 However, the Stoics do not maintain that the mere having of a cognitive impression constitutes knowledge (epistêmê). Indeed, not even assent to such an impression amounts to knowledge. Such assent is merely cognition or grasp (katalêpsis) of some individual fact. Real knowledge (epistêmê) requires cognition which is secure, firm and unchangeable by reason (Sextus Empiricus, 41C) – and, furthermore, worked into a systematic whole with other such cognitions (Arius Didymus, 41H). Weak and changeable assent to a cognitive impression is only an act of ignorance. It is not entirely clear where opinion or belief in general (doxa) stands in this categorization. Most Stoic sources define it as ‘assent to the incognitive’ (i.e. to an impression that does not firmly grasp its object) (see Sextus Empiricus, 41E) but some suggest that changeable assent to a cognitive impression might still count as opinion. There is a potential for serious confusion when we try to assimilate the Stoic view to contemporary epistemology. Modern definitions of knowledge make the agent’s belief that P a necessary but not sufficient condition for knowing that P. For the Stoics, doxa (involving ‘weak’ assent) and knowledge are incompatible. In any event, there is an absolute distinction between the wise and the ignorant. Only the Stoic sage’s assent to cognitive impressions clearly counts as knowledge for only a sage has the proper discipline always to avoid withdrawing assent, or assenting to things that one shouldn’t. The Stoics call this epistemic virtue ‘non-precipitancy’ (aproptôsia) and it underlies their claim that the Stoic sage never makes mistakes (41D).