V. There is a certain political science which is made up of many and important particulars. A very great and extensive portion of it is artificial eloquence, which men call rhetoric. For we do not agree with those men who think that the knowledge of political science is in no need of and has no connexion with eloquence; and we most widely disagree with those, on the other hand, who think that all political ability Is comprehended under the skill and power of a rhetorician. On which account we will place this oratorical ability in such a class as to assert that it is a part of political science. But the duty of this faculty appears to be to speak in a manner suitable to persuading men; the end of it is to persuade by language. And there is difference between the duty of this faculty and its end; that with respect to the duty we consider what ought to be done; with respect to the end we consider what is suitable to the duty. Just as we say, that it is the duty of a physician to prescribe for a patient in a way calculated to cure him; and that his end is to cure him by his prescriptions. And so we shall understand what we are to call the duty of an orator, and also what we are to call his end; since we shall call that his duty which he ought to do, and we shall term that his end for the sake of which he is bound to do his duty. We shall call that the material of the art, on which the whole art, and all that ability which is derived from art, turns. Just as if we were to call diseases and wounds the material of medicine, because it is about them that all medical science is concerned. And in like manner, we call those subjects with which oratorical science and ability is conversant the materials of the art of rhetoric. And these subjects some have considered more numerous, and others less so. For Gorgias the Leontine, who is almost the oldest of all rhetoricians, considered that an orator was able to speak in the most excellent manner of all men on every subject. And when he says this he seems to be supplying an infinite and boundless stock of materials to this art. But Aristotle, who of all men has supplied the greatest number of aids and ornaments to this art, thought that the duty of the rhetorician was conversant with three kinds of subjects; with the demonstrative, and the deliberative, and the judicial.