From Chapters 15 through 23 of The Prince, what sentence from the text can I use to best describe this part of the book?
The Prince by Machiavelli is a treatise about establishing and holding political power during the era of princes. It is topically divided into four sections although there are no structural headings or subheadings. It opens with a "Dedicatory Letter" to Lorenzo de Medici of Florence. The second section comprising Chapters 15 through 23 is the famous section in which Machiavelli upturns the ancient philosophy advocating moral greatness in leaders, thereby breaking with Aristotle and Plato. In contrast to this philosophy (not uniformly adhered to throughout antiquity), Machiavelli advocates the appearance, as opposed to fact, of moral qualities. Therefore, perhaps the best single sentence from the text of Chapters 15 through 23 to describe the third section of The Prince would be one Machiavelli writes in Chapter 18 (XVIII):
And I shall dare to say this also, that to have [the five moral qualities] and always to observe them is injurious, and that to appear to have them is useful; to appear merciful, faithful, humane, religious, upright, and to be so, but with a mind so framed that should you require not to be so, you may be able and know how to change to the opposite.