General supply consists in the commodities offered in exchange for other commodities; general demand likewise, if no money exists, consists in the commodities offered as purchasing power in exchange for other commodities. That is, one can not increase the demand for certain things without increasing the supply of some articles which will be received in exchange for the desired commodities. Demand is based upon the production of articles having exchange value, in its economic sense; and the measure of this demand is necessarily the quantity of commodities offered in exchange for the desired goods. General demand and supply are thus reciprocal to each other. But as soon as money, or general purchasing power, is introduced, Mr. Cairnes(208) defines “demand as the desire for commodities or services, seeking its end by an offer of general purchasing power; and supply, as the desire for general purchasing power, seeking its end by an offer of specific commodities or services.” But many persons find a difficulty because they insist upon separating the idea of supply from that of demand, owing to the fact that producers seem to be a distinct class in the community, different from consumers. That they are in reality the same persons can be easily explained by the following statement: “A certain number of people, A, B, C, D, E, F, etc., are engaged in industrial occupations—A produces for B, C, D, E, F; B for A, C, D, E, F; C for A, B, D, E, F, and so on. In each case the producer and the consumers are distinct, and hence, by a very natural fallacy, it is concluded that the whole body of consumers is distinct from the whole body of producers, whereas they consist of precisely the same persons.” But in regard to demand and supply of particular commodities (not general demand and supply), the increase of the demand is not necessarily followed by an increased supply, or _vice versa_. Out of the total production (which constitutes general demand) a varying amount, sometimes more, sometimes less, may be directed by the desires of men to the purchase of some given thing. This should be borne in mind, in connection with the future discussion of over-production. The identity of general demand with general supply shows there can be no general over-production: but so long as there exists the possibility that the demand for a particular commodity may diminish without a corresponding effect being thereby produced on the supply of that commodity, by a necessary connection, we see that there may be over-production of particular commodities; that is, a production in excess of the demand.