[Update: Tim Worstall has a beautiful response to this post, applying the Ricardian idea to trade between humans and robots (or the human owners of robots). Will the robots out-compete us in everything, causing mass unemployment? Nope. Robots will never have a comparative advantage in everything. Once again, Tim's Ricardian point seems totally obvious once you have seen it, but was totally non-obvious (to me) until he said it.]
Noah Smith says: "Maybe people are perfectly smart and rational enough to understand the David Ricardo idea,..."
I'm not so sure.
I must have been taught the David Ricardo idea two or three times, and I must have taught it to my students about 20 times, and then this happened:
I was chair of economics at Carleton. One of the jobs of the chair is to decide (with consultation etc.) which professor teaches which course. All courses are different, and all professors are different, and teaching is important, and I thought it was really important to make sure that each course was taught by the best professor for that course.
So I went down the list of courses, assigning the professor who I thought was best for that course. (Can you see my mistake? It's not that obvious, is it?)