On the Economic surface we have the category of Money. To the wage-worker Money is the medium for trading the commodities he helps to produce for those he wishes to consume. To the merchant, however, or the manufacturer, or other Economic specializer, Money presents also a variety of minute details for expert study. The business accountant, for example, must familiarize himself with numerous details in connection with Money in its minute relationships to his specialty. He might very likely confuse facts Economically different, yet as a special business matter practically identical--the Money measurement of a natural mineral deposit, for instance, with the Money measurement of its artificial equipment. As an accountant in that specialty he would be right in doing so. Or, in slavery days, for another instance, an accountant might properly have classified the Money measurement of a human chattel with that of a domesticated horse or a constructed house. He also would have been right; for, as an accountant, he would have been dealing with a customary classification of private property; and not with Economic science comprehensively. It is doubtless specialty work of such kinds that has involved the science of Economics in so much confusion, especially among advanced students who are prone to identify business conventionalities with Economic normalities. As a science, Economics cannot make its categories according to conventional maladjustments. It must make them according to essential differences. These admit of no categorical identification of mineral deposits with mining machinery, or of human beings with domesticated animals or buildings. Such classifications are as absurd in Economics as identifying sun and earth would be in astronomy, or bone with brain in anatomy. The science of Economics--that is to say, the science of Business in the broad social sense in contradistinction to the narrow private sense--neither requires nor permits such classifications as slaves with domesticated animals, or mining machinery with natural mineral deposits. It is obedient to the ancient but still vital maxim of philosophy that things which are essentially different must not be mixed nor things that are essentially the same be separated. Essentially different things in Economics are indeed confused by Money, which knows no difference, except in degree of Money measurement, between slaves and cattle, or mineral deposits and mining machinery. As the medium of trade, Money must measure the values of everything tradable; and custom may make tradable objects--human beings, for instance,--which in the science of Economics are no more within the normal boundaries of trade than are transfers by theft.