 It is not difficult to see how this shift in the understanding of persuasive impressions might lead to a re-evaluation of the conception of rationality shared by Stoics and, at least in part, by the radical skeptics. Clitomachus had agreed with the Stoics that avoiding error is a necessary condition for rationality, and consequently, given the apparent unavailability of knowledge, advocated suspending assent universally. But once Philo had begun to use persuasiveness as a rational method for evaluating impressions, the radically skeptical position started to look more like an abdication of rationality than its paradigm. Granted that nothing can be known—a view the mitigated skeptics continued to find extremely persuasive (see Cicero Ac. 2.148)—it still seems that we are rationally obliged to form the best beliefs we can, given the evidence available to us. And while this will involve the possibility of error, the refusal to use the evidence we have seems absurd when it is generalized to every case—and immoral in the case of ethics, since without some regulatory beliefs, it is unlikely that our lives could be virtuous or happy (see  section 4).