In philosophy, acatalepsy (from the Greek ἀκαταληψία "inability to comprehend" from alpha privative and καταλαμβάνειν, "to seize") is incomprehensibleness, or the impossibility of comprehending or conceiving a thing. It is the antithesis of the Stoic doctrine of katalepsis (i.e., the ability to apprehend). According to the Stoics, katalepsis was true perception, but to the Pyrrhonists and Academic Skeptics, no perception could be known to be true. All perceptions were thus acataleptic, i.e. what, if any, conformity between the object and the perception of that object was unknown and, for the Academic Skeptics, could never be known.