PREFACE

IX

“the impatience theory” which I am not, and not creditedwith being the author of those parts lacking any catch-word. It was this misunderstanding which led me, aftermuch search, to adopt the catchword “investment op-portunity” as a substitute for the inadequate term “pro-ductivity” which had come into such general use. 1

In economics it is difficult to prove originality; forthe germ of every new idea will surely be found over andover again in earlier writers. For myself, I would be satis-fied to have my conclusions accepted as true even iftheir origin should be credited by the critics wholly toearlier writers. While I hope I may be credited with acertain degree of originality, every thorough student ofthis subject will recognize in my treatment of interesttheory features of his own. My own theory is in somedegree every one’s theory. Every essential part of it wasat least foreshadowed by John Rae in 1834.

If this combined “impatience and opportunity” theorycan be said to be at all distinct from all others, it isbecause it explicitly analyses opportunity, and fits to-gether impatience and opportunity and income. Theincome concept plays the basic role in the theory ofinterest. I venture to hope that the theory, as here pre-sented, will be found not so much to overthrow as toco-ordinate previous theories, and to help in making thechain of explanation complete and strong.

1 This term, investment opportunity, seems to be the nearest expres-sion in popular language to suggest or denote the technical magnitude remployed in this book. The full expression for r is the rate of return fover cost, and both cost and return are differences between two optional'income streams. So far as I know, no other writer on interest has madeuse of income streams and their differences, or rates of return over costper annum. The nearest approximation to this usage seems to be inthe writings of Professor Herbert J. Davenport , particularly his Eco-nomics oj Entreprise, (Macmillan, New York , 1913) pages 368, 379,381, 394, 395, 396, 410, 411.