"Lucas and Sargent’s main argument was that Keynesian economics had ignored the full implications of the effect of expectations on behavior. The way to proceed, they argued, was to assume that people formed expectations as rationally as they could, based on the information they had. Thinking of people as having rational expectations had three major implications, all highly damaging to Keynesian macroeconomics. ... The first implication was that existing macroeconomic models could not be used to help design policy. … The second implication was that when rational expectations were introduced in Keynesian models, these models actually delivered very un-Keynesian conclusions. … The third implication was that if people and firms had rational expectations, it was wrong to think of policy as the control of a complicated but passive system. Rather, the right way was to think of policy as a game between policy makers and the economy. … To summarize: When rational expectations were introduced, Keynesian models could not be used to determine policy; Keynesian models could not explain long-lasting deviations of output from the natural level of output; the theory of policy had to be redesigned, using the tools of game theory."