 However, the claim that ancient skepticism is about belief, while modern skepticism is about knowledge, needs to be qualified. Ancient skepticism is not alone in being concerned with belief. Descartes speaks repeatedly of demolishing his opinions (for example, Med 2:12, AT 7:18; cf. Broughton, 2002, 33–61). Contemporary epistemology often pays equal attention to the notions of both belief and knowledge. The distinctive focus of ancient skepticism on belief becomes clearer once we consider a third concept that figures centrally in ancient discussions: the criterion of truth. The skeptics and their opponents discuss how one recognizes a true impression as true. Are there some evident things (some kind of impressions), which can be used as standards or criteria, so that nothing is to be accepted as true if it is not in agreement with these evident things? The Stoics and Epicureans formulate theories that conceive of such criteria. The skeptics respond critically to their proposals. Accordingly, the conception of a criterion of truth assumes as central a role in ancient debates as does the notion of knowledge in modern discussions. This debate includes in-depth analysis of sense perception and its relation to belief. According to Epicurus, all sense perceptions are true, but judgments based on these perceptions are true or false (Striker 1977, Vogt 2017). The Stoics explore differences between sense perception, illusion, and hallucination (Vasiliou 2019). Their account of the criterion of truth starts from perceptual impressions that qualify, or fail to qualify, as cognitive (Shogry 2018). The Stoics propose that we should accept only cognitive impressions, and accordingly we should only form beliefs based on a subset of true perceptual impressions.