Here we have three forces of gravity working perpendicularly to one another. Three earth-planes cut across each other at right angles, and human beings are living on each of them.

It is impossible for the inhabitants of different worlds to walk or sit or stand on the same floor, because they have differing perceptions of what is horizontal and what is vertical. Yet they may well share the use of the same staircase. On the top staircase illustrated here, two people are moving side by side and in the same direction, and yet one of them is going downstairs and the other upstairs. Contact between them is out of the question because they live in different worlds and therefore can have no knowledge of each other's existence.