Arrian's Discourses of Epictetus (2.3)

DIOGENES said well to one who asked from him letters of recommendation, That you are a man, he said, he will know as soon as he sees you; and he will know whether you are good or bad, if he is by experience skilful to distinguish the good and the bad; but if he is without experience, he will never know, if I write to him ten thousand times.[*] For it is just the same as if a drachma (a piece of silver money) asked to be recommended to a person to be tested. If he is skilful in testing silver, he will know what you are, for you (the drachma) will recommend yourself. We ought then in life also to have some skill as in the case of silver coin that a man may be able to say like the judge of silver, Bring me any drachma and I will test it. But in the case of syllogisms, I would say, Bring any man that you please, and I will distinguish for you the man who knows how to resolve syllogisms and the man who does not. Why? Because I know how to resolve syllogisms. I have the power, which a man must have who is able to discover those who have the power of resolving syllogisms. But in life how do I act? At one time I call a thing good, and at another time bad. What is the reason? The contrary to that which is in the case of syllogisms, ignorance and inexperience.