Confide not to a friend every secret thou possessest. How knowest thou that he will not some time become thy foe? Inflict not every injury thou canst upon an enemy because it is possible that one day he may become thy friend.
Saadi Shirazi
http://www.poemhunter.com/poem/ch-08-on-rules-for-conduct-in-life-admonition-02/
