In classical economics, Say's law, or the law of markets, is the claim that the production of a product creates demand for another product by providing something of value which can be exchanged for that other product. So, production is the source of demand. It is named after Jean-Baptiste Say. In his principal work, A Treatise on Political Economy "A product is no sooner created, than it, from that instant, affords a market for other products to the full extent of its own value." And also, "As each of us can only purchase the productions of others with his/her own productions – as the value we can buy is equal to the value we can produce, the more men can produce, the more they will purchase." Some maintain that Say further argued that this law of markets implies that a general glut (a widespread excess of supply over demand) cannot occur. If there is a surplus of one good, there must be unmet demand for another: "If certain goods remain unsold, it is because other goods are not produced." However, according to Petur Jonsson, Say does not claim a general glut cannot occur and in fact acknowledges that they can occur. Say's law has been one of the principal doctrines used to support the laissez-faire belief that a capitalist economy will naturally tend toward full employment and prosperity without government intervention. Say's law was generally accepted throughout the 19th century, though modified to incorporate the idea of a "boom-and-bust cycle". During the worldwide Great Depression of the 1930s, the theories of Keynesian economics disputed Say's conclusions with regards to times of high unemployment.
Theoretical contribution
Say's law did not posit that (as per the Keynesian formulation) "supply creates its own demand". Nor was it based on the idea that everything that is saved will be exchanged. Rather, Say sought to refute the idea that production and employment were limited by low consumption. Thus Say's law, in its original concept, was not intrinsically linked nor logically reliant on the neutrality of money (as has been alleged by those who wish to disagree with it), because the key proposition of the law is that no matter how much people save, production is still a possibility, as it is the prerequisite for the attainment of any additional consumption goods. Say's law states that in a market economy, goods and services are produced for exchange with other goods and services—"employment multipliers" therefore arise from production and not exchange alone—and that in the process a sufficient level of real income is created to purchase the economy's entire output, due to the truism that the means of consumption are by definition limited by the level of production. That is, with regard to the exchange of products within a division of labour, the total supply of goods and services in a market economy will equal the total demand derived from consumption during any given time period. In modern terms, "general gluts cannot exist", although there may be local imbalances, with gluts in some markets balanced out by shortages in others.
Formulation Say argued that economic agents offer goods and services for sale so that they can spend the money they expect to obtain. Therefore, the fact that a quantity of goods and services is offered for sale is evidence of an equal quantity of demand. Essentially Say's argument was that money is just a medium, people pay for goods and services with other goods and services. This claim is often summarized as "supply creates its own demand", although that phrase does not appear in Say's writings. Explaining his point at length, Say wrote:
It is worthwhile to remark that a product is no sooner created than it, from that instant, affords a market for other products to the full extent of its own value. When the producer has put the finishing hand to his product, he is most anxious to sell it immediately, lest its value should diminish in his hands. Nor is he less anxious to dispose of the money he may get for it; for the value of money is also perishable. But the only way of getting rid of money is in the purchase of some product or other. Thus the mere circumstance of creation of one product immediately opens a vent for other products. Say further argued that because production necessarily creates demand, a "general glut" of unsold goods of all kinds is impossible. If there is an excess supply of one good, there must be a shortage of another: "The superabundance of goods of one description arises from the deficiency of goods of another description." To further clarify, he wrote: "Sales cannot be said to be dull because money is scarce, but because other products are so. ... To use a more hackneyed phrase, people have bought less, because they have made less profit." Say's law should therefore be formulated as: Supply of X creates demand for Y, subject to people being interested in buying X. The producer of X is able to buy Y, if his products are demanded. Say rejected the possibility that money obtained from the sale of goods could remain unspent, thereby reducing demand below supply. He viewed money only as a temporary medium of exchange.
Money performs but a momentary function in this double exchange; and when the transaction is finally closed, it will always be found, that one kind of commodity has been exchanged for another.
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