  The focus of Arcesilaus’ attack on Zeno’s theory was its centerpiece, the theory of cognitive impressions. Zeno defined a cognitive impression as one that comes from what is, is stamped and impressed exactly in accordance with it, and is such that it could not be false (Sextus M. 7.248, Cicero Academica 2.77). This means, roughly, that an impression is cognitive if and only if [a] its propositional content is true, [b] it is caused in the appropriate way for correctly representing its object, and [c] its truth is thus warranted by the inimitable richness and detail of the representational character guaranteed by its causal history—such that [a] is entailed by [b]. Arcesilaus’ tactic was to grant that conditions [a] and [b] are often met, as Zeno claimed, but to argue that condition [c] never obtained (M. 7.154, Academica 2.77).  Although his detailed arguments for this have not survived, it is fairly clear from later Academic and Stoic arguments that he followed two main lines of attack. One line depended on the existence of indistinguishable—or, at any rate, indiscernibly distinct—objects, such as twins, or pairs of eggs, manufactured items (statues or impressions on wax of the same letter-seal), and grains of sand (Academica 2.54–8 &amp; 2.84–6, M.  7.408–10). Any of these could be mistaken for another no matter how good one’s impression of it was. The second depended on abnormal states of mind, such as dreams, illusions, and fits of madness (Academica 2.47–53 &amp; 2.88–90, M.  7.402–8). In either case, Arcesilaus argued that, whether the nature of the objects or of our minds is at fault, it is always possible to have a false impression with exactly the same propositional content and representational character as a true one that meets condition [b]. But if so, no impression can be self-warranting in virtue of the way in which its content is represented. So condition [c] never obtains. Hence, on the Stoic view of the requirements for cognition, there is no cognition. And if knowledge is derived entirely from perceptual cognition and concepts warranted by it, as Zeno supposed, it follows that nothing can be known. (Intricate Stoic-Academic debates on these issues lasted for another 150 years; they can best be traced through the arguments of Chrysippus and Carneades, preserved in some detail in Cicero’s Academica and Sextus M. 7.)