The present study offers a reconstruction of part of the epistemological debate between Stoics and the sceptical Academy, showing that the Stoic views were in part shaped by the philosophical exchange with their sceptical opponents and refined as a result of this confrontation. It focuses on the Stoic doctrine of the cognitive impression (a kind of impression that gives us reliable information about the world) and on the challenge posed by the ‘Aparallaxia Argument’. This argument seeks to establish that there are no cognitive impressions by claiming that, for every true impression, there could be an indistinguishable false impression (Indistinguishability Thesis). The thesis is structured in six chapters. The opening chapter introduces the Stoic theory of perception and the role that rational sense-impressions play in their theory of knowledge, whilst examining how the Stoics understand sense-impressions and their contents. The second chapter presents a detailed analysis of the Aparallaxia Argument and is restricted to only one of the type of cases invoked by the Academics to support their Indistinguishability Thesis, namely cases in which the putatively indistinguishable sense-impressions are caused by pairs of objects that are plausibly perceptually indiscernible, so that the perceiver could not distinguish the impression she is entertaining from a false impression despite being in a normal state of mind. The fact that the Academics seem to have raised this challenge once they had the Stoics agree that a true impression would not be cognitive if there could be a false impression from which it could not be distinguished suggests—so I argue—that, on the Stoic view, cognitive impressions can in principle be experientially distinguished from their false counterparts insofar as they exhibit a distinctive mark that is available to the perceiver’s awareness. The third chapter is devoted to the Stoic doctrine of the cognitive sense-impression. The first sections are concerned with the claim that cognitive sense-impressions exhibit a distinctive mark in virtue of which they can in principle be experientially distinguished from false sense-impressions. On the Stoic account, it is argued, the distinguishability of cognitive sense-impressions is due to the fact that they exhibit a degree of clarity that cannot be also exhibited by their false counterparts, which is ultimately due to their different respective causal histories. In the remaining sections it is explained why the Stoics reject the Indistinguishability Thesis and the cases of perceptually indiscernible objects invoked by the Academics. These sections offer a novel reconstruction of the Stoic theory of how conceptions are activated during the process giving rise to rational sense-impressions so that the corresponding concepts come to constitute the propositional content of these impressions, which allow us to understand better the Stoic reply to the Aparallaxia Argument. The fourth chapter addresses some of the Academic attacks on Stoic metaphysics insofar as they have implications for the Stoic epistemological position. It examines some of the views the Stoics hold with respect to objects and their perceptible qualities, which gives us a better idea of the complexity of the debate and the interconnectedness of Stoic views on metaphysics and epistemology. The fifth chapter shows that the Academics pursued a variety of different argumentative strategies to attack the Stoic epistemological position. This, in turn, supports the view that the Academics were flexible with respect to the strategies they adopted and their arguments could be easily adapted depending on the context and their intended target, which sits well with the noncommittal outlook of their scepticism. Whereas some strategies target the Stoic claim that appropriately-caused true impressions exhibit a distinctive degree of clarity that makes them in principle distinguishable from false impressions, other strategies question the perceiver’s ability to notice degrees of clarity that are only subtly different. The final chapter focuses on the Stoic appeal to the notion of expertise to support their claim that any two objects can be perceptually discriminated. After arriving at the conclusion that for the Stoics cognitive impressions require some degree of expertise, the chapter addresses further Academic attacks invited by this Stoic view.