Stephen W. Hawking, George F. R. Ellis. The Large Scale Structure of Space-time.
p4.While we expect that most of our readers
will have some acquaintance with General Relativity, we have
endeavoured to write this book so that it is self-contained apart from
requiring knowledge of simple calculus, algebra, and point set
Stephen W. Hawking. A Brief History of Time: from the Big Bang to black holes.
p1.A well-known scientist (some say it was
Bertrand Russell) once gave a public
lecture on astronomy. He described how the earth orbits around the sun
and how the sun, in turn, orbits around the center of a vast
collection of stars called our galaxy. At the end of the lecture, a
little old lady at the back of the room got up and said: "What
you have told us is rubbish. The world is really a flat plate
supported on the back of a giant tortoise." The scientist gave a
superior smile before replying, "What is the tortoise standing
on?" "You're very clever, young man, very clever," said
the old lady. "But it's turtles all the way down!"
[A fuller account of the origin of "turtles all the way down"
in Wikipedia (naturally)]