The material of these lectures, which I had the honor of delivering at Princeton University, on the Vanuxem Foundation, was prepared, during the War, at the Universities of Harvard, Poitiers, and Toronto. Certain portions of the work, relatively few, have already appeared in the form of articles, viz.: part of Chapter I in the Revue de Metaphysique et de Morale, July, 1918; Chapter IV, ii, in the Philosophical Review, July, 1918; Chapter V, iii, in the International Journal of Ethics, January, 1919; Chapter III, ii, and Chapter VII, i-v, in the Harvard Theological Review, October, 1918. These now take their place as integral parts of what may be regarded as a supplement to my History of Mediaeval Philosophy. The purpose of the study as here presented is to approach the Middle Ages from a new point of view, by showing how the thought of the period, metaphysics included, is intimately connected with the whole round of Western civilization to which it belongs. My work represents simply an attempt to open the way; it makes no pretense to exhaustive treatment of any of the innumerable problems involved in so vast a subject. -- Preface ([v]).