Most of the criticism of a wealth tax, an idea that is quickly becoming mainstream within the Democratic Party, revolves around its feasibility. The more compelling argument, however, is against it on principle: It would allow the federal government to undermine a central animating idea of American capitalism.

Set aside the questions of whether a wealth tax would be constitutional or how much revenue it would raise. Instead, focus on whether it could be made to work - and what it would mean if the answer is yes.