictetus, tr. by P.E Matheson, , at sacred-texts.com
That is a good answer of Diogenes to one who asked him for letters of introduction: 'You are a man, and that his eyes will tell him; but whether you are good or bad he will discover, if he has skill to distinguish the good from the bad; and if he has not that skill, he will never discover it, though I should write him ten thousand letters.' A drachma might just as well ask to be introduced to some one in order to be tested. If the man is a judge of silver, you will introduce yourself. We ought, therefore, to have some faculty to guide us in life, as the assayer has in dealing with silver, that I may be able to say as he does, 'Give me any drachma you please, and I will distinguish.' Now I can deal with a syllogism and say, 'Bring any one you like, and I will distinguish between him who can analyse syllogisms and him who cannot.' Why? Because I know how to analyse them: I have the faculty a man must have who is to recognize those who can handle syllogisms aright. But when I have to deal with life, how do I behave? Sometimes I call a thing good, sometimes evil. And the reason is just this, that whereas I have knowledge of syllogisms, I have no knowledge or experience of life.