My excellent publishers, who have always been keenly and wisely alive to my interests as an author, suggested several times after the success of the first book was reasonably assured, that a second and smaller one should be written out, with an especial eye to the needs of high schools and academies and colleges for a text-book within moderate limits, yet soundly based and covering in full outline the whole subject. This is the origin of the "Introduction to Political Economy," first published in 1877, twelve years after the other. Its success as a text-book and as a book of reading for young people has already justified, and will doubtless continue to justify in the future, the forethought of its promoters. It has found a place in many popular libraries, and in courses of prescribed reading. Twice it has been carefully corrected and somewhat enlarged, and is now in its final form. In the preface to the later editions of the "Introduction" may be found the following sentence, which expresses a feeling not likely to undergo any change in the time to come:--"I have long been, and am still, ambitious that these books of mine may become the horn-books of my countrymen in the study of this fascinating Science." Why, then, should I have undertaken of my own motion a new and third book on Political Economy, and attempted to mark the completion of the third cycle of a dozen years each of teaching it, by offering to the public the present volume? One reason is implied in the title, "_Principles of Political Economy_." There are three extended historical chapters in the earlier book, occupying more than one-quarter of its entire space, which were indeed novel, which cost me wide research and very great labor, and which have also proven useful and largely illustrative of almost every phase of Economics; but I wanted to leave behind me one book of about the same size as that, devoted exclusively to the Principles of the Science, and using History only incidentally to illustrate in passing each topic as it came under review. For a college text-book as this is designed to become, and for a book of reading and reference for technical purposes, it seems better that all the space should be taken up by purely scientific discussion and illustration. This does not mean, however, that great pains have not been taken in every part to make this book also easily intelligible, and as readable and interesting as such careful discussions can be made.