A theoretical statement is a statement following necessarily and logically from some one or more known first principles. Thus, we know that two sides of a triangle are longer than the third, so we say it follows _theoretically_ that a straight road from London to Brighton is quicker motoring than going round by Lewes. But the number of first principles at work in the actual world is indefinitely large. Therefore one must test any one theoretical conclusion by practice: by seeing how it works. Because, side by side with the one or two first principles upon which our theory is built, there are an indefinitely large number of other first principles which come into play in the real world. Thus there is, in motoring, the principle that speed varies with road surface. So the way round by Lewes may be quicker than the straight road if it has a better surface. There is yet another principle that speed is checked by turnings in the road, and it may prove that on trial the two ways are about equal. Or again: we know that the tidal wave is raised on either side of the earth, and that there is, therefore, about twelve hours of even ebb and flow, six hours each on the average and taking the world as a whole: because the earth takes twenty-four hours to go round. But if you were to act upon that first principle _only_ in any one part of the world, and to say without testing the thing in practice, “I can calculate the tide theoretically,” you would very often wreck your ship. For many other principles come into play in the matter of the tide besides this twelve-hour period. In one case the tide will be delayed by shoals or by the current of a river. In another there may be two or three tides meeting. In a third the sea will be so locked that there will be hardly any tide for many hours, and then a rush at the end--and so on. Now it is just the same with Economics. Your economic first principle makes you come to such and such a _theoretical_ conclusion. But there are a lot of other first principles at work, and they may modify the effect _in practice_ to any extent. When people object to “theoretical dreaming,” as they call it, they mean the bad habit of thinking that one conclusion from one particular set of first principles is sufficient and will apply to any set of circumstances. It never does. One has always to watch the thing in practice, and see what other forces come in.