数学的本质在于它的自由。

"To write is to wrestle with the demons of the past."

Courtesy costs nothing. Lower the toilet seat after use. Let the people in the elevator exit before you enter. Return shopping carts to their designated areas. When you borrow something， return it better shape (filled up， cleaned) than when you got it.

想象力是人类最具颠覆性的能力。

老时相信一切，中年怀疑一切，青年知道一切。