Thanks for your good question,my knowledge about this follows:

Say's Law of Market is often stated roughly as Say's Law which implies the economic system is always operating at the full-employment,the "(aggregate) demand creates its own (aggregate) supply".

Say's Law interpreted by Oscar Lange as Say's Identity in 1942.Under the assumption that the demand and supply functions for commodities are homogeneous degree at zero to money prices,in other words,the variations of money price do no affects to the economic activities.

Say's Equality was defined to mean if the demand for money moves out the equilibrium with the supply of goods, the processes of economy will bring the both into equilibrium immediately.Say's Equality moves back to Say's Law (of Market).

[此贴子已经被作者于2006-4-15 14:28:12编辑过]

多谢斑竹.

我可不可这么理解斑竹的话:
Say's Law of Market就是通常所说的萨伊定律.
Say's Identity等价于货币中性或货币面纱论.
Say's Equality指完全竞争的市场会自动趋向充分就业均衡状态.
不知道理解对没有?
BTW这句话Say's Equality moves back to Say's Law (of Market)没懂.
Say's Equality与Say's Law两者是过程与结果的关系,还是包含与被包含的关系,或是其它关系呢?

你的理解基本是我所理解的那样.我记得这三个词好像是这样的过程:Say's Law到Say's Identity,是被特定化的过程,加入了特定的涵义,主要是被用于货币理论.后来又提出了Say's Equality,回到了市场机制.