The key to the modern concept of the market may be found in the famous observation of the 18th-century British economist Adam Smith that “The division of labour depends upon the extent of the market.” He foresaw that modern industry depended for its development upon an extensive market for its products. The factory system developed out of trade in cotton textiles, when merchants, discovering an apparently insatiable worldwide market, became interested in increasing production in order to have more to sell. The factory system led to the use of power to supplement human muscle, followed in turn by the application of science to technology, which in an ever-accelerating spiral has produced the scope and complexity of modern industry.

The economic theory of the late 19th century, which is still influential in academic teaching, was, however, concerned with the allocation of existing resources between different uses rather than with technical progress. This theory was highly abstract. The concept of the market was most systematically worked out in a general equilibrium system developed by the French economist Léon Walras, who was strongly influenced by the theoretical physics of his time. His system of mathematical equations was ingenious, but there are two serious limitations to the mechanical analogy upon which they were based: it omitted the factor of time—the effect upon peoples’ present behaviour of their expectations about the future; and it ignored the consequences for the human beings concerned of the distribution of purchasing power among them. Though economists have always admitted the abstract nature of the theory, they generally have accepted the doctrine that the free play of market forces tended to bring about full employment and an optimum allocation of resources. On this view, unemployment could only be caused by wages being too high. This doctrine was still influential in the Great Depression of the 1930s.

The change in view that was to become known as the Keynesian Revolution was largely an escape to common sense, as opposed to abstract theory. In a private-enterprise economy, investment in industrial installations and housing construction is aimed at profitability in the future. Because investment therefore depends upon expectations, unfavourable expectations tend to fulfill themselves—when investment outlay falls off, workers become unemployed; incomes fall, purchases fall, unemployment spreads to the consumer goods industries, and receipts are reduced all the more. The operation of the market thus generates instability. The market may also generate instability in an upward direction. A high level of effective demand leads to a scarcity of labour; rising wages raise both costs of production and incomes so that there is a general tendency to inflation.

While the English economist John Maynard Keynes was attacking the concept of equilibrium in the market as a whole, the notion of equilibrium in the market for particular commodities was also being undermined. Traditional theory had conceived of a group of producers as operating in a perfect market for a single commodity; each produced only a small part of the whole supply; for each, the price was determined by the market; and each maximized its profits by selling only as much as would make marginal cost equal to price—that is to say, only so much that to produce a little more would add more to costs than it would to proceeds. Each firm worked its plant up to capacity—i.e., to the point where profitability was limited by rising costs. This state of affairs, known as “perfect competition,” is quite contrary to the general run of business experience, particularly in bad times when under-capacity working is prevalent. A theory of imperfect competition was invented to reconcile the traditional theory with under-capacity working but was attacked as unrealistic. The upshot was a general recognition that strict profit maximizing is impossible in conditions of uncertainty; that prices of manufactures are generally formed by adding a margin to direct costs, large enough to yield a profit at less than capacity sales; and that an increase in capacity generally has to be accompanied by a selling campaign to ensure that it will be used at a remunerative level.

Once it is recognized that competition is never perfect in reality, it becomes obvious that there is great scope for individual variations in the price policy of firms. No precise generalization is possible. The field is open for study of what actually happens, and exploration is going on. Meanwhile, however, textbook teaching often continues to seek refuge in the illusory simplicity of the traditional theory of market behaviour.