The real difficulty in formulating the general definition of supply and demand is that it seems to take on the appearance of a tautology. First consider the supply — the product available in the market, or that which can be delivered to it. To avoid dwelling upon useless detail, we shall here consider only the mass annually reproduced in every given branch of production and ignore the greater or lesser faculty possessed by the different commodities to be withdrawn from the market and stored away for consumption, say, until next year. This annual reproduction is expressed by a certain quantity — in weight or numbers — depending on whether this mass of commodities is measured in discrete elements or continuously. They are not only use-values satisfying human wants, but these use-values are available in the market in definite quantities. Secondly, however, this quantity of commodities has a specific market-value, which may be expressed by a multiple of the market-value of the commodity, or of its measure, which serves as unit. Thus, there is no necessary connection between the quantitative volume of the commodities in the market and their market-value, since, for instance, many commodities have a specifically high value, and others a specifically low value, so that a given sum of values may be represented by a very large quantity of one commodity, and a very small quantity of another. There is only the following connection between the quantity of the articles available in the market and the market-value of these articles: On a given basis of labour productivity the production of a certain quantity of articles in every particular sphere of production requires a definite quantity of social labour-time; although this proportion varies in different spheres of production and has no inner relation to the usefulness of these articles or the special nature of their use-values. Assuming all other circumstances to be equal, and a certain quantity a of some commodity to cost b labour-time, a quantity na of the same commodity will cost nb labour-time. Further, if society wants to satisfy some want and have an article produced for this purpose, it must pay for it. Indeed, since commodity-production necessitates a division of labour, society pays for this article by devoting a portion of the available labour-time to its production. Therefore, society buys it with a definite quantity of its disposable labour-time. That part of society which through the division of labour happens to employ its labour in producing this particular article, must receive an equivalent in social labour incorporated in articles which satisfy its own wants. However, there exists an accidental rather than a necessary connection between the total amount of social labour applied to a social article, i.e., between the aliquot part of society's total labour-power allocated to producing this article, or between the volume which the production of this article occupies in total production, on the one hand, and the volume whereby society seeks to satisfy the want gratified by the article in question, on the other. Every individual article, or every definite quantity of a commodity may, indeed, contain no more than the social labour required for its production, and from this point of view the market-value of this entire commodity represents only necessary labour, but if this commodity has been produced in excess of the existing social needs, then so much of the social labour-time is squandered and the mass of the commodity comes to represent a much smaller quantity of social labour in the market than is actually incorporated in it. (It is only where production is under the actual, predetermining control of society that the latter establishes a relation between the volume of social labour-time applied in producing definite articles, and the volume of the social want to be satisfied by these articles.) For this reason, these commodities must be sold below their market-value, and a portion of them may even be altogether unsaleable. The reverse applies if the quantity of social labour employed in the production of a certain kind of commodity is too small to meet the social demand for that commodity. But if the quantity of social labour expended in the production of a certain article corresponds to the social demand for that article, so that the produced quantity corresponds to the usual scale of reproduction and the demand remains unchanged, then the article is sold at its market-value. The exchange, or sale, of commodities at their value is the rational state of affairs, i.e., the natural law of their equilibrium. It is this law that explains the deviations, and not vice versa, the deviations that explain the law.