On true and false friendship
You sent a letter to me through the hand of a "friend" of yours, as you call him. And in your very next sentence you warn me not to discuss with him all the matters that concern you, saying that even you yourself are not accustomed to do this; in other words, you have in the same letter affirmed and denied that he is your friend.
If you used this word of in the popular sense, and called him "friend" in the same way in which we speak of candidates for political office as "honourable gentlemen," and as we greet all whom we meet casually, whose name we have forgotten at the moment, with the salutation, "My Dear ..." - so be it.
But if you consider any man a friend whom you do not trust as you trust yourself, you are mistaken and you do not fully understand what true friendship means. Indeed, I would have you discuss everything with a friend; but first of all discuss the man himself. Before a friendship is formed, you must judge, after the friendship is settled, you must trust. Those persons who put the cart before the horse, violate the rules of Theophrastus, judging a man after they have made him their friend, instead of making him their friend after they have judged him. Ponder for a long time whether you shall admit a given person to your friendship; but when you have decided to admit him, welcome him with all your heart and soul.
Image nabbed from here.