Buried for more than forty years in a Latin text written for seminarians at the Gregorian University in Rome, Bernard Lonergan's important work on systematic theology, De Deo Trino: Pars systematica, is presented here for the first time in a facing-page edition that includes the original Latin along with a precise English translation. De Deo Trino, or The Triune God, the second part of which is the pars systematica, continues a particular strand in trinitarian theology, namely, the tradition that appeals to a psychological analogy for understanding trinitarian processions and relations. The psychological analogy dates back to St Augustine but was significantly developed by St Thomas Aquinas. Lonergan advances it to a new level of understanding by bringing to it his extensive exploration of cognitional theory and deliberative process. Suggestions for a further development of the analogy appear in Lonergan's late work, but these cannot be fully comprehended and implemented without the background provided in this volume. With this definitive translated edition, one of the masterpieces of systematic theology, will at last be available to contemporary scholars.