The teaching of St. Thomas and the other great Schoolmen of the Middle Ages forms the groundwork of the book. This corpus of doctrine is scarcely yet accessible outside its Latin sources. As typical of the Ifuller scholastic text-books the excellent treatise of the Spanish author, Urraburu,1 has been most frequently «consulted. Much assistance has also been derived from Kleutgen's s'losop/zie dew Voreez't,2 a monumental work which ought to have been long since translated into English. And finally, the excellent treatise in the Louvain Cow's d'e by the present Cardinal Archbishop of Mechlin,3 has been consulted with profit and largely followed in many places. The writer freely and gratefully acknowledges his indebtedness to these and other authors quoted and referred to in the course of the present volume.