Certain market transactions, such as selling one’s kidneys, seem morally repugnant to many people. At a conference discussion on what makes certain transactions morally repugnant, a professor of psychology said, “The problem is not that economists are unreasonable people, it’s that they’re evil people. . . . They work in a different moral universe.” The psychologist argued that the bur-den of proof should be “on someone who wants to include a transaction in the marketplace.” Contrast this view with the view inherent in the First Fundamental Theorem of Welfare Economics.