In his Meditations, Descartes (1596-1650) proposed discarding any kind of belief that could be doubted, that might be false. Initially, he was inclined to doubt all the evidences of his senses (pointing out that it seemed impossible to tell for sure whether he was at any point aswake or asleep). The doubt that Descartes introduced into philosophy has been a characteristic feature as many philosophers since have supposed that we have no secure rational basis for believing in the existence of a world external to our sense experience, etc. See the Private Language Argument.