Here is a Paul Krugman article on Say’s Law published on 30 January 2009. Here is the entire note:

If there was one essential element in the work of John Maynard Keynes, it was the demolition of Say’s Law — the assertion that supply necessarily creates demand. Keynes showed that the fact that spending equals income, or equivalently that saving equals investment, does not imply that there’s always enough spending to fully employ the economy’s resources, that there’s always enough investment to make use of the saving the economy would have had it it were at full employment.

Getting to that realization was an awesome intellectual achievement. That’s why it’s deeply depressing to find, not that people like Eugene Fama disagree with Keynes’s conclusions — that’s OK, no theory is sacred — but that they’re obviously completely unaware of the whole argument.

One of Brad DeLong’s commentators compares what’s going on to the discovery that some eminent biologists are creationists, but it’s actually worse than that: it’s like discovering that some eminent biologists have never heard of the theory of evolution and the concept of natural selection.

How did we get to this point?

Interestingly, there is a comment by E. Roy Weintraub which supports Krugman and Keynes on Say’s Law:

Robinson’s Marxist turn, with Sraffa, is irrelevent to the current mess. Keynes (with Henderson) in ‘Can Lloyd George Do It?’ in 1929 laid out all that we need to do now with respect to economic policy. The theorization of it all in 1936 may have helped convince Keynes’s ‘fellow economists’ (Say’s Law idiots and so on), but practical people understood correctly that unemployment benefits, and public works transportation, housing, etc. would create jobs quickly. Alas, Lloyd George did not win, and FDR hardly got Keynes’s message. Schacht and Hitler did, but we don’t want to go there…

— E. Roy Weintraub

Well, of course even Henderson did not get the message which is ironic since the message was co-authored by Henderson himself.