"Emptiness," said the Tibetan philosopher Tsongkhapa, in 1397, "is the track on which the centered person moves." The word he uses for track is shul. This term is defined as "an impression", a mark that remains after that which made it has passed by — a footprint, 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. All these are shul.