And let us discuss the two first divisions very briefly.  For it is not so much that they have not an important reference to the highest perfection, as that they are indispensable, and almost common to other studies also.  For to plan and decide on what you will say are important points, and are as it were the mind in the body; still they are parts of prudence rather than of eloquence; and yet what matter is there in which prudence is not necessary?  This orator, then, whom we wish to describe as a perfect one, must know all the topics suited to arguments and reasons of this class.  For since whatever can possibly be the subject of any contest or controversy, gives rise to the inquiry whether it exists, and what it is, and what sort of thing it is; while we endeavour to ascertain whether it exists, by tokens; what it is, by definitions; what sort of thing it is, by divisions of right and wrong; and in order to be able to avail himself of these topics the orator,—­I do not mean any ordinary one, but the excellent one whom I am endeavouring to depict,—­always, if he can, diverts the controversy from any individual person or occasion.  For it is in his power to argue on wider grounds concerning a genus than concerning a part; as, whatever is proved in the universal, must inevitably be proved with respect to a part.  This inquiry, then, when diverted from individual persons and occasions to a discussion of a universal genus, is called a thesis.  This is what Aristotle trained young men in, not after the fashion of ordinary philosophers, by subtle dissertations, but in the way of rhetoricians, making them argue on each side, in order that it might be discussed with more elegance and more copiousness; and he also gave them topics (for that is what he called them) as heads of arguments, from which every sort of oration might be applied to either side of the question.

XV.  This orator of ours then (for what we are looking for is not some declaimer out of a school, or some pettifogger from the forum, but a most accomplished and perfect orator), since certain topics are given to him, will run through all of them; he will use those which are suitable to his purpose according to their class; he will learn also from what source those topics proceed which are called common.  Nor will he make an imprudent use of his resources, but he will weigh everything, and make a selection.  For the same arguments have not equal weight at all times, or in all causes.  He will, therefore, exercise his judgment, and he will not only devise what he is to say, but he will also weigh its force.  For there is nothing more fertile than genius, especially of the sort which has been cultivated by study.  But as fertile and productive corn-fields bear not only corn, but weeds which are most unfriendly to corn, so sometimes from those topics there are produced arguments which are either trifling, or foreign to the subject, or useless; and the judgment of the orator has great room to exert itself in making a selection