The AS / AD Model Is Logically Inconsistent; It Has Two Inconsistent Supply Analyses: One Implicitly Built into the Slope of the AD Curve, the Other Explicitly Behind the AS Curve

In short, the logical problem here is that one cannot derive an AD [aggregate demand] curve from the Keynesian model, because the Keynesian model includes a dynamic interactive effect between supply and demand in the form of the multiplier. The Keynesian model has embodied in it what Robert Clower (1994) calls Hansen's Law—the proposition that demand creates its own supply. This analysis of supply might be totally wrong, but there is no denying that the Keynesian model has assumptions of supply responses in it. As you move along the 45-degree line, supply is changing independently of any change in the wage/price ratio.

Given that the Keynesian model includes assumptions about supply, one cannot logically add another supply analysis to the model unless that other supply analysis is consistent with the Keynesian model assumption about supply. The AS [aggregate supply] curve used in the standard AS / AD model is not; thus the model is logically inconsistent. It has two inconsistent supply analyses: one implicitly built into the slope of the AD curve, the other explicitly behind the AS curve.

--David Colander, “The Stories We Tell: A Reconsideration of AS / AD Analysis,”Journal of Economic Perspectives 9, no. 3 (Summer 1995): 176.