  The Stoic theory of knowledge represented a radical shift in epistemology, since it offered an empirically-based route to the kind of wisdom Socrates had sought (see Frede 1999). Its basis was three novel claims made by Zeno, the founder of the Stoa (see Cicero Academica 1.40–2). First, Zeno proposed a new psychological theory: to form a belief of any kind is to give one’s assent to one’s ‘impression’ (or ‘appearance’: ‘phantasia’ in Greek) about the matter. Secondly, he claimed that some of our perceptual impressions are ‘cognitive’ or self-warranting, so that assenting to them constitutes a cognition or apprehension (‘katalêpsis’) of their objects. And, thirdly, he argued that we ought to restrict our assent to just cognitive impressions, since it is contrary to reason to form ‘opinions’—that is, mere beliefs whether true or false—by assenting to inadequately warranted, non-cognitive impressions. But, given that there are cognitive impressions, we can attain infallible knowledge or wisdom by restricting our assent to them, since our thoughts will then be constituted entirely by cognitions derived from perception or from concepts warranted by perceptual cognitions.