Using the somewhat simple thesis that we can learn from our mistakes despite our fallibility as a basis, Karl Popper developed a non-justificationist epistemology in which knowledge grows through criticizing rather than justifying our theories. However, there is much controversy among philosophers over the validity and feasibility of his non-justificationism. In this paper, I first consider the problem of the bounds of reason which, arising from justificationism, disputes Popper’s non-justificationist epistemology. Then, after examining in turn three views of rationality that are intended to solve this problem, viz. comprehensive rationalism, critical rationalism, and comprehensively critical rationalism, I argue that Popper’s non-justificationism is justified on the ground that it can solve the problem in the form of comprehensively critical rationalism. Finally, I argue that the implementation of such a non-justificationist theory means exposing to criticism various philosophical presuppositions that work against criticism. Copyright © 2007 Institute of Philosophy, Jagiellonian University.