In epistemology, Chisholm took as his fundamental primitive concept the idea we express when we say that believing p is more reasonable for S than believing q would be. He tried to define other terms of epistemic evaluation by appeal to this one. Furthermore, he assumed that we in fact do know many of the things we take ourselves to know. He thought that a person can be justified in believing many foundational propositions about his own current mental state. He took one of the central projects of epistemology to be the formulation of epistemic principles that would show how we become justified in believing things about the external world, the past, and other problematic things on the basis of this present, internal, self-oriented foundation.