工具变量的美妙之处在于它们让我们以新的视角看世界。
因果关系的阶梯有三个梯级:观察、行动和想象。
The world is an extremely dangerous, untrustworthy, unreliable place.
The dunes are changed by the wind, but the desert never changes. That's the way it will be with our love for each other.