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.