"When a man is riding through this desert by night and for some reason
-falling asleep or anything else -he gets separated from his companions
and wants to rejoin them, he hears spirit voices talking to him as if they were his companions,
sometimes even calling him by name.

For this reason bands of travelers make a point of keeping very close together.
Before they go to sleep they set up a sign pointing in the direction in which they have to travel,
and round the necks of all their beasts they fasten little bells,
so that by listening to the sound they may prevent them from straying off the path."

Marco Polo (1254-1324)