Around computers it is difficult to find the correct unit of time to measure progress. Some cathedrals took a century to complete. Can you imagine the grandeur and scope of a program that would take as long?

相关系数的可能误差不是一个固定的量，而是取决于样本的大小。

墨翟我上没有侍奉国君的差事，下没有耕田种地的艰难，我怎么敢放弃读书呢？ 

人心比山川更险恶，比了解天象更困难。