By the middle of the nineteenth century, English-speaking economists generally shared a perspective on value theory and distribution theory. The value of a bushel of corn, for example, was thought to depend on the costs involved in producing that bushel. The output or product of an economy was thought to be divided or distributed among the different social groups in accord with the costs borne by those groups in producing the output. This, roughly, was the "Classical Theory" developed by Adam Smith, David Ricardo, Thomas Robert Malthus, John Stuart Mill, and Karl Marx.
But there were difficulties in this approach. Chief among them was that prices in the market did not necessarily reflect the "value" so defined, for people were often willing to pay more than an object was "worth." The classical "substance" theories of value, which took value to be a property inherent in an object, gradually gave way to a perspective in which value was associated with the relationship between the object and the person obtaining the object. Several economists in different places at about the same time (the 1870s and 1880s) began to base value on the relationship between costs of production and "subjective elements, " later called "supply" and "demand." This came to be known as the Marginal Revolution in economics, and the overarching theory that developed from these ideas came to be called neoclassical economics. (The first to use the term "neoclassical economics" seems to have been the American economist Thorstein Veblen.)
The framework of neoclassical economics is easily summarized. Buyers attempt to maximize their gains from getting goods, and they do this by increasing their purchases of a good until what they gain from an extra unit is just balanced by what they have to give up to obtain it. In this way they maximize "utility"—the satisfaction associated with the consumption of goods and services. Likewise, individuals provide labor to firms that wish to employ them, by balancing the gains from offering the marginal unit of their services (the wage they would receive) with the disutility of labor itself—the loss of leisure. Individuals make choices at the margin. This results in a theory of demand for goods, and supply of productive factors.
Similarly, producers attempt to produce units of a good so that the cost of producing the incremental or marginal unit is just balanced by the revenue it generates. In this way they maximize profits. Firms also hire employees up to the point that the cost of the additional hire is just balanced by the value of output that the additional employee would produce.
The neoclassical vision thus involves economic "agents, " be they households or firms, optimizing (doing as well as they can), subject to all relevant constraints. Value is linked to unlimited desires and wants colliding with constraints, or scarcity. The tensions, the decision problems, are worked out in markets. Prices are the signals that tell households and firms whether their conflicting desires can be reconciled.