自然实验可以为经济问题提供有价值的见解。

在这个世界上，没有人能使你倒下，假如你自己的信念还站立的话。

写作是狗一样的生活，但却是唯一值得过的生活。

A mathematician's work is never done; there are always more problems to solve.

The consequences of this problem make themselves felt in bookshops. When you go to a large bookshop and look at the self-help sections, as I sometimes do -- if you analyze self-help book sproduced in the world today, there are basically two kinds. The first kind tells you, "You can do it! You can make it! Anything's possible!" The other kind tells you how to cope with what we politely call "low self-esteem," or impolitely call, "feeling very bad about yourself."

Every ton of coal we extract carries equal weight in responsibility.