Enchiridion 52

The first and most necessary division in philosophy is that which has to do with the application of the principles, as, for example, Do not lie. The second deals with the demonstrations, as, for example. How comes it that we ought not to lie? The third confirms and discriminates between these processes, as, for example, How does it come that this is a proof? For what is a proof, what is logical consequence, what contradiction, what truth, what falsehood? Therefore, the third division is necessary because of the second, and the second because of the first; while the most necessary of all, and the one in which we ought to rest, is the first. But we do the opposite; for we spend our time in the third division, and all our zeal is devoted to it, while we utterly neglect the first. Wherefore, we lie, indeed, but are ready with the arguments which prove that one ought not to lie.

Epictetus, Enchiridion 52, translated by W. A. Oldfather

The first and most necessary place (part, τόπος) in philosophy is the use of theorems (precepts, θεωρήματα), for instance, that we must not lie: the second part is that of demonstrations, for instance, How is it proved that we ought not to lie: the third is that which is confirmatory of these two and explanatory, for example, How is this a demonstration? For what is demonstration, what is consequence, what is contradiction, what is truth, what is falsehood? The third part (topic) is necessary on account of the second, and the second on account of the first; but the most necessary and that on which we ought to rest is the first. But we do the contrary. For we spend our time on the third topic, and all our earnestness is about it: but we entirely neglect the first. Therefore we lie; but the demonstration that we ought not to lie we have ready to hand.

Epictetus, Enchiridion 52, translated by George Long

The first and most necessary department of philosophy deals with the application of principles; for instance, 'not to lie'. The second deals with demonstrations; for instance, 'How comes it that one ought not to lie?' The third is concerned with establishing and analysing these processes; for instance, 'How comes it that this is a demonstration? What is demonstration, what is consequence, what is contradiction, what is true, what is false?' It follows then that the third department is necessary because of the second, and the second because of the first. The first is the most necessary part, and that in which we must rest. But we reverse the order: we occupy ourselves with the third, and make that our whole concern, and the first we completely neglect. Wherefore we lie, but are ready enough with the demonstration that lying is wrong.

Epictetus, Enchiridion 52, translated by P. E. Matheson