Are Recessions Caused by the Supply of Commodities in the Aggregate Surpassing the Demand?

The central issue in the debates over Say's Law was thus whether recessions might be due to 'a supply of commodities in the aggregate surpassing the demand', that is, whether recessions are caused by deficient effective demand. The answer, according to Say's Law, was no. Moreover, as the quotation from Mill also shows, Say's Law also denies the possibility of 'a general overproduction of wealth', which is the same phenomenon as a deficiency of demand. Demand deficiency and over-production are two ways of describing the same phenomenon: too much production relative to the demand for it. It was this possibility which Say's Law was formulated to deny.

--Steven Kates, introduction to Say's Law and the Keynesian Revolution: How Macroeconomic Theory Lost its Way (Cheltenham, UK: Edward Elgar, 2009), 1-2.