I first met Patrick Heelan when I was doing my Ph.D. in history and philosophy of science at the University of Pittsburgh, where he was a visiting fellow in 1982–3. As both a Jesuit and a hermeneutical phenomenologist, he was a dangerous person to have in such an avowedly godless and positivist environment. (Adolf Grünbaum was still prowling the corridors of the Cathedral of Learning.) Nevertheless, I was impressed by Heelan's independence of mind and originality of thought, the two qualities that over the years I have come to value the most in people. At the time, I was especially impressed by his recent book, Space-Perception and the Philosophy of Science. It remains the best defense of the incommensurability thesis that is ordinarily associated with Thomas Kuhn and Paul Feyerabend. However, in what follows, I tackle the issues raised in a recent, but no less challenging, work. Here Heelan takes on Pope John Paul II's 1998 encyclical, Fides et Ratio, by arguing for a liberalization of the Church's philosophical stance toward scientific inquiry.¹