This chapter looks at a third how-possible question, namely: (HP _apk ). How is a priori knowledge possible? If how-possible questions are obstacle-dependent, the key to making any progress with (HP _apk ) is to identify the obstacles to the acquisition or existence of a priori knowledge which give this question its bite. Having identified these obstacles, the prospect for a multi-levels response to (HP _apk ) can then be examined. Predictably, the basic idea of this chapter is that such a response promises to cast at least as much light on (HP _apk ) as on other epistemological how-possible questions. The chapter identifies a means of coming to know things a priori, shows how obstacles to the acquisition of a priori knowledge by the suggested means can be overcome or dissipated, and considers whether it is either possible or necessary to give a substantive account of what makes it possible to come to know things a priori by these means.
Keywords: reflection, how-possible question, reasoning, calculation, idealism, explanation, rationalism