Fr. Stephen L. Brock, Ph.D., Distinguished Fellow of the Principium Institute, is a priest of the Prelature of Opus Dei (ordained 1992). He is Ordinary Professor of Medieval Philosophy at the Pontifical University of the Holy Cross in Rome, where he has taught since 1990. From 2010 to 2014 he served there as vice-dean of the Philosophy Department. He received a B.A. from the University of Chicago in 1979 and a Ph.D. in Medieval Studies from the University of Toronto in 1988. In 1999 he was a visiting professor in the School of Philosophy of The Catholic University of America. Since 2008 he has been an Ordinary Member of the Pontifical Academy of St. Thomas Aquinas. He is the author of Action & Conduct: Thomas Aquinas and the Theory of Action (T&T Clark, 1998); articles in English, Italian, and Spanish, on various aspects of Aquinas’s thought; and most recently, The Philosophy of Saint Thomas Aquinas: A Sketch (Wipf & Stock, 2015). He serves on the boards of the journals Acta Philosophica, Nova et Vetera, and The American Catholic Philosophical Quarterly. Once each year since 2014 he has directed week-long intensive seminars for graduate students on topics in Aquinas’s philosophy, sponsored by the Lumen Christi Institute (Chicago). Most recently, he has been invited to offer a course in the Philosophy Department at the University of Chicago, in the spring of 2017, under the auspices of the Templeton Foundation project “Virtue, Happiness, and Meaning in Life,” directed by C. Vogler and J. Frey.