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The final chapter intends to revive reason or rationality as a source of the a priori in a roughly Kantian spirit. Therefore, the two notions of apriority introduced in Chapter 6 and their dynamic character are more thoroughly explained. This helps grasping the conceptual a priori and its three forms of appearance. However, concepts are not the only source of the a priori. The rational structure of reasons is so as well, as is unfolded in a series of principles. The starting point is the Basic Empiricist principle that is the core of all empiricist attitudes and that is shown to entail two coherence principles expressing something like the unity of science. Then the chapter proceeds to the fundamental issue of the truth-conduciveness of reasons and argues that the Basic Empiricist principle should be strengthened to the Basic Belief-Truth and Basic Reason-Truth Connection, which are further amended by certain stability assumptions. On the basis of Chapter 14, these stronger principles provably entail a weak principle of causality. The moral of all this is: there is substantial and rigorous theorizing about the a priori beyond the merely conceptual a priori.

Conditional ranks also allow explicating a subjective notion of (deductive or non-deductive) reason, i.e., positive relevance, which is at the base of all current notions of (theoretical) reasons. The chapter introduces and defends this explication; it distinguishes sufficient, necessary, insufficient, and supererogatory reasons (in an unconditional and a conditional version); it states the formal behavior of reasons thus explicated; it explains how such reasons can be weighed; and it shows that it is reasons precisely in this sense that drive the dynamics of belief. Moreover, it explains what maximal certainty and unrevisability mean in ranking-theoretic terms and thus provides a dynamic approach to two notions of apriority: (traditional) unrevisable apriority and so-called defeasible apriority.