Jean Piaget, the great child psychologist—or, as he preferred to call himself, “genetic epistemologist”—tells a wonderful story about two little boys. (He doesn’t say so, but I expect that they were his own children.) One night when the moon was full, the older, who was about four, led his younger brother into the front garden of his house and ordered him to walk back and forth. As little brother faithfully did so, big brother carefully observed him—and the moon. “I was trying to see if the moon follows him when he walks,” the older brother explained. “But it doesn’t, it only follows me.”
