  Stoic epistemology attempts to show how it is possible for human beings to attain wisdom, which the Stoics take to be a condition entirely free of opinion, i.e., false or insecure judgment (see the  section on Logic in the entry on  Stoicism).  For this to be possible, they maintain, there must be a criterion of truth (see Striker 1990). In their theory, it is what they called a cognitive impression that plays this part.  This 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 (cf. Annas 1990, Frede 1999, Sedley 2002). Cognitive impressions, then, are true impressions which are, in addition, distinguished by a special character belongs only to true impressions (though not to all of them) and which enables human beings to discriminate cognitive from non-cognitive impressions. In the paradigm case, which the definition has in view, cognitive impressions are perceptual, but in a broader sense of the term, non-perceptual impressions that afford an equally secure grasp of their contents can also be called cognitive. According to the Stoics, by confining assent to cognitive impressions, it is possible to avoid error entirely.