 Perhaps the most famous topic considered under the Stoic heading of logic is that of the criterion of truth and the Stoics’ disputes with the skeptical New Academy about it. According to Chrysippus, the criterion of truth is the ‘cognitive impression’ (phantasia katalêptikê, lit. an impression that firmly grasps its object) (Diog. Laert., 40A). A criterion or canon of truth is an instrument for definitely determining that something is true, and the Hellenistic schools all provide some view on how it is that we are to measure or evaluate whether something is true or not. The Stoics’ cognitive impression is an impression which (according to Zeno’s definition, cf. Cicero, SVF I.59) “arises from that which is; is stamped and impressed in accordance with that very thing; and of such a kind as could not arise from what is not” (Sextus Empiricus, 40E). Recall that among the powers of the commanding faculty is the capacity to assent or withhold assent to impressions. The fact that it is always within our power to withhold assent means that if we are sufficiently disciplined, we are capable of avoiding error. In itself, it does not mean that we are capable of attaining knowledge, for there might not be any impressions that one can be confident in assenting to. The cognitive impression was supposed to fill that role: when you experience one of these, provided that you recognize it as such, you can, on its basis, assert definitely that the matter in question is true. It was initially supposed that such an impression commanded one’s assent by its very nature: it “all but seizes us by the hair” and drags us to assent. But this optimistic assessment seems to have been qualified in the face of criticism by members of the Skeptical Academy – perhaps, even if there are such impressions, it is not so easy to be sure when one is experiencing one.