To grasp Say's point requires two intellectual jumps.
The first is to see past money, which can obscure what is really going on in an economy.
The second is to jump from micro to macro, from a worm's eye view of individual plants and specific customers to a panoramic view of the economy as a whole.
Firms, like coal plants and cotton mills, sell their products for money.
But in order to obtain that money, their customers must themselves have previously sold something of value.
Thus, before they can become a source of demand, customers must themselves have been a source of supply.
What most people sell is their labour, one of several “productive services” on offer to entrepreneurs.
By marshalling these productive forces, entrepreneurs can create a new item of value, for which other equally valuable items can then be exchanged.
It is in this sense that production creates a market for other products.
In the course of making his merchandise, a producer will pay wages to his workers, rent to his landlord, interest to his creditors, the bills of his suppliers and any residual profits to himself.
These payments will at least equal the amount the entrepreneur can get for selling his product.
The payments will therefore add as much to spendable income as the recipients' joint enterprise has added to supply.
That supply creates demand in this way may be easy enough to grasp.
But in what sense does supply create its “own” demand?
The epigram seems to suggest that a coal plant could buy its own coal—like a subsistence farmer eating the food he grows.
In fact, of course, most producers sell to, and buy from, someone else.
But what is true at the micro level is not true at the macro level.
At the macro level, there is no someone else.
The economy is an integrated whole.
What it purchases and distributes among its members are the self-same goods and services those members have jointly produced.
At this level of aggregation, the economy is in fact not that different from the subsistence farmer.
What it produces, what it earns, and what it buys is all the same, a “harvest” of goods and services, better known as gross domestic product.
它所生产的、所赚取的，所购买的全都是相同的，是商品和服务的“收成”，即更为人们所熟知的国内生产总值（gross domestic product）。
How then did Say explain the woes of his age, the stuffed warehouses, clogged ports and choked markets?
He understood that an economy might oversupply some commodities, if not all.
That could cause severe, if temporary, distress to anyone involved in the hypertrophied industries.
But he argued that for every good that is too abundant, there must be another that is too scarce.
The labour, capital and other resources devoted to oversupplying one market must have been denied to another more valuable channel of industry, leaving it under-resourced.