Val'ue, one of the fundamental conceptions of political economy, has been variously defined, but is now generally considered to be the power, given by an article to its possessor, to command, in exchange for it, the labor or the product of labor of others. Obviously, a multitude of influences and qualities enter into the value of an article, and the number and relation of these factors have been the source of many controversies. Some have declared that usefulness is the one characteristic feature of valuable articles, but it has been justly replied that air is eminently useful, but has no economic value, since it cannot be exchanged, except under extraordinary conditions. Again, the labor cost of an article, it has been claimed, is really the source of its value, and the scientific socialists base their whole system upon this theory. Others have said that scarcity, or the difficulty of attainment, regulates value. Most modern economists agree that the two fundamental factors in value are utility, that is, the quality by which an article satisfies a human desire, and scarcity, or difficulty of attainment, which makes it necessary for a person to sacrifice labor or the product of labor in order to obtain it. These two factors, considered together, constitute what is known as the "law of supply and demand." Value, then, depends upon the relation between demand and supply, if demand is understood to mean the quantity of a given article which would be taken at a given price, and supply to mean the quantity of that article which could be had at that price. Evidently there can be no absolute standard of value, as there can be no absolute measure of desire or of difficulty; but to avoid rapid and violent fluctuation, values are measured by reference to some one article, that is, an article is said to be more or less valuable than a given article, according as its possessor would be willing to give more or less of it in exchange for the given article. Usually this article, or medium of measurement of value, is a precious metal and is called money. The expression of the value of an article in money is generally called its price. See POLITICAL ECONOMY.