好不容易在cartoonbank找到了Mike Twohy的这幅漫画，就是想说明这个问题。

漫画主要描述的是，A man sits on a park bench reading the paper while a vender approaches with his stand, which has an umbrella on it. The man buys something from the vender. The man is back seated on the bench reading and the umbrella is now over him. The vender walks on. The umbrella on his stand is gone.
 

如果要追溯，我们可以先从亚当·斯密那个时代说起。在1977年，亚当·斯密是第一个在《国富论》（Wealth of Nations）中提到市场资源会被最有效率地使用的。亚当斯密的原文如下：...every individual necessarily labours to render the annual revenue of the society as great as he can. He generally, indeed, neither intends to promote the public interest, nor knows how much he is promoting it. By preferring the support of domestic to that of foreign industry, he intends only his own security; and by directing that industry in such a manner as its produce may be of the greatest value, he intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. Nor is it always the worse for the society that it was no part of it. By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it. I have never known much good done by those who affected to trade for the public good.

我们同样可以在这幅漫画中感受到这只“看不见的手”（the Invisible Hand）。卖饮料的小贩同时拥有一把大伞。对于这两样东西，他心里自然有一个最低价（边际成本）。同样，公园里坐在长椅上看报纸的男士对饮料和阳伞也有一个可承受的最高价格（边际收益）。很显然，这位男士需要一把阳伞，阳伞的边际收益超过了它的价格，而饮料的价格超过了它的边际收益。他和小贩便进行了交易，于是产生了生产者剩余和消费者剩余。小贩卖出了阳伞，他的生产者剩余大于机会成本；男士购买了阳伞，获得了消费者剩余。在第三幅图中，两个人都比第一副图更有余裕(better off），阳伞被更有价值地利用了。这就是楼主所说的“交易增加福利”。

 

[此贴子已经被作者于2008-2-20 20:02:47编辑过]

请恕我驽钝，我无法理解楼上的漫画和解释，与“交换可以增加福利”之间存在的明显的或者隐含的联系。漫画内容显示了一次交易，一次供给/消费的成交，但却没有直观的体现“交换”。也许两个作为生产者的人之间的“物物交换”更能体现这个命题的含义吧？从初等经济学角度也不难理解这个命题：一则交换使得作为生产者的经济个体更能从事最擅长的工作，从而使整个社会的生产可能性边界扩大；二则作为消费者的个体能有更多的消费选择，直接使每个消费者效用水平提高成为可能。

为什么一定要“物物交换”才能说明问题呢？这里要体现的不是用何种方式交易（以物易物也好，货币购买也好），而是交易如何使得整体的福利增加——这个过程。

我已经解释过了：“这位男士需要一把阳伞，阳伞的边际收益超过了它的价格，而饮料的价格超过了它的边际收益。他和小贩便进行了交易，于是产生了生产者剩余和消费者剩余。小贩卖出了阳伞，他的生产者剩余大于机会成本；男士购买了阳伞，获得了消费者剩余。在第三幅图中，两个人都比第一副图更有余裕(better off），阳伞被更有价值地利用了。”

顶 谢谢

月下美丽的梦 解释的不错，赞一下。