Natural experiments are a powerful tool for understanding causal relationships in the real world.
We kid ourselves if we think that the ratio of procedure to data in an active data-base system can be made arbitrarily small or even kept small.
用程序证明四色定理不会改变数学——它只是证明了这个挑战了一个世纪的定理是一个有限的问题。
解释:人有知识,犹如树木有枝叶,树木有了枝叶,能带给人阴凉,人有了知识,就可以为人民做更多的事。