The present work is substantially a dissertation presented to the Faculty of the Graduate School of the University of Toronto. While aware of the numerous imperfections of the work I have decided, on the urging of many colleagues, to publish it at this time because of the current relevance of the subject-matter and especially of the collection of texts. I am happy to acknowledge my indebtedness to the faculty of the Pontifical Mediaeval Institute of Toronto and especially to the Reverend Ignatius Eschmann, O.P., who first suggested the idea of this study and whose encouragement and assistance brought it to completion. My thanks are due also to the Reverend George Klubertanz, S.J., and Mr. Paul Mathews, both of the Department of Philosophy of Saint Louis University, and .. for invaluable secretarial assistance, to Mrs. Savina Tonella and Miss Agnes Kutz. R.J. HENLE, S.J. Saint Louis December, 1954 TABLE OF CONTENTS GENERAL INTRODUCTION. . . . . . . . . . XIII .