This highly original contribution examines one of the most controversial concepts in the history of economics - the true meaning of the Law of Markets. This has been a contentious issue since the publication of Keynes's General Theory, but has also divided economists since it first emerged almost two centuries ago in the writings of James Mill. This book discusses the change in the understanding of the nature of the business cycle wrought by the General Theory whose major innovation in overturning Say's Law was to introduce demand deficiency into mainstream economic thought. The volume provides a robust and innovative exposition of the crucial point of division between classical and Keynesian economics, demonstrating that the role of demand deficiency was the fundamental issue at stake. Steven Kates first discusses Keynes's interpretation of Say's Law before documenting its development within classical theory. He then charts the development of post-General Theory interpretations of Say's Law, challenging Keynes's definition which was captured in the phrase `supply creates its own demand'. The author also attempts to unravel the vast literature on the progress made by Keynes between his Treatise on Money published in 1930 and the General Theory, published six years later. He suggests that the crucial point in the origins of the General Theory was Keynes's discovery of Malthus's writings on Say's Law at the very depths of the Great Depression in 1932. This provocative book will be required reading for scholars and students interested in the history of economic thought, the history of macroeconomics and the Keynesian revolution.