The Realm of Reason is a manifesto for a new rationalism in philosophy. Christopher Peacocke develops an original theory of what makes a thinker entitled to form a given belief. The theory is articulated in three principles of rationalism, which together imply that all entitlement has an element that is independent of experience. Peacocke elaborates this rationalism in detail for the classical issues of perceptual knowledge, induction, and the status of moral thought. Hisnew generalized approach to epistemology has applications throughout philosophy, and it will interest all concerned with knowledge, truth, and rationality.