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.Needless to say, such an effort will require a kind of "mining energy" of an archeologist of ideas who knows how to recover what was once known (or at least suspected) from time immemorial but has now been forgotten. But Josef Pieper does more than bring to bear on this issue his famous powers of excavation; he also makes meaningful the concept of sin to the ways of thinking and speaking of our time.Readers of his work already know Pieper as an extraordinarily fitting master in this art of making "the wisdom of the ages" a living reality today. And in this work he 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 of "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 mystery of existence itself, is the most impenetrable mystery of all.