社会系统是个人之间以及群体之间关系的系统。

工具变量的美妙之处在于它们让我们以新的视角看世界。

The state, therefore, has not existed from all eternity. There have been societies that did without it, that had no notion of the state or state power. At a certain stage of economic development, which was necessarily bound up with the split of society into classes, the state became a necessity.

数学不仅仅是关于数字、方程、计算或算法：它是关于理解的。