  Roughly speaking, Sextus is pointing out to the Epicureans and the Stoics, both of whom attach great weight to the deliverance of the senses, that there is no way that one’s intellect can distinguish amongst impressions those ones which are faithful to the way the world is: one only has access to one’s impressions, and not the external world, so one cannot discern which of one’s impressions are faithful to the external world, and assent to them alone. The intellect is impotent when it comes to deciding whether perceptual impressions are to be an adequate criterion. (Sextus might be misunderstanding the Stoic position here; see Frede 1983.)