【译文】 孔子说：“我十五岁立志于学习，三十岁有所建树，四十岁不困惑，五十理解自然规律，六十明辨是非，七十随心所欲，不违规。”

数学中最重要的是提出正确的问题。

The same principles which at first view lead to skepticism， pursued to a certain point， bring men back to common sense.

Change the way you look at things and the things you look at change.

To find out what one is fitted to do, and to secure an opportunity to do it, is the key to happiness.