By TIMOTHY WILLIAMSON
Imagine being a slave in ancient Rome. Now remember being one. The second task, unlike the first, is crazy. If, as I’m guessing, you never were a slave in ancient Rome, it follows that you can’t remember being one — but you can still let your imagination rip. With a bit of effort one can even imagine the impossible, such as discovering that Dick Cheney and Madonna are really the same person. It sounds like a platitude that fiction is the realm of imagination, fact the realm of knowledge.