Robert Blumen writes in today’s Mises Daily:
It is acknowledged by both sides that, if Say’s Law is true, then Keynes’s entire system is wrong. Keynes knew this, so he took upon himself the task of refuting Say’s Law as the very first thing in his General Theory. Keynes’s argument was that Say’s Law is only valid under the conditions of full employment, but that it does not hold when there are unemployed resources; in that case we are in the Keynesian Zone where the laws of economics are turned upside down.
But, as Stephen Kates explains in his book Say’s Law and the Keynesian Revolution (subtitled How Economics Lost its Way), Keynes failed in his attempt to overturn Say’s Law. Kates shows beyond any dispute that Say and his fellow classical economists were well aware that there could be unemployed resources, and that Say’s Law was still valid in that case.