I would like to thank Giovanni Arcioni, John Barrow, Roman
Jackiw, Nemanja Kaloper, Jorma Louko, and Vincent Moncrief for
useful comments. This review is dedicated to the memory of Bryce
DeWitt, who taught me, and the world, much of what we know about
quantum gravity. This work was supported in part by Department of
Energy grant DE-FG02-91ER40674.