1.17.1
Since reason is the faculty which analyses and perfects the rest, and it ought itself not to be unanalysed, by what should it be analysed? for it is plain that this should be done either by itself or by another thing. Either then this other thing also is reason, or something else superior to reason; which is impossible. But if it is reason, again who shall analyse that reason? For if that reason does this for itself, our reason also can do it. But if we shall require something else, the thing will go on to infinity and have no end. Reason therefore is analysed by itself. Yes: but it is more urgent to cure (our opinions ) and the like. Will you then hear about those things? Hear. But if you should say, "I know not whether you are arguing truly or falsely," and if I should express myself in any way ambiguously, and you should say to me, "Distinguish," I will bear with you no longer, and I shall say to you, "It is more urgent." This is the reason, I suppose, why they (the Stoic teachers) place the logical art first, as in the measuring of corn we place first the examination of the measure. But if we do not determine first what is a modius, and what is a balance, how shall we be able to measure or weigh anything?