Such choice must necessarily be somewhat arbitrary, for it is not always an easy matter to fix upon the best representative of each doctrine. Especially is this the case in a science like economics, where the writers, unknown to one another, not infrequently repeat the same ideas, and it becomes a matter of some difficulty to decide the claim to priority. But although it may be difficult to hit upon the exact moment at which a certain idea first made its appearance, it is comparatively easy to determine when such an idea attracted general attention or took its place in the hierarchy of accepted or scarcely disputed truths. This has been our criterion. With regard to those whose names do not figure in our list, although quite worthy of a place in the front rank, we cannot believe that they will suffer much through this temporary eclipse, especially in view of the partiality of the age for the pioneers. That we are not unduly optimistic in this matter may be inferred from the numerous attempts recently made to discover the _poetæ minores_ of the science, and to make amends for the scant justice done them by the more biased historians of the past. Not only was selection necessary in the case of authors, but a similar procedure had to be applied to the doctrines. It must be realised, however, that a selection of this character does not warrant the conclusion that the doctrines dealt with are in any way superior to those which are not included, either from the standpoint of moral value, of social utility, or of abstract truth, for we are not of the number who think with J. B. Say that the history of error can serve no useful purpose.[4] We would rather associate ourselves with Condillac when he remarks: “It is essential that everyone who wishes to make some progress in the search for truth should know something of the mistakes committed by people like himself who thought they were extending the boundaries of knowledge.” The study of error would be thoroughly well justified even though the result were simply a healthy determination to avoid it in future. It would be even more so if Herbert Spencer’s version of the saying of Shakespeare, that there is no species of error without some germ of truth in it, should prove correct. One cannot, moreover, be said to possess a knowledge of any doctrine or to understand it until one knows something of its history, and of the pitfalls that lay in the path of those who first formulated it. A truth received as if it has fallen from the sky, without any knowledge of the efforts whereby it has been acquired, is like an ingot of gold got without toil—of little profit.