For much of the modern period, theologians and philosophers of religion have struggled with the problem of proving that it is rational to believe in God. Drawing on the thought of Thomas Aquinas, this book lays the foundation for an innovative effort to overturn the longstanding problem of proving faith's rationality, and to establish instead that rationality requires to be explained by appeals to faith. To this end, Schumacher advances the constructive argument that rationality is not only an epistemological question concerning the soundness of human thoughts, which she defines in terms of 'intellectual virtue'. Ultimately, it is an ethical question whether knowledge is used in ways that promote an individual's own flourishing and that of others. That is to say, rationality in its paradigmatic form is a matter of moral virtue, which should nonetheless entail intellectual virtue. This conclusion sets the stage for Schumacher's argument in a companion book, Theological Philosophy, which explains how Christian faith provides an exceptionally robust rationale for rationality, so construed, and is intrinsically rational in that sense.