" Emptiness is the track on which the centered person moves," said a Tibetan sage six hundred years ago, and the book where I found this edict followed it with an explanation of the word " track" in Tibetan: shul, " a mark that remains after that which made it has passed by - a footprint for example. In other contexts, shul is used to describe the scarred hollow in the ground where a house once stood, the channel worn through rock where a river runs in flood, the indentation in the grass where an animal slept last night. "