It turns out that, 220 years later, the French Revolution has something to contribute to mankind other than a warning about bloody ideological passions. In a ruling loaded with arguments from the 1789 Declaration of the Rights of Man and of the Citizen, France's Constitutional Court on Saturday invalidated the new 75% top French tax rate on grounds of—get this—unfairness.

The 75% rate on incomes above €1 million is Socialist President François Hollande's signature economic proposal, and the opposition had challenged its legality...