 By far the most influential and widespread variety of Evidentialism is epistemic (see Chisholm 1957, Adler 2002, Conee and Feldman 2004, Shah 2006). The central thesis of epistemic Evidentialism is that the norms of evidence governing belief are somehow based in the nature and aims of theoretical reason itself. To believe on insufficient evidence is at bottom an epistemic failure—a failure to use our cognitive faculties in such a way that we are likely to acquire significant knowledge and avoid significant unjustified belief. Some philosophers in this tradition also defend Locke's proportionality thesis according to which our degree of belief must be in proportion to the strength of our evidence (see White 2005). 