The "Art" of Rhetoric (1.2.12-1.2.16)

Now, it is possible to draw conclusions and inferences partly from what has been previously demonstrated syllogistically, partly from what has not, which however needs demonstration, because it is not probable.[*] The first of these methods is necessarily difficult to follow owing to its length, for the judge is supposed to be a simple person; the second will obtain little credence, because it does not depend upon what is either admitted or probable. The necessary result then is that the enthymeme and the example are concerned with things which may, generally speaking, be other than they are, the example being a kind of induction and the enthymeme a kind of syllogism, and deduced from few premises, often from fewer than the regular[*] syllogism; for if any one of these is well known, there is no need to mention it, for the hearer can add it himself. For instance, to prove that Dorieus[*] was the victor in a contest at which the prize was a crown,

But since few of the propositions of the rhetorical syllogism are necessary, for most of the things which we judge and examine can be other than they are, human actions, which are the subject of our deliberation and examination, being all of such a character and, generally speaking, none of them necessary; since, further, facts which only generally happen or are merely possible can only be demonstrated by other facts of the same kind, and necessary facts by necessary propositions (and that this is so is clear from the Analytics[*]), it is evident that the materials from which enthymemes are derived will be sometimes necessary, but for the most part only generally true; and these materials being probabilities and signs, it follows that these two elements must correspond to these two kinds of propositions, each to each.[*]

For that which is probable is that which generally happens, not however unreservedly, as some define it, but that which is concerned with things that may be other than they are, being so related to that in regard to which it is probable

As to signs, some are related as the particular to the universal, others as the universal to the particular. Necessary signs are called tekmeria; those which are not necessary have no distinguishing name.