Say's law, or the law of markets, is the controversial assertion, found in classical economics, that aggregate production necessarily creates an equal quantity of aggregate demand. It was stated by the French economist Jean-Baptiste Say (1767–1832), who wrote:
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.
As each of us can only purchase the productions of others with his 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.
Say further argued that the law of markets implies that a "general glut" (the term used in Say's time for 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." 
Along with Adam Smith's concept of the "invisible hand", Say's law has been one of the doctrines used to support the laissez-faire belief that a capitalist economy will naturally tend toward full employment and prosperity without government intervention.
Over the years, at least two objections to Say's law have been raised:
- General gluts do in fact occur, particularly during recessions and depressions.
- Economic agents may collectively choose to increase the amount of money they hold, thereby reducing demand but not supply.
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 novel theories of Keynesian economics disputed Say's conclusions. The debate between classical and Keynesian economics continues today.
Scholars disagree on the surprisingly subtle question of whether it was Say who first stated the principle, but by convention, "Say's law" has been another name for the law of markets ever since John Maynard Keynes used the term in the 1930s.
- 1 History of Say's law
- 2 Consequences
- 3 Assumptions and criticisms
- 4 Say's law as a theoretical point of departure
- 5 Modern interpretations
- 6 See also
- 7 References
- 8 Further reading
- 9 External links
History of Say's law
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. 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, he 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."
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.
Early writers on political economy held a variety of opinions on what we now call Say’s law. James Mill and David Ricardo both supported the law in full. Thomas Malthus and John Stuart Mill questioned the doctrine that general gluts cannot occur.
James Mill and David Ricardo restated and developed Say's law. Mill wrote, "The production of commodities creates, and is the one and universal cause which creates, a market for the commodities produced." Ricardo wrote, "Demand depends only on supply."
Thomas Malthus, on the other hand, rejected Say’s law because he saw evidence of general gluts.
We hear of glutted markets, falling prices, and cotton goods selling at Kamschatka lower than the costs of production. It may be said, perhaps, that the cotton trade happens to be glutted; and it is a tenet of the new doctrine on profits and demand, that if one trade be overstocked with capital, it is a certain sign that some other trade is understocked. But where, I would ask, is there any considerable trade that is confessedly under-stocked, and where high profits have been long pleading in vain for additional capital?
John Stuart Mill also recognized general gluts. He argued that during a general glut, there is insufficient demand for all non-monetary commodities and excess demand for money.
When there is a general anxiety to sell, and a general disinclination to buy, commodities of all kinds remain for a long time unsold, and those which find an immediate market, do so at a very low price... At periods such as we have described... persons in general... liked better to possess money than any other commodity. Money, consequently, was in request, and all other commodities were in comparative disrepute... As there may be a temporary excess of any one article considered separately, so may there of commodities generally, not in consequence of over-production, but of a want of commercial confidence.
Mill rescued the claim that there cannot be a simultaneous glut of all commodities by including money as one of the commodities.
In order to render the argument for the impossibility of an excess of all commodities applicable... money must itself be considered as a commodity. It must, undoubtedly, be admitted that there cannot be an excess of all other commodities, and an excess of money at the same time.
Contemporary economist Brad DeLong considers that Mill’s argument refutes the assertions that a general glut cannot occur, and that a market economy naturally tends towards an equilibrium in which general gluts do not occur. What remains of Say's law, after Mill's modification, are a few less controversial assertions:
- In the long run, the ability to produce does not outstrip the desire to consume.
- In a barter economy, a general glut cannot occur.
- In a monetary economy, a general glut occurs not because sellers produce more commodities of every kind than buyers wish to purchase, but because buyers increase their desire to hold money.
Say himself never used many of the later, short definitions of Say's law, and thus the law actually developed through the work of many of his contemporaries and successors. The work of James Mill, David Ricardo, John Stuart Mill, and others evolved Say's law into what is sometimes called "law of markets", which was a key element of the framework of macroeconomics from the mid-19th century until the 1930s.
Say's law and the Great Depression
The Great Depression posed a challenge to Say's law. In the United States, unemployment rose to 25%. The quarter of the labor force that was unemployed constituted a supply of labor for which the demand predicted by Say's law did not exist.
John Maynard Keynes argued in 1936 that Say's law is simply not true, and that demand, rather than supply, is the key variable that determines the overall level of economic activity. According to Keynes, demand depends on the propensity of individuals to consume and on the propensity of businesses to invest, both of which vary throughout the business cycle. There is no reason to expect enough aggregate demand to produce full employment.
Say's law today
Today, the vast majority of economists believe Say's law is incorrect. 
However, some proponents of the heterodox Austrian school of economics maintain that the economy tends to full-employment equilibrium, and that recessions and depressions are the result of government intervention in the economy.  Some proponents of real business cycle theory maintain that high unemployment is due to a reduced labor supply rather than reduced demand. In other words, people choose to work less when economic conditions are poor, so that involuntary unemployment does not actually exist.  
Furthermore, some politicians today believe Say's law. For example, French President François Hollande declared, "Supply actually creates demand." 
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A number of laissez-faire consequences have been drawn from interpretations of Say's law. However, Say himself advocated public works to remedy unemployment and criticized Ricardo for neglecting the possibility of hoarding if there was a lack of investment opportunities.
Recession and unemployment
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Say argued against claims that businesses suffer because people do not have enough money. He argued that the power to purchase can only be increased through more production.
James Mill used Say's law against those who sought to give the economy a boost via unproductive consumption. In his view, consumption destroys wealth, in contrast to production, which is the source of economic growth. The demand for a product determines the price of the product.
According to Keynes (see more below), if Say's law is correct, widespread involuntary unemployment (caused by inadequate demand) cannot occur. Classical economists in the context of Say's law explain unemployment as arising from insufficient demand for specialized labour—that is, the supply of viable labour exceeds demand in some segments of the economy.
When more goods are produced by firms than are demanded in certain sectors, the suppliers in those sectors lose revenue as result. This loss of revenue, which would in turn have been used to purchase other goods from other firms, lowers demand for the products of firms in other sectors, causing an overall general reduction in output and thus lowering the demand for labour. This results in what contemporary macroeconomics call structural unemployment, the presumed mismatch between the overall demand for labour in jobs offered and the individual job skills and location of labour. This differs from the Keynesian concept of cyclical unemployment, which is presumed to arise because of inadequate aggregate demand.
Such economic losses and unemployment were seen by some economists, such as Marx and Keynes himself, as an intrinsic property of the capitalist system. The division of labor leads to a situation where one always has to anticipate what others will be willing to buy, and this leads to miscalculations. However, this theory alone does not explain the existence of cyclical phenomena in the economy, because such miscalculations would happen with constant frequency, and to such a large scale that thousands of businesses in multiple sectors would simultaneously miscalculate (as during an economic bubble).[according to whom?]
Assumptions and criticisms
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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 limited ex vi termini 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.
Nevertheless, for some neoclassical economists, Say's law implies that economy is always at its full employment level. This is not necessarily what Say proposed.
In the Keynesian interpretation, the assumptions of Say's law are:
- a barter model of money ("products are paid for with products");
- flexible prices—that is, all prices can rapidly adjust upwards or downwards; and
- no government intervention.
Under these assumptions, Say's law implies that there cannot be a general glut, so that a persistent state cannot exist in which demand is generally less than productive capacity and high unemployment results. Keynesians therefore argued[who?][when?] that the Great Depression demonstrated that Say's law is incorrect. Keynes, in his General Theory, argued that a country could go into a recession because of "lack of aggregate demand".
Because historically there have been many persistent economic crises, one may reject one or more of the assumptions of Say's law, its reasoning, or its conclusions. Taking the assumptions in turn:
- Circuitists and some post-Keynesians dispute the barter model of money, arguing that money is fundamentally different from commodities and that credit bubbles can and do cause depressions. Notably, the debt owed does not change because the economy has changed.
- Keynes argued that prices are not flexible; for example, workers may not take pay cuts if the result is starvation.
- Laissez-faire economists[who?] argue that government intervention is the cause of economic crises, and that left to its devices, the market will adjust efficiently.
As for the implication that dislocations cannot cause persistent unemployment, some theories of economic cycles accept Say's law and seek to explain high unemployment in other ways, considering depressed demand for labour as a form of local dislocation. For example, advocates of Real Business Cycle Theory argue that real shocks cause recessions and that the market responds efficiently to these real economic shocks.
Role of money
It is not easy to say what exactly Say's law says about the role of money apart from the claim that recession is not caused by lack of money. The phrase "products are paid for with products" is taken to mean that Say has a barter model of money; contrast with circuitist and post-Keynesian monetary theory.
One can read Say as stating simply that money is completely neutral, although he did not state this explicitly, and in fact did not concern himself with this subject. Say's central notion concerning money was that if one has money, it is irrational to hoard it.
a tenacious fallacy, consequent on the inveterate evasion of the plain fact that men want for their goods, not merely some other goods to consume, but further, some credit or abstract claim to future wealth, goods, or services. This all want as a surplus or bonus, and this surplus cannot be represented for all in present goods.—John M. Robertson, The Fallacy of Saving, p. 98
Here Robertson identifies his critique as based on Say's theory of money: people wish to accumulate a "claim to future wealth", not simply present goods, and thus the hoarding of wealth may be rational.
For Say, as for other classical economists, it is quite possible for there to be a glut (excess supply, market surplus) for one product alongside a shortage (excess demand) of others. But there is no "general glut" in Say's view, since the gluts and shortages cancel out for the economy as a whole. But what if the excess demand is for money, because people are hoarding it? This creates an excess supply for all products, a general glut. Say's answer is simple: there is no reason to engage in hoarding money. According to Say, the only reason to have money is to buy products. It would not be a mistake, in his view, to treat the economy as if it were a barter economy. To quote Say:
Nor is [an individual] less anxious to dispose of the money he may get ... But the only way of getting rid of money is in the purchase of some product or other.
An alternative modern view[by whom?] that gives an equivalent result is that all money that is held is done so in financial institutions (markets), so that any increase in the holding of money increases the supply of loanable funds. Then, with full adjustment of interest rates, the increased supply of loanable funds leads to an increase in borrowing and spending. Consequently, any negative effects on demand that results from the holding of money are canceled out, and Say's law still applies.
In Keynesian terms, followers of Say's law would argue that on the aggregate level, there is only a transactions demand for money. That is, there is no precautionary, finance, or speculative demand for money. Money is held for spending, and increases in money supplies lead to increased spending.
Some classical economists did see that a loss of confidence in business or a collapse of credit will increase the demand for money, which will decrease the demand for goods. This view was expressed both by Robert Torrens and John Stuart Mill. This would lead demand and supply to move out of phase and lead to an economic downturn in the same way that miscalculation in productions would, as described by William H. Beveridge in 1909.
However, in classical economics, there was no reason for such a collapse to persist. In this view, persistent depressions, such as that of the 1930s, are impossible in a free market organized according to laissez-faire principles. The flexibility of markets under laissez faire allows prices, wages, and interest rates to adjust so as to abolish all excess supplies and demands; however, since all economies are a mixture of regulation and free-market elements, laissez-faire principles (which require a free market environment) cannot adjust effectively to excess supply and demand.
Say's law as a theoretical point of departure
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The whole of neoclassical equilibrium analysis implies that Say's law in the first place functioned to bring a market into this state: that is, Say's law is the mechanism through which markets equilibrate uniquely. Equilibrium analysis and its derivatives of optimization and efficiency in exchange live or die with Say's law. This is one of the major, fundamental points of contention between the neoclassical tradition, Keynes, and Marxians. Ultimately, from Say's law they deduced vastly different conclusions regarding the functioning of capitalist production.
The former, not to be confused with "new Keynesian" and the many offsprings and syntheses of the "General Theory", take the fact that a commodity–commodity economy is substantially altered once it becomes a commodity–money–commodity economy, or once money becomes not only a facilitator of exchange (its only function in marginalist theory) but also a store of value and a means of payment. What this means is that money can be (and must be) hoarded: it may not re-enter the circulatory process for some time, and thus a general glut is not only possible but, to the extent that money is not rapidly turned over, probable.
A response to this in defense of Say's law (echoing the debates between Ricardo and Malthus, in which the former denied the possibility of a general glut on its grounds) is that consumption that is abstained from through hoarding is simply transferred to a different consumer—overwhelmingly to factor (investment) markets, which, through financial institutions, function through the rate of interest.
Keynes' innovation in this regard was twofold: First, he was to turn the mechanism that regulates savings and investment, the rate of interest, into a shell of its former self (relegating it to the price of money) by showing that supply and investment were not independent of one another and thus could not be related uniquely in terms of the balancing of disutility and utility. Second, after Say's law was dealt with and shown to be theoretically inconsistent, there was a gap to be filled. If Say's law was the logic by which we thought financial markets came to a unique position in the long run, and if Say's law were to be discarded, what were the real "rules of the game" of the financial markets? How did they function and remain stable?
To this Keynes responded with his famous notion of "Animal Spirits": markets were ruled by speculative behavior, influenced not only by one's own personal equation but also by one's perceptions of the speculative behavior of others. In turn, others' behavior was motivated by their perceptions of others' behavior, and so on. Without Say's law keeping them in balance, financial markets were thus inherently unstable. Through this identification, Keynes deduced the consequences of the macroeconomy of long-run equilibrium being attained not at only one unique position that represented a "Pareto Optima" (a special case), but through a possible range of many equilibria that could significantly under-employ human and natural resources (the general case).
For the Marxian critique, which is more fundamental, one must start at Marx's initial distinction between use value and exchange value, use value being the use somebody has for a commodity, and exchange value being what an item is traded for on a market. In Marx's theory, there is a gap between the creation of surplus value in production and the realization of that surplus value via a sale. To realize a sale, a commodity must have a use value for someone, in order for them to purchase the commodity and complete the cycle M–C–M'. Capitalism, which is purely interested in value (money as wealth), must create use value. The capitalist has no control over whether or not the value contained in the product is realized through the market mechanism. This gap between production and realization creates the possibility for capitalist crisis. As the realization of capital is only possible through a market, Marx criticized other economists, such as David Ricardo, who argued that capital is realized via production. Thus, in Marx's theory, there can be general overproductive crises within capitalism.
Given these concepts and their implications, Say's law does not hold in the Marxian framework. Moreover, the theoretical core of the Marxian framework contrasts with that of the neoclassical and Austrian traditions.
Conceptually, the distinction between Keynes and Marx is that for Keynes the theory is but a special case of his general theory, whereas for Marx it never existed at all.
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A modern way of expressing Say's law is that there can never be a general glut. Instead of there being an excess supply (glut or surplus) of goods in general, there may be an excess supply of one or more goods, but only when balanced by an excess demand (shortage) of yet other goods. Thus, there may be a glut of labor ("cyclical" unemployment), but this is balanced by an excess demand for produced goods. Modern advocates of Say's law see market forces as working quickly, via price adjustments, to abolish both gluts and shortages. The exception is when governments or other non-market forces prevent price adjustments.
According to Keynes, the implication of Say's law is that a free-market economy is always at what Keynesian economists call full employment (see also Walras' law). Thus, Say's law is part of the general world view of laissez-faire economics—that is, that free markets can solve the economy's problems automatically. (These problems are recessions, stagnation, depression, and involuntary unemployment.)
Some proponents of Say's law argue that such intervention is always counterproductive. Consider Keynesian-type policies aimed at stimulating the economy. Increased government purchases of goods (or lowered taxes) merely "crowd out" the production and purchase of goods by the private sector. Contradicting this view, Arthur Cecil Pigou, a self-proclaimed follower of Say's law, wrote a letter in 1932 signed by five other economists (among them Keynes) calling for more public spending to alleviate high levels of unemployment.
Keynes versus Say
Keynes summarized Say's law as "supply creates its own demand", or the assumption "that the whole of the costs of production must necessarily be spent in the aggregate, directly or indirectly, on purchasing the product" (from chapter 2 of his General Theory).
Keynesian economists, such as Paul Krugman, stress the role of money in negating Say's law: Money that is hoarded (held as cash or analogous financial instruments) is not spent on products. To increase monetary holdings, someone may sell products or labor without immediately spending the proceeds. This can be a general phenomenon: from time to time, in response to changing economic circumstances, households and businesses in aggregate seek to increase net savings and thus decrease net debt. To increase net savings requires earning more than is spent—contrary to Say's law, which postulates that supply (sales, earning income) equals demand (purchases, requiring spending). Keynesian economists argue that the failure of Say's law, through an increased demand for monetary holdings, can result in a general glut due to falling demand for goods and services.
This provides an explanation for recessionary spirals and economic depressions. Without sufficient demand for the products of labor, the availability of jobs will be low; without enough jobs, working people will receive inadequate income, implying insufficient demand for products. Thus, an aggregate demand failure involves a vicious circle: if a worker supplies more labor time (in order to buy more goods), he or she may be frustrated because no one is hiring—that is, because there is no increase in the demand for the worker's products until after he or she gets a job and earns an income. (Of course, most employees get paid after working, after some of the product is sold.) Further, unlike the Say's law story above, there are interactions between different markets (and their gluts and shortages) that go beyond the simple price mechanism, to limit the quantity of jobs supplied and the quantity of products demanded.
Some classical economists[who?] suggested that hoarding (increases in money-equivalent holdings) would always be balanced by dis-hoarding. This requires equality of saving (abstention from purchase of goods) and investment (the purchase of capital goods). However, Keynes and others argued that hoarding decisions are made by different people and for different reasons than are decisions to dis-hoard, so that hoarding and dis-hoarding are unlikely to be equal at all times, as indeed they are not. Decreasing demand (consumption) does not necessarily stimulate capital spending (investment).
Some[who?] have argued that financial markets, and especially interest rates, could adjust to keep hoarding and dis-hoarding equal, so that Say's law could be maintained, or that prices could simply fall, to prevent a decrease in production. (See the discussion of "excess saving" under "Keynesian economics".) But Keynes argued that in order to play this role, interest rates would have to fall rapidly and that there were limits on how quickly and how low they could fall (as in the liquidity trap, where interest rates approach zero and cannot fall further). To Keynes, in the short run, interest rates were determined more by the supply and demand for money than by saving and investment. Before interest rates could adjust sufficiently, excessive hoarding would cause the vicious circle of falling aggregate production (recession). The recession itself would lower incomes so that hoarding (and saving) and dis-hoarding (and real investment) could reach a state of balance below full employment.
Worse, a recession would hurt private real investment—by hurting profitability and business confidence—through what is called the accelerator effect. This means that the balance between hoarding and dis-hoarding would be pushed even further below the full-employment level of production.
Keynesians argue that this kind of vicious circle can be broken by stimulating the aggregate demand for products using various macroeconomic policies mentioned in the introduction above, making Say's law true in practice when it is false in theory . Increases in the demand for products lead to increased supply (production) and an increased availability of jobs, and thus to further increases in demand and in production. This cumulative causation is called the multiplier process.
- Demand side economics, the New Keynesian perspective
- Fiscal policy
- List of eponymous laws
- Parable of the broken window
- Treasury view, a related critical view of fiscal policy
- Walras' law
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