Galen is committed to the possibility of secure knowledge of the physical world. In The Best Teaching (Opt. Doc.), in response to contemporary Greek epistemological debates and in particular to Academic Skepticism, he rejects arguments that encourage us to doubt one’s natural evaluative faculty (kritērion), or to advance arguments on both sides of any proposition in order to reach suspension of judgement; he also attacks the Stoic terminology of the “apprehensive” or “kataleptic” (katalēptikē) impression, a kind of cognitive experience which is so clear as to be self-guaranteeing. (But he seems to regard the distinction between “kataleptic” and his preferred term, “securely knowable”, as purely verbal, rather than of philosophical interest. Relatedly, he mentions, but does not seem to feel a serious philosophical challenge from, the Skeptical objection that one may believe one has a reliable perception also in dreams or in mentally deranged states.)