For the Stoics, as for most ancient philosophers who reflected on signs, signs were examined as part of a theory of inference. A sign was a symptom, and the medical usage is often overt in examples; or a sign is a premise of an argument, from which something unknown can be inferred. For the Stoics, though, this went beyond the realm of logic into the realm of epistemology: Signs were means of disclosing knowledge, mechanisms for the advance of knowledge. In “If it is day, then it is light” the day does not qualify as a sign, because both day and light are known by immediate experience. Signs move from the evident to the hidden. As Giovanni Manetti puts it in his book on classical theories of the sign (which he begins, importantly, with chapters on Mesopotamian and Greek divination), “the sign must enable the inferential passage from an easily accessibly piece of knowledge, for example, ‘s/he has brought up bronchial cartilage,’ to much less accessibly knowledge, such as, in this example, ‘s/he has a lung wound.’” The theory of signs moves beyond medical practice in attempting to provide “a solid logical-formal structure which serves to exclude any possibility of false inferences.”

This background clarifies the achievement of Augustine’s (by all accounts original) development of a sign-theory of language. When he brought language under the heading of signs, Augustine brought this accepted notion of signification with him: Signs enable transition from the known to the unknown; if language is a system of signs, language discloses what is hidden. Marcia Colish sums the effect: “a twofold linguistic transformation was in order: the faculty of human speech was to be recast as a Pauline mirror, faithfully mediating God to man in the present life; and the agencies appoint for the translation of man’s partial knowledge by faith into his complete knowledge of God by direct vision were to be redefined as modes of verbal expression.” Colish calls this a “new covenant between speech and knowledge.”