1945] HAYEK:THE USE OF KNOWLEDGE IN SOCIETY 523 as such.Indeed,there are few points on which the assumptions made (usually only implicitly)by the "planners"differ from those of their opponents as much as with regard to the significance and frequency of changes which will make substantial alterations of production plans necessary.Of course,if detailed economic plans could be laid down for fairly long periods in advance and then closely adhered to,so that no further economic decisions of importance would be required,the task of drawing up a comprehensive plan governing all economic activity would appear much less formidable. It is,perhaps,worth stressing that economic problems arise always and only in consequence of change.So long as things continue as before,or at least as they were expected to,there arise no new problems requiring a decision,no need to form a new plan.The belief that changes,or at least day-to-day adjustments,have become less im- portant in modern times implies the contention that economic problems also have become less important.This belief in the decreasing im- portance of change is,for that reason,usually held by the same people who argue that the importance of economic considerations has been driven into the background by the growing importance of technological knowledge. Is it true that,with the elaborate apparatus of modern production, economic decisions are required only at long intervals,as when a new factory is to be erected or a new process to be introduced?Is it true that,once a plant has been built,the rest is all more or less mechanical, determined by the character of the plant,and leaving little to be changed in adapting to the ever-changing circumstances of the moment? The fairly widespread belief in the affirmative is not,so far as I can ascertain,borne out by the practical experience of the business man.In a competitive industry at any rate-and such an industry alone can serve as a test-the task of keeping cost from rising requires constant struggle,absorbing a great part of the energy of the manager. How easy it is for an inefficient manager to dissipate the differentials on which profitability rests,and that it is possible,with the same technical facilities,to produce with a great variety of costs,are among the commonplaces of business experience which do not seem to be equally familiar in the study of the economist.The very strength of the desire,constantly voiced by producers and engineers,to be able to proceed untrammeled by considerations of money costs,is eloquent testimony to the extent to which these factors enter into their daily work. One reason why economists are increasingly apt to forget about the constant small changes which make up the whole economic picture is probably their growing preoccupation with statistical aggregates,which