Epictetus, Golden Sayings 57

That was a good reply which Diogenes made to a man who asked him for letters of recommendation.

"That you are a man, he will know when he sees you; whether a good or bad one, he will know if he has any skill in discerning the good or bad. 

"But if he has none, he will never know, though I write him a thousand times."

It is as though a piece of silver money desired to be recommended to someone to be tested. If the man be a good judge of silver, he will know; the coin will tell its own tale.