For instance: we find out, when we do the elements of Arithmetic, that solid bodies vary with the cube of their linear measurements. That is the general abstract principle; but the _use_ of it is in real life when we come (for instance) to measuring boats. We learn there from Arithmetic that, with boats of similar shape, a boat twice as long as another will be eight times as big; it is also by using the elements of Arithmetic that we can keep household accounts and do all the rest of our work. It is precisely the same with Economics. We are perpetually coming upon political problems which Economics illustrate and to which economic science furnishes the answer--or part of the answer--and that is where the theoretical elements of Economics have practical importance. For instance: once we know the elementary economic principle that rent is a surplus, we appreciate that it does not enter into cost of production. We do not try to make things cheaper by compulsorily lowering rent. Or, again, when we have learned the nature of money we can appreciate the dangers that come from using false money. In these political applications of Economics we also come upon what is much more important than mere politics, and that is the question of right and wrong. We see that such and such a thing ought to be so as a matter of justice; but we may blunder, as many great reformers have blundered, in trying to do the right thing and failing to do it, because we have not made a proper application of our economic science. And the opposite is also true: that is, a knowledge of Economics prevents their being wrongly applied by those who desire evil. Many men take refuge in the excuse that, with the best will in the world, they cannot work such and such a social reform because economic science prevents their doing what they know to be right. If we know our Economics properly we can refute these false arguments, to the great advantage of our own souls and of our fellow-men. For instance: it is clearly our duty to-day to alleviate the fearful poverty in which most Englishmen live. A great many people who ought to know better say, or pretend, that economic laws prevent our doing this act of justice. Economic laws have no such effect; and an understanding of Economics clears us in this matter, as we shall see later on. We have hitherto been following the statement and examination of economic laws: that is, the _theoretical_ part of our study and its necessary foundation. Now we go on to the _practical_ part, or “Applied Economics,” which is the effect of those laws on the lives of men. Before leaving this Introduction I think it is important to get quite clear the difference between what is called “theoretical” study and the practical application of such study. People are very often muddle-headed about this, and the more clearly we think about it the better.