“I almost worship him as if he were a god. I have never felt such an extravagant admiration for anybody.” So the 22-year-old Bertrand Russell wrote to his fiancée Alys Pearsall Smith in November 1894.

The naturalistic fallacy, which was coined by the English philosopher George Edward Moore in the early 20th century though first identified much earlier by the Scottish philosopher David Hume, is ...