I call those necessary signs from which a logical syllogism can be constructed, wherefore such a sign is called tekmērion ; for when people think that their arguments are irrefutable, they think that they are bringing forward a tekmērion , something as it were proved and concluded; for in the old language tekmar and peras have the same meaning (limit, conclusion).
 Among signs, some are related as the particular to the universal; for instance, if one were to say that all wise men are just, because Socrates was both wise and just. Now this is a sign, but even though the particular statement is true, it can be refuted, because it cannot be reduced to syllogistic form. But if one were to say that it is a sign that a man is ill, because he has a fever, or that a woman has had a child because she has milk, this is a necessary sign. This alone among signs is a tekmērion ; for only in this case, if the fact is true, is the argument irrefutable. Other signs are related as the universal to the particular, for instance, if one were to say that it is a sign that this man has a fever, because he breathes hard; but even if the fact be true, this argument also can be refuted, for it is possible for a man to breathe hard without having a fever. We have now explained the meaning of probable, sign, and necessary sign, and the difference between them; in the Analytics Aristot. Pr. Anal. 2.27 . we have defined them more clearly and stated why some of them can be converted into logical syllogisms, while others cannot. We have said that example is a kind of induction and with what kind of material it deals by way of induction. It is neither the relation of part to whole, nor of whole to part, nor of one whole to another whole, but of part to part, of like to like, when both come under the same genus, but one of them is better known than the other. For example, to prove that Dionysius is aiming at a tyranny, because he asks for a bodyguard, one might say that Pisistratus before him and Theagenes of Megara did the same, and when they obtained what they asked for made themselves tyrants. All the other tyrants known may serve as an example of Dionysius, whose reason, however, for asking for a bodyguard we do not yet know. All these examples are contained under the same universal proposition, that one who is aiming at a tyranny asks for a bodyguard. We have now stated the materials of proofs which are thought to be demonstrative. But a very great difference between enthymemes has escaped the notice of nearly every one, although it also exists in the dialectical method of syllogisms. For some of them belong to Rhetoric, some syllogisms only to Dialectic, and others to other arts and faculties, some already existing and others not yet established. Hence it is that this escapes the notice of the speakers, and the more they specialize in a subject, the more they transgress the limits of Rhetoric and Dialectic. But this will be clearer if stated at greater length. I mean by dialectical and rhetorical syllogisms those which are concerned with what we call topics, which may be applied alike to Law, Physics, Politics, and many other sciences that differ in kind, such as the topic of the more or less, which will furnish syllogisms and enthymemes equally well for Law, Physics, or any other science whatever, although these subjects differ in kind. Specific topics on the other hand are derived from propositions which are peculiar to each species or genus of things; there are, for example, propositions about Physics which can furnish neither enthymemes nor syllogisms about Ethics, and there are propositions concerned with Ethics which will be useless for furnishing conclusions about Physics; and the same holds good in all cases. The first kind of topics will not make a man practically wise about any particular class of things, because they do not deal with any particular subject matter; but as to the specific topics, the happier a man is in his choice of propositions, the more he will unconsciously produce a science quite different from Dialectic and Rhetoric. For if once he hits upon first principles, it will no longer be Dialectic or Rhetoric, but that science whose principles he has arrived at. The common topics do not deal with particular subject matter, as the specific topics do. In making use of the latter, the better (that is, in regard to a special science) the propositions chosen by a man, the more he will without knowing it quit the domain of Rhetoric and Dialectic, and become a professor of that special science whose first principles he has hit upon.