Say’s Law of Markets is based on the obvious fact that because nothing can be consumed until and unless it is first produced, we must first produce before we can consume. If we wish to consume what others produce, we can only do so (absent gift or alms) by offering something we have produced for what others have produced.
Thus, recalling our discussion of contracts and that they consist of offer, acceptance, and consideration, “money” serves as the medium by means of which we exchange what we produce for what others produce, that is, enter into and fulfill contractual obligations, the basis of a functional civil order. As Say explained when refuting the scarcity-based economics of the Reverend Thomas Malthus,
“All those who, since Adam Smith, have turned their attention to Political Economy, agree that in reality we do not buy articles of consumption with money, the circulating medium with which we pay for them. We must in the first instance have bought this money itself by the sale of our produce.
“To a proprietor of a mine, the silver money is a produce with which he buys what he has occasion for. To all those through whose hands this silver afterwards passes, it is only the price of the produce which they themselves have raised by means of their property in land, their capitals, or their industry. In selling them they in the first place exchange them for money, and afterwards they exchange the money for articles of consumption. It is therefore really and absolutely with their produce that they make their purchases: therefore it is impossible for them to purchase any articles whatever, to a greater amount than those they have produced, either by themselves or through the means of their capital or their land.
“From these premises I have drawn a conclusion which appears to me evident, but the consequences of which appear to have alarmed you. I had said — As no one can purchase the produce of another except with his own produce, as the amount for which we can buy is equal to that which we can produce, the more we can produce the more we can purchase. From whence proceeds this other conclusion, which you refuse to admit — That if certain commodities do not sell, it is because others are not produced, and that it is the raising produce alone which opens a market for the sale of produce.
“I know that this proposition has a paradoxical complexion, which creates a prejudice against it. I know that one has much greater reason to expect to be supported by vulgar prejudices, when one asserts that the cause of too much produce is because all the world is employed in raising it. — That instead of continually producing, one ought to multiply barren consumptions, and expend the old capital instead of accumulating new. This doctrine has, indeed, probability on its side; it can be supported by arguments, facts may be interpreted in its favor. But, Sir, when Copernicus and Galileo taught, for the first time, that the sun, although we see it rise every morning in the east, magnificently pass over our heads at noon, and precipitate itself towards the west in the evening, still does not move from its place, they had also universal prejudice against them, the opinions of the Ancients, and the evidence of the senses. Ought they on that account to relinquish those demonstrations which were produced by a sound judgment? I should do you an injustice to doubt your answer.
“Besides, when I assert that produce opens a vent for produce; that the means of industry, whatever they may be, left to themselves, always incline themselves to those articles which are the most necessary to nations, and that these necessary articles create at the same time fresh populations, and fresh enjoyments for those populations, all probability is not against me.” (Jean-Baptiste Say, Letters to Malthus. London: Sherwood, Neely, and Jones, 1821, 2-3.)