chiefly in analysis, have a like certainty, although, as with all the causes of which it takes cognizance are at all times acting other causes, it can never predict exact results but only tendencies.
And, although in the study of political economy we can not use that potent method of experiment by artificially produced conditions which is so valuable in the physical sciences, yet, not only may we find, in the diversity of human society, experiments already worked out for us, but there is at our command a method analogous to that of the chemist, in what may be called mental experiment. You may separate, combine, or eliminate conditions in your own imagination, and test in this way the working of known principles. This, it seems to me, is the great tool of political economy. It is a method with which you must be familiar and doubtless use every day, though you may never have analyzed the process. Let me illustrate what I mean by something which has no reference to political economy.
When I was a boy I went down to the wharf with another boy to see the first iron steamship which had ever crossed the ocean to our port. Now, hearing of an iron steamship seemed to us then a good deal like hearing of a leaden kite or a wooden cooking-stove. But, we had not been long aboard of her, before my comrade said in a tone of contemptuous disgust: "Pooh! I see how it is. She's all lined with wood; that's the reason she floats." I could not controvert him for the moment, but I was not satisfied, and, sitting down on the wharf when he left me, I set to work trying mental experiments. If it was the wood inside of her that made her float, then the more wood the higher she would float; and, mentally, I loaded her up with wood. But, as I was familiar with the process of making boats out of blocks of wood, I at once saw that, instead of floating higher, she would sink deeper. Then, I mentally took all the wood out of her, as we dug out our wooden boats, and saw that thus lightened she would float higher still. Then, in imagination, I jammed a hole in her, and saw that the water would run in and she would sink, as did our wooden boats when ballasted with leaden keels. And, thus I saw, as clearly as though I could have actually made these experiments with the steamer, that it was not the wooden lining that made her float, but her hollowness, or, as I would now phrase it, her displacement of water.
Now, just such mental operations as these you doubtless perform every day, and in doing so you employ the method of imaginative experiment which is so useful in the investigations of political economy. You can, in this way, turn around in your mind a proposition or phenomenon and look on all sides of it, can isolate, analyze, recombine, or subject it to the action of a mental magnifying glass which will reveal incongruities as a reductio ad absurdum. Let me again illustrate:
Before I had ever read a line of political economy, I happened once to hear a long and well-put argument in favor of a protective tariff. Up to that time I had supposed that "protection to domestic industry"
