There’s another boat story, an ancient Greek paradox that provides a powerful metaphor for this process. After slaying the Minotaur in Crete, the hero Theseus sailed back to Athens in a well-worn ship. As each plank decayed, it was replaced by new and stronger timber, until every plank in the ship had been changed. Was it then still the same ship? And if not, then at what point—with which plank—did the ship’s identity shift? It’s a paradox that Plutarch called “the logical question of things that grow.”