Epicureans say that all sense-impressions are true and that reason is based on sense-impressions. But lots of people believe that not all sense-impressions are true. The Epicureans explain why people might hold this false belief as follows: the belief at the root of such mistakes is often the belief that sense impressions can conflict. But this latter belief is often the result of what the Epicureans think is our natural and often helpful tendency to generalise and extrapolate in forming beliefs on the basis of our sense-impressions.