Perhaps the most distinctive element of Chisholm’s theory of knowledge is a set of tightly linked epistemic principles. These serve to display the relation between directly evident foundational knowledge and indirectly evident beliefs about the external world, the past, and other matters about which we can have knowledge. In his doctoral dissertation, Chisholm made his earliest attempt to formulate a set of such principles. [PPS] contains a more sophisticated system of principles. In the three editions of [TK] Chisholm presented even more carefully worked out sets of proposed epistemic principles.