“Thus the facts we must use and the principles we must apply are common facts that are known to all and principles that are recognized in every-day life. Starting from premises as to which there can be no dispute, we have only to be careful as to our steps in order to reach conclusions of which we may feel sure. We cannot experiment with communities as the chemist can with material substances, or as the physiologist can with animals. Nor can we find nations so alike in all other respects that we can safely attribute any difference in their conditions to the presence or absence of a single cause without first assuring ourselves of the tendency of that cause. But the imagination puts at our command a method of investigating economic problems which is within certain limits hardly less useful than actual experiment. We may test the working of known principles by mentally separating, combining or eliminating conditions.”