ject that, in a pre-eminent degree, it calls for the exercise of imagination. "That is just what we have always said," the scoffers at political economy say at once; "so does novel-writing call for imagination, and a novelist is about as well fitted for the economist's position as the usual abstract thinker who masquerades as a teacher of political economy." To this it is to be replied that imagination is one of the chief requisites for mathematical study also; that a novelist is not necessarily a good mathematician, goes without saying. The simplest propositions of solid geometry require the exercise of imagination, as, for example, in the picturing of forms and solids with intersecting planes. The most logical student of the severest mathematical processes is called on for the exercise of this species of imagination. And so it is in political economy. In learning the subject, the perception of a simple general principle is often absurdly easy, but, for its assimilation into our own thinking, it is necessary that it should have become an interpreter of facts everywhere about us. To this end, it is essential for us to apply the abstraction, or general principle, in every possible case, to some concrete phenomenon. Very often, in order to show the action of this single principle operating by itself, we must separate all conflicting agencies from the situation—just as the physicist experiments in a vacuum exhausted of air, for the purpose of learning the full effect of a force, like gravity, when acting by itself. The economist, however, is not able to reproduce a given situation to the eye or ear, as is the physicist. He can not pile before him the exports of the United States or England, or summon before him the laboring-class or the capitalists of a country; he must, therefore, picture to himself the actual facts, just as the geometrician does the solid, and see how the operating principle works. This is very far from "theoretical dreaming." It is at once a most difficult process, and a most excellent discipline in learning how to think on such subjects. To illustrate my meaning in a simple way, it is one thing to say that in order to have value a commodity must satisfy some desire, and be hard to get; and quite another thing to be able to call up in the mind an image which will show the application of the principle. For example, to a shipwrecked sailor on a rocky island a bag of gold has no value, for it can not keep him alive. It is largely by such mental exercise as this that a student best succeeds in assimilating the body of principles which make up the science of political economy. It has been frequently said to me, "I can understand the statements of the writer easily, but I do not seem to be able to use the idea when called upon to explain things in a different connection." This is exactly the difficulty, as it is also, by struggling with the difficulty, the disciplinary gain of our study. To understand an abstract principle, without the ability to see it in the concrete form, and test its truth, is of little gain to any one. This would in truth make a "doctrinaire." The only "practical man," in any conceivable sense, is he who, while seeing
