For example, if the world is really round, we can predict that any of us, going on and on in the same direction, should eventually find ourselves back where we started.

The driver and three of his mates, who seemed to be along just for the ride, were pointing vaguely back in the direction from which we had just come.

If they begin to wander off in the wrong direction I have my trusty shepherd's staff to prod them back in the right direction.