This work has been long in preparationever since in 1902-1903 Professor James Harvey Robinson, when my mind was still in the making, suggested the study of magic in medieval universities as the subject of my thesis for the master's degree at Columbia Universityand has been foreshadowed by other publications, some of which are listed under my name in the preliminary bibliography. Since this was set up in type there have also appeared: "Galen: the Man and His Times," in The Scientific Monthly, January, 1922; "Early Christianity and Natural Science," in The Biblical Review, July, 1922; "The Latin Pseudo-Aristotle and Medieval Occult Science," in The Journal of English and Germanic Philology, April, 1922; and notes on Daniel of Morley and Gundissalinus in The English Historical Review. For permission to make use of these previous publications in the present work I am indebted to the editors of the periodicals just mentioned, and also to the editors of The Columbia University Studies in History, Economics, and Public Law, The American Historical Review, Classical Philology, The Monist, Nature, The Philosophical Review, and Science. The form, however, of these previous publications has often been altered in embodying them in this book, and, taken together, they constitute but a fraction of it. Book I greatly amplifies the account of magic in the Roman Empire contained in my doctoral dissertation. Over ten years ago I prepared an account of magic and science in the twelfth and thirteenth centuries based on material available in print in libraries of this country and arranged topically, but I did not publish it, as it seemed advisable to supplement it by study abroad and of the manuscript material, and to adopt an arrangement by authors. The result is Books IV and V of the present work. My examination of manuscripts has been done especially at the British Museum, whose rich collections, perhaps because somewhat inaccessibly catalogued, have been less used by students of medieval learning than such libraries as the Bodleian and Bibliothèque Nationale. I have worked also, however, at both Oxford and Paris, at Munich, Florence, Bologna, and elsewhere; but it has of course been impossible to examine all the thousands of manuscripts bearing upon the subject, and the war prevented me from visiting some libraries, such as the important medieval collection of Amplonius at Erfurt. However, a fairly wide survey of the catalogues of collections of manuscripts has convinced me that I have read a representative selection. Such classified lists of medieval manuscripts as Mrs. Dorothea Singer has undertaken for the British Isles should greatly facilitate the future labors of investigators in this field. Although working in a rather new field, I have been aided by editions of medieval writers produced by modern scholarship, and by various series, books, and articles tending, at least, in the same direction as mine. Some such publications have appeared or come to my notice too late for use or even for mention in the text: for instance, another edition of the De medicamentis of Marcellus Empiricus by M. Niedermann; the printing of the Twelve Experiments with Snakeskin of John Paulinus by J.W.S. Johnsson in Bull. d. l. société franç. d'hist. d.l. méd., XII, 257-67; the detailed studies of Sante Ferrari on Peter of Abano; and A. Franz, Die kirchlichen Benediktionen im Mittelalter, 1909, 2 vols. The breeding place of the eel (to which I allude at I, 491) is now, as a result of recent investigation by Dr. J. Schmidt, placed "about 2500 miles from the mouth of the English Channel and 500 miles north-east of the Leeward Islands" (Discovery, Oct., 1922) instead of in the Mediterranean.