In the first place, Money is the only Valuable that may belong to two out of the three possible categories into which Valuables may be scientifically thrown. All Valuables are either Commodities, or Services, or Credits. These categories never change places. Once a Commodity always a commodity, so long as value can be predicate of it; a personal Service can never take on any other valuable form; and a Credit is ever a credit, and nothing else, until it is annihilated by Fulfilment. Now Money is the only Valuable that ever appears in two of these forms. The same Dollar indeed cannot be both a Commodity and a Credit; but some Dollars are a Commodity cut out from gold and silver, and some other Dollars (so-called) are a Credit issued by Government or parties responsible to government; while Money as a general term properly enough covers both kinds of Dollars, the Commodity-Dollar and the Credit-Dollar. In other words, Money is of two kinds, and only two kinds, either a Piece of valuable metal stamped as to weight and fineness by the image and inscription of Cæsar,--a Commodity; or a Promise to pay to somebody some of these pieces,--a Credit. This unique peculiarity of Money, by which, always a Valuable, it may appear and does appear in two out of three possible predicaments of Valuables, makes a little difficulty at the outset of its discussion, and requires continued care in formulating its scientific propositions. In the second place, a more considerable difficulty, and yet a slight one still, is found in the fact that the choices and the legislations of men have more to do in shaping the propositions of Money than in most other economical propositions. It is true, that Nature and men coöperate in the determination of every case of Value whatsoever; while there is a difference in the cases, though perhaps not a distinction, in respect to the fixedness and universality of the natural laws involved, in contrariety to the purely human impulses concerned. The Providential elements in Economics, both the social and the physical, are of course relatively fixed and unchangeable, otherwise Science could not grapple with and classify them; and so also are those principles of Human Nature related to exchanges, which may be said to be _universal_ in their character,--such as, for example, the preference to receive a larger rather than a less return-service, and to render a smaller rather than a larger effort; and at the same time there are other principles of human nature related to exchanges much more _variable_ in their character than these, such, for instance, as the nation's choice of the kind of Money it will use, or the kind of Taxation it will impose. It certainly follows from this, that some Economical laws must be more _general_ than others, owing to a less variation in the human impulses concerned in them: it follows, for example, that the law of landed rents, or the law of the approach of the price of raw materials to that of the