Of modern Scholastic writers the author is most in debted to Professors Meyer and Cathrein, to Rev. Joseph Rickaby, Taparelli, Schififini, and Rev. Dr. Walter mcdonald, Maynooth College, whose treatises on Ethics have been of immense help to him in the preparation of his work. His most grateful thanks are due to the Rev. John Waters, b.a., of Clonliffe College, for his kindness in reading this book, and for very many valuable criticisms and suggestions. Father Waters' wide and minute acquaintance with the writings of St. Thomas Aquinas was always at the author's disposal in the difficult task of discovering and comparing the various references made by St. Thomas to any special Ethical question, references often occurring in places very wide apart, and under unpromising titles — titles apparently outside the, subject under discussion. Thanks are also due to those gentlemen who so kindly undertook the tedious and un interesting task of proof-reading, and to many others also for help given of various kinds.