But the comparison between things like or equal to each other has no elation or submission; for it is on equal terms:  but there are many things which are compared on account of their very equality; which are usually concluded in this manner:  “If to assist one’s fellow-citizens with counsel and personal aid deserves equal praise, those men who act as counsellors ought to enjoy an equal glory with those who are the actual defenders of a state.”  But the first premiss is certainly the case; therefore so must the consequent be.

Every rule necessary for the discovery of arguments is now concluded; so that as you have proceeded from definition, from partition, from observation, from words connected with one another, from genus, from species, from similarity, from difference, from contraries, from accessories, from consequents, from antecedents, from things inconsistent with one another, from causes, from effects, from a comparison with greater, or lesser, or equal things,—­there is no topic of argument whatever remaining to be discovered.

XIX.  But since we originally divided the inquiry in such a way that we said that other topics also were contained in the very matter which was the subject of inquiry; (but of those we have spoken at sufficient length:) that others were derived from external subjects; and of these we will say a little; although those things have no relation whatever to your discussions.  But still we may as well make the thing complete, since we have begun it.  Nor are you a man who take no delight in anything except civil law; and since this treatise is dedicated to you, though not so exclusively but that it will also come into the hands of other people, we must take pains to be as serviceable as possible to those men who are addicted to laudable pursuits.

This sort of argumentation then which is said not to be founded on art, depends on testimony.  But we call everything testimony which is deduced from any external circumstances for the purpose of implanting belief.  Now it is not every one who is of sufficient weight to give valid testimony; for authority is requisite to make us believe things.  But it is either a man’s natural character or his age which invests him with authority.  The authority derived from a man’s natural character depends chiefly on his virtue; but on his age there are many things which confer authority; genius, power, fortune, skill, experience, necessity, and sometimes even a concourse of accidental circumstances.  For men think able and opulent men, and men who have been esteemed during a long period of their lives, worthy of being believed Perhaps they are not always right; but still it is not easy to change the sentiments of the common people; and both those who form judgments and those who adopt vague opinions shape everything with reference to them.  For those men who are eminent for those qualities which I have mentioned, seem to be eminent for virtue itself.  But in the other circumstances also which I have just enumerated, although there is in them no appearance of virtue, still sometimes belief is confirmed by them, if either any skill is displayed,—­for the influence of knowledge in inspiring belief is very great; or any experience—­for people are apt to believe those who are men of experience.