 The principle of the prima facie credibility of mnemic presentation of past experience can’t itself be justified inductively for Lewis, on pain of circularity. Nor did he think it is simply a postulate — something we have to assume for empirical knowledge to be possible. Instead, he argued that it is constitutive of the lived world of experience and something for which there is no meaningful alternative. Sceptical alternatives designed to undermine the principle are ones that are inaccessible to knowledge and thus ones for which there is no criterion in experience. So it is an “analytic statement” that the past is knowable, and a similar claim was made for the relevance of past experience to the future, and thus for the knowability of empirical reality. The philosophical problem for Lewis (1946, 360–2) is to formulate correctly the criteria that “delimit empirical reality and explicate our sense of it”. 