 The principal target of the arguments was the epistemology of the Stoics, and it was out of their engagement on that front that the form of the skeptical position defended by the Academics first emerged. The Stoics maintained that it is possible for human beings to achieve a condition of wisdom entirely free of opinion, i.e., false or insecure belief. According to them, all of a wise human being’s judgments are instances of knowledge, based on an unshakable grasp of the truth. Knowledge of this kind was possible according to them because there exist so-called cognitive impressions (katalêptikai phantasiai), which function as the criterion of truth (cf. Frede, 1999; Sedley, 2002). A cognitive impression they define as an impression from what is, stamped and impressed in exact accordance with what is, and such as could not be from what is not. This seems to mean, in the paradigm case of a perceptual impression, that it is an impression that arises in conditions which: