The first and most necessary place (part) in philosophy is the use of theorems (precepts), 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.