The paper examines the meaning of “sign inferences”. First the reader will be reminded that sign inferences were used in all philosophical schools, but the meaning of “sign inference” is different in the various schools. After examining Quintilianus’ text one can come to the conclusion that he spoke of sign inferences in terms of the Aristotelian logic. In Aristotelian logic sign inferences were used to the effect of conviction, but, from point of view of logic, they were not valid. Thus in rhetoric Aristotle intentionally admitted invalid proofs as “rhetoric arguments”.