From an interview with Milton and Rose Friedman in the Wall Street Journal:
...I made a reflexive apology for not being an economist myself. "You mean you're not a trained economist," was Mr. Friedman's comeback. "I have found, over a long time, that some people are natural economists. They don't take a course, but they understand--the principles seem obvious to them. Other people may have Ph.D.s in economics, but they're not economists. They don't think like an economist. Strange, but true."