Let us see how this will work out. Suppose we have selected one of the numerous subsidiary problems suggested by the general inquiry, and obtained such full and complete information about one particular industry that we can tabulate the wages of the workers for a long series of years. We may do the same for other industries, some of them coming under the Statute of Apprenticeship, others not. If all the industries belong to one economic area over which, so far as we can tell from general statistics of wages and prices, and other information, fairly homogeneous conditions prevailed, we may be able to reach some useful conclusions as to the operation of the act. But it would be absurd to suppose that we could reach those conclusions by simple reference to the trades themselves. We cannot assume that the fluctuations in wages were due to the action or inaction of magistrates without the most careful examination of the other influences affecting the trades. In economic affairs the argument _post hoc propter hoc_ never leads to the whole truth, and is frequently quite misleading. We cannot suppose that the policy of the Merchant Adventurers' Company had nothing to do with the woollen industry; that the export trade in woollen cloth was quite independent of the foreign exchanges and international trade relations in those times; that the effect on wages of the state of the currency, the influx of new silver, the character of the harvests, and many other influences can be conveniently ignored. In studying, therefore, such an apparently simple question as the effect of an act of parliament on wages in a small group of trades we want a general theory which we can use as a kind of index of the factors we have to consider. Difficulties due to want of evidence.