ut 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 mentione