Now our supposed observer and classifier, having noted on London Bridge the sale of material commodities, and in the Lowell Mill the sale of personal services, and within the New York Clearing-House the sale of credit-claims, has seen in substance everything that ever was or ever will be exhibited in the world of trade. He may rest. There is no other class of salable things than these three. Keen eyes and minds skilled in induction have been busy for two millenniums and a half more or less to find another class of things bought and sold among men, and have not yet found it or any trace of it. This work has been perfectly and scientifically done. The generalization is completed for all time. The _genus_, then, with which Political Economy deals from beginning to end, has been discovered, can be described, and is easily and completely separable for its own purposes of science from all other kinds and classes and _genera_ of things, namely, Salable things or (what means precisely the same) Valuable things or (what is exactly equivalent) Exchangeable things. In other words, the sole and single class of things, with which the Science of Political Economy has to do, is Valuables, whose origin and nature and extent and importance it is the purpose of the present chapter to unfold. We have fully seen already that this Genus, Valuables, is sub-divided into three _species_, and three only, namely, Commodities, Services, Credits. A little table here may help at once the eye and the mind:-- ECONOMICS. _The Genus_ _Valuables_ { _Commodities_ _The Species_ { _Services_ { _Credits_ If only these three species of things are ever bought and sold, then it certainly follows that only six kinds of commercial exchanges are possible to be found in the world, namely these:-- 1. _A commodity for a commodity._ 2. _A commodity for a personal service._ 3. _A commodity for a credit-claim._ 4. _A personal service for another service._ 5. _A personal service for a credit-claim._ 6. _One credit-claim for another._ Though the kinds of possible exchanges are thus very few, the exchanges themselves in one or other of these six forms and in all of them are innumerable on every business day in every civilized country of the globe. And this point is to be particularly noted, that while buying and selling in these forms has been going on everywhere since the dawn of authentic History, it has gone on all the while in ever-increasing volume, it is increasing now more rapidly and variously than ever, and moreover all signs foretell that it will play a larger and still larger part in the affairs of men and nations as this old world gains in age and unity.