<u><big>Consumer surplus</big></u> equals buyers’ willingness to pay for a good minus the amount they actually pay, and it <u>measures the benefit buyers get from participating in a market.</u> Consumer surplus can be computed by finding the area below the demand curve and above the price.
上文涉及到的重要概念：Willingness to pay (支付意愿) is the maximum amount that a buyer will pay for a good. Consumer surplus (消费者剩余) is the amount a buyer is willing to pay for a good minus the amount the buyer actually pays for it.
上文涉及到的数学公式：消费者剩余 ＝ 支付意愿 － 实际价格.
<u><big>Producer surplus</big></u> equals the amount sellers receive for their goods minus their costs of production, and it <u>measures the benefit sellers get from participating in a market.</u> Producer surplus can be computed by finding the area below the price and above the supply curve.
上文涉及到的重要概念：Cost (代价,生产成本,机会成本) is the value of everything a seller must give up to produce a good. Producer surplus (生产者剩余) is the amount a seller is paid for a good minus the seller’s cost of providing it.
上文涉及到的数学公式：生产者剩余 ＝ 实际价格 － 生产成本.
<u>An allocation of resources that maximizes[^fn] the sum</u> of consumer and producer surplus <u>is said to be <big>efficient</big>.</u> Policymakers are often concerned with the efficiency, as well as the equality, of economic outcomes.
上文涉及到的重要概念：Efficiency (效率) is the property of a resource allocation of maximizing the total surplus received by all members of society. Equality (公平) is the property of distributing economic prosperity uniformly among the members of society.
[^fn]:由此可见，作者 N. Gregory Mankiw (曼昆) 是个美国人。
The equilibrium of supply and demand maximizes the sum of consumer and producer surplus. That is, <u><big>the invisible hand</big></u>[^fn1] of the marketplace <u>leads buyers and sellers to allocate resources efficiently</u>.
上文涉及到的重要概念：Welfare economics (福利经济学) is the study of how the allocation of resources affects economic well-being. 关于「福利经济学」这一概念，我们需要注意的是，它关注的是 “how to affect” 的问题（正如我们用 Italic 斜体 突出的字词）。
[^fn1]:语出 Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations (1776) —— 亚当·斯密《国富论》。
<u>Markets do not allocate resources efficiently in the presence of <big>market failures</big></u> such as market power or externalities.
必须澄清的是：「市场失败」不等于「自由市场一定是失败的」。「市场失败」说的是，如果存在市场势力和外部性，那么这种市场的自发均衡状态不是最有效率的情况。当我们从前提得出结论时，如果我们假设了这一前提是真的，那么按照合乎逻辑的步骤、最后得出的结论也一定是真的。问题在于「前提可能是错的」，即使说「前提可能不符合现实情况」。如果是这样的，那么「假前提不能保证结论一定真」[^fn2]，即使说「结论可能是真的，但这种真是偶然的」。从而，当我们提及「市场失败」时，我们是想表达「市场在某些情况下是无法有效配置资源的」这层意思。常见的2种情况，一个是「存在市场势力」，一个是「存在外部性」。
[^fn2]:注意，我说的是「不能保证」，我的意思是「从假前提得出的结论可真可假」。这里要注意的是，如果前提是真的，那么合乎逻辑的结论一定是真的；如果前提是假的，那么合乎逻辑的结论可真可假，即使说，它「真假不定」。直白地说，「真假不定」的句子对我们的生活没有任何指导意义 —— 它是无意义的信息。
