这一节读起来很（英国式的）有趣：
- 作者通篇没有提及什么是萨伊定律，也没有提及
- 作者认为正确的Lionel Robbins版本到底是啥内容
- 而且大部分内容，其实在五卷本的第九章已经提过了。
Anyway，我们先看什么是萨伊定律，按照经济学帮帮忙网站的说法：
Say’s law states that the production of goods creates its own demand.
In 1803, John Baptiste Say explained his theory.
“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."
所以是X产品的供给创造了Y的需求 —— 要注意，这里没有货币的作用。
然而，更进一步：
John Maynard Keynes summarised Say’s law in his 1936, General Theory of Money:
“From the time of Say and Ricardo the classical economists have taught that supply creates its own demand...”
就变成了所谓的“供给创造需求” —— 创造自己的需求，要注意，凯恩斯的理论是有流动性陷阱的。
对比两者的说法，其实关键点在于：
- 货币的作用，是简单的中介，还是真的能引导需求（既然有陷阱，当然能引导），以及
- 两个版本定律的实际经济情况 —— 原有版本是经济正常运行状态，后者是大萧条，背景也不相同。
