The studies united in this volume, although in the first instance published separately in the course of a number of years, form part of a single comprehensive plan. For this republication the exposition has been slightly revised and a few gaps have been filled in, but the main argument is unchanged. Their arrangement is now systematic, in the order in which the argument develops, rather than the accidental one of their first appearance. The book thus begins with a theoretical discussion of the general issues and proceeds to an examination of the historical role played by the ideas in question. This is not mere pedantry or a device for avoiding unnecessary repetition but, it seems to me, essential in order to show the true significance of the particular development. But I am quite aware that as a result the opening sections of the book are relatively more difficult than the rest, and that it might have been more politic to put the more concrete matter in the forefront. I still believe that most readers who are interested in this kind of subject will find the present arrangement more convenient. But any reader who has little taste for abstract discussion may do well to read first the second part which has given the title to this volume. I hope he will then find the general discussion of the same problems in the first study more interesting.
These two major sections of the volume were first published in parts in Economica for 1942-44 and for 1941 respectively. The third study, written more recently as a lecture, appeared first in Measure for June 1951 but was prepared from notes collected at the same time as those for the first two essays. I have to thank the editors of both of these journals and the London School of Economics and Political Science and the Henry Regnery Company of Chicago as their respective publishers for permission to reprint what first appeared under their auspices.