Aristotle, Rhetoric
J. H. Freese, Ed.

[18] 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 Analytics¹ we have defined them more clearly and stated why some of them can be converted into logical syllogisms, while others cannot.