In Britain the Luddites broke stocking frames to stop machines taking their jobs.
On the other hand, global demand was damaged by failed ventures in South America and debilitated by the eventual downfall of Napoleon.
In Britain government spending was cut by 40% after the Battle of Waterloo in 1815.
Some 300,000 discharged soldiers and sailors were forced to seek alternative employment.
The result was a tide of overcapacity, what Say's contemporaries called a “general glut”.
Britain was accused of inundating foreign markets, from Italy to Brazil, much as China is blamed for dumping products today.
In 1818 a visitor to America found “not a city, nor a town, in which the quantity of goods offered for sale is not infinitely greater than the means of the buyers”.
It was this “general overstock of all the markets of the universe” that came to preoccupy Say and his critics.
In trying to explain it, Say at first denied that a “general” glut could exist.
Some goods can be oversupplied, he conceded.
But goods in general cannot.
His reasoning became known as Say's law: “it is production which opens a demand for products”, or, in a later, snappier formulation: supply creates its own demand.
他的推理成为了著名的萨伊定律（Say's law）:“正是生产开启对产品的需求”，或者是用后来更时髦的公式来说，就是：供给创造自己的需求（supply creates its own demand）。
This proposition, he admitted, has a “paradoxical complexion, which creates a prejudice against it”.
To the modern ear, it sounds like the foolhardy belief that “if you build it, they will come”.
Rick Perry, America's energy secretary, was ridiculed after a recent visit to a West Virginia coal plant for saying, “You put the supply out there and the demand will follow.”