Book excerpt: From the 30 year General Glut debate at the start of the 19th century which focused solely on its truth, to the Keynesian revolution and Keynes's successful attempt to convince his fellow economists that Say's Law was wrong, it remains the most controversial principle in the history of economic theory. The central question - not resolved to this day - is this: can demand deficiency ever be the cause of recession and, if so, are greater levels of unproductive spending an appropriate response? The thrust of the argument is that if Say's Law is valid, much of modern macroeconomic theory is fatally flawed. This book explores the validity of this problematic principle, reminding us that this 200-year debate has not yet been laid to rest.