Like Robert Waldmann, I have always taught that the Phillips curve was initially promoted as a permanent tradeoff between inflation and unemployment. It was thought to be a menu of choices that allowed most any unemployment rate to be achieved so long as we were willing to accept the required inflation rate (a look at scatterplots from the UK and the US made it appear that the relationship was stable).

However, the story goes, Milton Friedman argued this was incorrect in his 1968 presidential address to the AEA. Estimates of the Phillips curve that produced stable looking relationships were based upon data from time periods when inflation expectations were stable and unchanging. Friedman warned that if policymakers tried to exploit this relationship and inflation expectations changed, the Phillips curve would shift in a way that would give policymakers the inflation they were after, but the unemployment rate would be unchanged. There would be costs (higher inflation), but not benefits (lower unemployment). When subsequent data appeared to validate Friedman's prediction, the New Classical, rational expectations, microfoundations view of the world began to gain credibility over the old Keynesian model (though the Keynesians eventually emerged with a New Keynesian model that has microfoundations, rational expectations, etc., and overcomes some of the problems with the New Classical model).

Robert Waldmann argues that the premise of this story -- that Samuelson and Solow thought the Phillips curve represented a permanent, exploitable tradeoff between inflation and unemployment -- is wrong: