Lewis's account of the role of sensory experience in empirical knowledge rests on the theses: (1) that one's apprehension of what is given in sensory experience is certain; (2) that unless there were such certain apprehension of the given, No knowledge would be possible; (3) that justification of one's other justified empirical beliefs always derives from one's apprehension of the given. I show that all three theses are false. That they are false provides further motivation for the theory of justification (...) I call "foundherentist", Which is fallibilist and permits mutual support (see my "theories of knowledge; an analytic framework", "proceedings of the aristotelian society, Lxxxiii", 1982-3). (shrink)