In ordinary conversation, including among the 'educated,' the word 'sin' rarely gets mentioned except when one is trying to be coy or facetious. As Thomas Mann once said, 'sin' is nowadays 'an amusing word used only when one is trying to get a laugh.' But this small work will interpret sin in its true - that is, serious - meaning. What will emerge from its analysis is the discovery that the concept of sin can still serve to unlock the mystery of existence, at least for a thinking that wants to press down to the very foundations. In this work Pieper brings Plato, Augustine, and Thomas Aquinas into a living dialogue with T. S. Eliot, Andre Gide, even with Jean-Paul Sartre. As he shows in this powerful work, none of these writers leaves any doubt that the fact of sin is central: It is the willful denial of one's own life-ground, a denial that alone rightly bears the name 'sin.' Paradoxically, this reality is both willed and yet also pre-given, that is, both adventitious and yet somehow innate to our existence - a paradox which, next to the mystey of existence itself, is the most impenetrable mystery of all.