The truth of all sensations

The first criterion of truth is sensation. But sceptics cast doubt on the veridicality of the senses, often by bringing up examples of conflicting appearances. The same wall that seems white to me will (supposedly) appear yellow to a man with jaundice. An oar seems bent when stuck partway under water but straight after being pulled out, and a tower seems round in the distance but square close up. The common-sense position is that some of these sensations are true, others false. Since the wall is really white, the oar really straight and the tower really square, the sensations that report these things are true, while the ones that report otherwise are false. But the sceptical response to this common-sense position is that we have no criterion to distinguish reliably between true and false sensations. And since we have no good way of deciding which sensations are true and which are false, we should suspend judgement.

Although Epicurus himself did not confront the sceptical challenge as formulated in precisely this way – it was put this way by later sceptics, the Academics and the Pyrrhonians – the general problem of conflicting appearances was featured prominently in earlier disputes about the reliability of the senses, and many people concluded on the basis of sensory variability that the senses could not be relied on.