It is the aim of this essay to examine and present in as concise a form as possible the principles and rules which guided and regulated men in their economic and social relations during the period known as the Middle Ages. The failure of the teaching of the so-called orthodox or classical political economists to bring peace and security to society has caused those interested in social and economic problems to inquire with ever-increasing anxiety into the economic teaching which the orthodox economy replaced; and this inquiry has revealed that each system of economic thought that has from time to time been accepted can be properly understood only by a knowledge of the earlier system out of which it grew. A process of historical inquiry of this kind leads one ultimately to the Middle Ages, and it is certainly not too much to say that no study of modern European economic thought can be complete or satisfactory unless it is based upon a knowledge of the economic teaching which was accepted in mediæval Europe. Therefore, while many will deny that the economic teaching of that period is deserving of approval, or that it is capable of being applied to the conditions of the present day, none will deny that it is worthy of careful and impartial investigation. There is thus a demand for information upon the subject dealt with in this essay. On the other hand, the supply of such information in the English language is extremely limited. The books, such as Ingram's _History of Political Economy_ and Haney's _History of Economic Thought_, which deal with the whole of economic history, necessarily devote but a few pages to the Middle Ages. Ashley's _Economic History_ contains two excellent chapters dealing with the Canonist teaching; but, while these chapters contain a mass of most valuable information on particular branches of the mediæval doctrines, they do not perhaps sufficiently indicate the relation between them, nor do they lay sufficient emphasis upon the fundamental philosophical principles out of which the whole system sprang. One cannot sufficiently acknowledge the debt which English students are under to Sir William Ashley for his examination of mediæval opinion on economic matters; his book is frequently and gratefully cited as an authority in the following pages; but it is undeniable that his treatment of the subject suffers somewhat on account of its being introduced but incidentally into a work dealing mainly with English economic practice. Dr. Cunningham has also made many valuable contributions to particular aspects of the subject; and there have also been published, principally in Catholic periodicals, many important monographs on special points; but so far there has not appeared in English any treatise, which is devoted exclusively to mediæval economic opinion and attempts to treat the whole subject completely. It is this want in our economic literature that has tempted the author to publish the present essay, although he