数学的本质在于它的自由。
The essence of mathematics lies in its freedom.
We write to remember, but also to forget.
儿童有一种特殊的敏感性，集中体现他去吸收周围的一切。
自由的代价是永恒的警惕。
I am always glad to think that my education was, for the most part, informal, and had its roots in the soil.
Every program is a part of some other program and rarely fits.