 According to the Stoics, by restricting one’s assent in the perceptual sphere to impressions of this kind, one can avoid ever assenting to a false perceptual impression, a key necessary condition if one is to attain knowledge. As the criterion of truth, cognitive impressions are the ultimate basis of all knowledge, by confining one’s assent to which, and meeting certain further conditions, the Stoics maintained, one can avoid error and opinion entirely.