The first and most necessary topic in philosophy is that of the use of moral theorems, such as, "We ought not to lie;" the second is that of demonstrations, such as, "What is the origin of our obligation not to lie;" the third gives strength and articulation to the other two, such as, "What is the origin of this is a demonstration." For what is demonstration? What is consequence? What contradiction? What truth? What falsehood? The third topic, then, is necessary on the account of the second, and the second on the account of the first. But the most necessary, and that whereon we ought to rest, is the first. But we act just on the contrary. For we spend all our time on the third topic, and employ all our diligence about that, and entirely neglect the first. Therefore, at the same time that we lie, we are immediately prepared to show how it is demonstrated that lying is not right.
Epictetus, Enchiridion #51