【正文】 s opinion much better worked out by Marshall than by any other writer generally read. But Marshall himself has adopted a cautious, almost antitheoretical attitude toward fundamentals。 mathematical economics in particular seems likely to remain little more than a cult, a closed book to all except a few of the initiated. In the great mass of economic literature there is certainly still wanting the evidence of a prehensive grasp of general principles and even more of the meaning and importance of general principles in a scientific program. There is still a need for thoroughgoing and critical parison and contrast of theoretical assumptions with the conditions of real life and of theoretical conclusions with concrete facts. The makers and users of economic analysis have in general still to be made to see that deductions from theory are necessary, not because literally true— that in the strict sense they are useful because not literally true— but only if they bear a certain relation to literal truth and if all who work with them constantly bear in mind what that relation is. It must be admitted that even the pure theorists have not generally been assiduous in emphasizing the practical significance of their work and its relation to the outside body of the science。 the ignorant will not in general defer to the opinion of the informed, and in the absence of voluntary deference it is usually impossible to give an objective demonstration. If our social science is to yield fruits in an improved quality of human life, it must for the most part be sold to the masses first. The necessity of making its literature not merely accurate and convincing, but as nearly foolproof as possible, is therefore manifest. Whether or not the use of the method of exact science is as necessary in the field of social phenonena as the present writer believes, it will doubtless be conceded。 it is imperative that the contrast between these simplified assumptions and the plex facts of life be made as conspicuous and as familiar as has been done in mechanics. The present essay is an attempt in the direction indicated above. We shall endeavor to search out and placard the unrealities of the postulates of theoretical economics, not for the purpose of discrediting the doctrine, but with a view to making clear its theoretical limitations. There are several reasons why the approximate character of theoretical economic laws and their inapplicability without empirical correction to real situations should be especially emphasized as pared, for instance, with those of mechanics. The first reason is historical and has already been indicated. The limitations of the results have not always been clear, and theorists themselves as well as writers in practical economics and statecraft have carelessly used them without regard for the corrections necessary to make them fit concrete facts. Policies must fail, and fail disastrously, which are based on perpetual motion reasoning without the recognition that it is such. In the second place, the allowances and corrections necessary in the case of theoretical economics are vastly greater than in the case of mechanics, and the importance of not losing sight of them is correspondingly accentuated. The general principles do not bring us so close to reality。s judgment theorists of the past and present are to be justly criticized not for following the theoretical method and studying a simplified and idealized form of petitive anization, but for not following it in a sufficiently selfconscious, critical, and explicit way. In their discussions of methodology the historic economists have, indeed, been as clear and explicit as could be desired,*6 but in the use of the method as much cannot, unfortunately, be said. It should go without saying that in the use of the scientific method of reasoning from simplified premises, it is imperative that it be clear to the reasoner and be made unmistakable to those who use his work what his procedure is and what presuppositions are involved. Two supreme difficulties have underlain controversies regarding method in the past. The first is the strong aversion of the masses of humanity, including even a large proportion of scholars, to all thinking in general terms. The second difficulty, on the other side, is the fact referred to above, that the persons employing methods of approximation in economics have not themselves adequately and always recognized, and still less have they made clear to their readers, the approximate character of their conclusions, as descriptions of tendency only, but have frequently hastened to base principles of social and business policy upon very inplete data. The evil results of the failure to emphasize the theoretical character of economic speculation are apparent in every field of practical economics. The theorist not having definite assumptions clearly in mind in working out the principles, it is but natural that he, and still more the practical workers building upon his foundations, should fet that unreal assumptions were made, and should take the principles over bodily, apply them to concrete cases, and draw sweeping and wholly unwarranted conclusions from them. The clearly untenable and often vicious character of such deductions naturally works to discredit theory itself. This, of course, is wrong。 but it is a very fundamental section, in a sense the first of all, the foundation and prerequisite of those that follow. And this also may very well hold good of a body of pure theory in economics。 but much remains to be done to establish a systematic and coherent view of what is necessary to perfect petition, just how far and in what ways its conditions deviate from those of real life and what corrections have accordingly to be made in applying its conclusions to actual s