EPICTETUS. DISCOURSES. Book ii. §24. ¶1.
DO we know, then, what man is? What is his nature; what our idea of him is; and how far our ears are open in respect to this matter? Nay, do you understand what nature is; or are you able, and in what degree, to comprehend me, when I come to say, "But I must use demonstration to you"? How should you? Do you comprehend what demonstration is; or how a thing is demonstrated, or by what methods; or what resembles a demonstration, and yet is not a demonstration? Do you know what true or false is? What is consequent to a thing, and what contradictory? Or unsuitable, or dissonant?