The book will be read to little advantage if two salient ideas are not seized on and submitted to thorough examination: those of Philosophical Sin and of Material Sin. A firm grasp of what these two things mean must, the author is convinced, preserve the student from a deal of confusion and of error that are by no means uncommon. Possibly even the professor; some reviewers certainly. Those, for instance, who profess to find light and leading in the ethical writings of Kant, might decide whether the external act of murder or robbery, committed in good faith and with whatever motive you will, is wrong, according to that philosopher; might explain, if it is, where the categorical imperative comes in to make it wrong; and show, if it is not, how Kant's is anything else than an ethics-madeeasy system of moral science. Disciples of the Schoolmen, too, who admit no ethical quality in any act which is not free, might tell us whether the lunatic who attacks one with sword or gun is or is not an unjust aggressor; if not, why one may kill him in self defence; and if his assault is unjust, how it is not thereby stamped as ethical. Those, too, who require for moral goodness reference of an act to God as last end, might say how the concept of philosophical morality is not as intrinsically absurd as that of a square circle; seeing that the act of one who knows no God, and can refer no act to Him, can no more be conceived as good, in the sense of being referred to God as last end, than a geometrical figure can be conceived as being at once square and circular Similarly, those who defend probabilism on the ground that a doubtful law cannot bind, might ask themselves whether non-observance of such a law may result in material sin, however you may have convinced yourself that the non-observance is justified; and might, in case they find it so, think out the question how a law that does not bind can beget even material sin. These are some of the questions that occurred to me when reflecting on the two fundamental concepts of this book: Material Sin and Philosophical Sin. The book itself is the result of these and other such reflections.