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 conceptions 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 two different worlds and therefore can have no
knowledge of each other's existence.
