'Walras' Law' generally means that total demand, including the demand for money, is equal to total supply, including money. This is merely a definition and has no economic implications. 'Say's Identity' refers to the proposition that the total demand for goods is always equal to the total supply of goods. Therefore, variations in the demand for money do not affect the level of economic activity. It is this proposition which is generally seen as the meaning of Say's Law contained in the General Theory. Finally, 'Say's Equality' means that while the demand for goods may move out of equilibrium with the supply of goods, the processes of the economy will rapidly bring the two back into equilibrium. This proposition is generally seen as the meaning of Say's Law held by classical economists.
The significant point is that the classical meaning of Say's Law and the modern are very different. And in this it is important to recognise that, although there is a modern meaning to Say's Law, if it is not what classical economists meant by it, then it has no intrinsic value as a means to understand pre-Keynesian economic theory. Say's Law should have only the meaning attached to it by economists who believed it was a valid principle of economic analysis. Sowell (1972: 5, 37), for example, argues that the modern interpretation is as valid as any other. Modern interpretations, which do not explain what classical economists actually meant, may have value in their own terms, but they should not be confused with the classical meaning of the law of markets.
--Steven Kates, introduction to Say's Law and the Keynesian Revolution: How Macroeconomic Theory Lost its Way (Cheltenham, UK: Edward Elgar, 2009), 5.