 The Stoics’ epistemology was designed to accommodate their belief that it must be possible, in principle, to achieve the sort of inerrant wisdom Socrates had desired. The Stoic theory depends on three innovations (see, e.g., Cicero Ac. 1.40–2.). (1) They defined the formation of a belief as a matter of assenting to an occurrent thought or impression that something is, or is not, the case. (2) They isolated a certain kind of perceptual (and perhaps also non-perceptual) impression as “cognitive” (cataleptic), because it was self-warranting, such that assenting to these impressions constituted a “cognition” (catalepsis—lit. a “grasp”) of the state of affairs they represented. And (3) they took secure knowledge of the sort Socrates sought to be the state of an agent whose beliefs are constituted entirely by perceptual and non-perceptual cognitions. As a result, given their ethical doctrine that secure knowledge was necessary and sufficient for happiness, and since, according to (3), such knowledge could be achieved by restricting one’s assent to cognitive impressions, the Stoics identified the avoidance of opinion and error—i.e., assent to non-cognitive impressions, whether inadequately warranted (opinion) or false (error)—as the overriding principle of rationality.