Early one summer morning, a young Australian philosophy graduate was awoken by a phone call. It was the eminent professor David Lewis, calling from Princeton University’s philosophy department. “You’ve been admitted to our graduate program,” Lewis said. “Do you have any questions?” Still in the fog of sleep and desperate to think of something to ask, the student blurted out: “How many Australians are in the department?” After an uncomfortably long pause, Lewis replied: ‘Depends how you count Australians.’ This anecdote, recounted by the erstwhile student Alan Hayek, shows that even the most commonplace concepts can turn out to be polyvalent. Especially if you let a philosopher loose on them.