 This bare outline of Philo’s final position is sufficient to show its novelty, and thus explain its poor reception by Antiochus and the Academics alike (see Ac. 2.11–12, 2.18). His new conception of knowledge was a radical innovation in three respects. First, given his acceptance of the standard Academic arguments against self-warranting cognitive impressions, it offered a fallibilist theory, which allows for unqualified assent to knowledge claims on the basis of impressions that might nonetheless be false. Antiochus’ criticism suggests that he defended this view by pointing out that the theoretical or counterfactual possibility of error was epistemically irrelevant in cases where the impression is in fact true and our positive evidence for it is, in practice, conclusive (see Barnes 1989). But however he justified it, Philo’s new conception of knowledge challenged the fundamental epistemological principle of the Hellenistic period that knowledge must be certain. Secondly, Philo’s new theory implied that there is in fact a good deal of knowledge. It thus constituted an apparently radical rejection of the basic thesis that all the skeptical Academics had shared, that nothing can be known. And, thirdly, Philo’s theory implied a revised principle of rationality. The mitigated skeptics had responded to the Stoic inactivity objection by identifying rationality with forming defeasible beliefs on the basis of the available evidence. But, by qualifying the assent of the rational agent as “measured” or provisional, they insulated rationality itself from practical error: whether or not the belief is false, an agent who forms it on the appropriate grounds is never mistaken in holding it provisionally. On Philo’s new position, however, rationality is compatible with outright error because it is rational to give straightforward, unqualified assent to impressions that might nonetheless be false. The Roman Books thus marked Philo’s final rejection of the joint Stoic and Academic thesis that the avoidance of error is an overriding principle of rationality.