Milton Friedman emphasized expectations errors as the main cause of deviation in unemployment from the natural rate. For Friedman, the notion that there was a unique Natural rate was equivalent to his assertion that there is only one level of unemployment at which inflation can be fully anticipated (when actual and expected inflation are the same). Edmund Phelps focused more in detail on the labor market structures and frictions that would cause aggregate demand changes to feed into inflation, and for sluggish expectations, into the determination of the unemployment rate. Also, his theories gave insights into the causes of a too high natural rate of unemployment (i.e., why unemployment could be structural or classical).