Does that seem to you a truism? Do I seem almost impertinent in
asking you to remember it? So much the better. I shall be saved
unnecessary trouble hereafter.

But some one may say, and will have a right to say, "Stop--the lower
thing may have been thrust under the upper one." Quite true: and
therefore I said only that the lower one was most probably put there
first. And I said "most probably," because it is most probable that
in nature we should find things done by the method which costs least
force, just as you do them. I will warrant that when you want to
hide a thing, you lay something down on it ten times for once that
you thrust it under something else. You may say, "What? When I want
to hide a paper, say, under the sofa-cover, do I not thrust it
under?"

No, you lift up the cover, and slip the paper in, and let the cover
fall on it again. And so, even in that case, the paper has got into
its place first.

Now why is this? Simply because in laying one thing on another you
only move weight. In thrusting one thing under another, you have not
only to move weight, but to overcome friction. That is why you do
it, though you are hardly aware of it: simply because so you employ
less force, and take less trouble.

And so do clays and sands and stones. They are laid down on each
other, and not thrust under each other, because thus less force is
expended in getting them into place.

There are exceptions. There are cases in which nature does try to