Abstract: To be justified, a belief must be ‘based’ on its grounds. For an externalist, being based on grounds is being ‘caused’ by these grounds (by a non-deviant route). For an internalist (normally), a belief is said to be based on basic beliefs. But then, the believer would need to believe that his belief is caused and/or rendered probable by the basic beliefs; and then maybe actual causing is not even necessary. Further, basic beliefs need to be understood rather as basic propositions that come to the believer with different degrees of prior probability, which measure their initial strengths—only some of these are sufficiently strong to form beliefs. Some actual basic propositions are, for a priori reasons, not rightly basic (e.g. because they are beliefs in a logically impossible proposition). But in general (following the Principle of Credulity), a basic proposition, of whatever strength, is (to that strength) rightly basic.

Keywords: basic belief, belief, externalism, grounded belief, internalism, Lehrer, Plantinga, Principle of Credulity,