A man who once wrote in Dublin[1] complained of the difficulty of composing a learned work so far from the Bodleian and British Museum, and I have often felt the same way. When able to visit foreign collections or the largest libraries in this country, or when books have been sent for my use for a limited period, I have spent all the available time in the collection of material, which has been written up later as opportunity offered. Naturally one then finds many small and some important points which require verification or further investigation, but which must be postponed until one’s next vacation or trip abroad, by which time some of the smaller points are apt to be forgotten. Of such loose threads I fear that more remain than could be desired. And I have so often caught myself in the act of misinterpretation, misplaced emphasis, and other mistakes, that I have no doubt there are other errors as well as omissions which other scholars will be able to point out and which I trust they will. Despite this prospect, I have been bold in affirming my independent opinion on any point where I have one, even if it conflicts with that of specialists or puts me in the position of criticizing my betters. Constant questioning, criticism, new points of view, and conflict of opinion are essential in the pursuit of truth. After some hesitation I decided, because of the expense, the length of the work, and the increasing unfamiliarity of readers with Greek and Latin, as a rule not to give in the footnotes the original language of passages used in the text. I have, however, usually supplied the Latin or Greek when I have made a free translation or one with which I felt that others might not agree. But in such cases I advise critics not to reject my rendering utterly without some further examination of the context and line of thought of the author or treatise in question, since the wording of particular passages in texts and manuscripts is liable to be corrupt, and since my purpose in quoting particular passages is to illustrate the general attitude of the author or treatise. In describing manuscripts I have employed quotation marks when I knew from personal examination or otherwise that the Latin was that of the manuscript itself, and have omitted quotation marks where the Latin seemed rather to be that of the description in the catalogue. Usually I have let the faulty spelling and syntax of medieval copyists stand without comment. But as I am not an expert in palaeography and have examined a large number of manuscripts primarily for their substance, the reader should not regard my Latin quotations from them as exact transliterations or carefully considered texts. He should also remember that there is little uniformity in the manuscripts themselves. I have tried to reduce the bulk of the footnotes by the briefest forms of reference consistent with clearness—consult lists of abbreviations and of works frequently cited by author and date of publication—and by