Distinctions of Reason and Reasonable Distinctions is an intellectual biography of John Wallis (1616-1703), professor of mathematics at Oxford for over half a century. His career spans the political tumult of the English Civil Wars, the religious upheaval of the Church of England, and the fascinating developments in mathematics and natural philosophy. His ability to navigate this terrain and advance human learning in the academic world was facilitated by his use of the Jesuit Francisco Suarez’s theory of distinctions. This Roman Catholic’s philosophy in the hands of a Protestant divine fostered an instrumentalism necessary to bridge the old and new. With this tool, Wallis brought modern science into the university and helped form the Royal Society.