Diogenes of Sinope | Epictetus, Discourses, 3.21
<blockquote>You must approach the task in another fashion: it is momentous and full of mystery, not a chance gift which any one can command. The care of the young demands, it may be, more than wisdom: yes, by Zeus, one must have a certain readiness and special fitness, and a certain habit of body, and above all the counsel of God advising one to discharge this duty, as He counselled Socrates to examine men, and Diogenes to rebuke men in royal fashion, and Zeno to instruct and lay down precepts. You open a doctor's consulting-room with nothing but some drugs, without ever taking the trouble to acquire a knowledge of when or how they are applied. 'See, that's his remedy, eye-salve' (you say): 'I have that too.' Have you also the faculty of using it? Do you know when and how and to whom it will do good? Why then do you play at hazard with matters of highest moment, why are you reckless, why do you take in hand a task unsuited to your powers? Leave it to those who can do it and do it with distinction.
Source: The Discourses of Epictetus, tr. by P.E Matheson, [1916]</blockquote>