 It is tempting, for modern readers, to divide Aquinas’s work into the theological and philosophical, but lines here are difficult to draw. The medieval university did formally distinguish between the higher-level discipline of theology and the undergraduate arts faculty, whose curriculum consisted largely in the works of Aristotle. But many of the questions of the theologians—such as the workings of intellect, the nature of freedom, the systematization of ethical theory—are now seen as paradigmatically philosophical. At the same time, much of Aristotle, and therefore much of the arts faculty curriculum, looks to us more scientific than philosophical. Aquinas, unlike his teacher Albert the Great, only occasionally dwelled on what we now think of as scientific questions. Conversely, as a professor of theology, he considered at great length various matters that depend on  divine revelation,  such as the nature of the Trinity and the character of Christ’s humanity. A complete understanding of Aquinas’s thought thus requires spending considerable time with such properly theological questions. At the same time, it is appropriate today to number Aquinas among the philosophers, because he was particularly interested in and had particularly innovative things to say about what we now think of as philosophical questions. The tremendous energy he put into commenting on Aristotle’s works is a testimony both to the importance he gives philosophy for a proper understanding of theology, and to his confidence that progress in philosophy will only benefit the Christian faith. As he famously wrote, “if anything is found in the words of the philosophers that is contrary to the faith, this is not philosophy but rather an abuse of philosophy, due to a failure of reason” (Comm. Boethius De trinitate 2.3c).