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. Buried for more than forty years in a Latin text written for seminarian students at the Gregorian University in Rome, Bernard Lonergan's 1964 masterpiece of systematic-theological writing, De Deo trino: Pars systematica, is only now being published in an edition that includes the original Latin along with an exact and literal translation. De Deo trino , or The Triune God, is the third great installment on one particular strand in trinitarian theology, namely, the tradition that appeals to a psychological analogy for understanding trinitarian processions and relations. The analogy dates back to St Augustine but was significantly developed by St Thomas Aquinas. Lonergan advances it to a new level of sophistication by rooting it in his own highly nuanced cognitional theory and in his early position on decision and love. Suggestions for a further development of the analogy appear in Lonergan's late work, but these cannot be understood and implemented without working through this volume. This is truly one of the great masterpieces in the history of systematic theology, perhaps even the greatest of all time.Bernard Lonergan (1904-1984), a professor of theology, taught at Regis College, Harvard University, and Boston College. An established author known for his Insight and Method in Theology, Lonergan received numerous honorary doctorates, was a Companion of the Order of Canada in 1971 and was named as an original members of the International Theological Commission by Pope Paul VI.