The Discourses of Epictetus, tr. by P.E Matheson, , at sacred-texts.com
When one of his audience said, 'Convince me that logic is useful,' he said,
Would you have me demonstrate it?
Well, then, must I not use a demonstrative argument?
And, when the other agreed, he said, How then shall you know if I impose upon you? And when the man had no answer, he said, You see how you yourself admit that logic is necessary, if without it you are not even able to learn this much—whether it is necessary or not.