这节课重点讲解合同法中“约因”（consideration）的法律概念，并以里程碑式案例*哈默诉西德韦案*（*Hamer v. Sidway*, 1891年）作为主要例子。约因是承诺在法律上作为合同得以强制执行的一项基本要求。本讲对比了定义约因的传统方法和现代方法，重点强调了从“利益-损害”概念向“交易对价”概念的转变。 *哈默诉西德韦案*涉及叔叔承诺如果侄子在21岁之前戒酒、戒烟、戒脏话和戒赌博，将支付给侄子5000美元。侄子履行了他的承诺，但叔叔的遗产管理人后来拒绝支付，理由是缺乏约因。核心法律问题是，侄子对可能有害的行为的克制是否构成了充分的约因。 本讲解释说，约因本质上是“为换取他人的承诺而给予的东西”，即*回报*（quid pro quo）。叔叔的遗产管理人辩称，侄子没有放弃任何有价值的东西，因为他只是在克制有害行为，因此，这种论点未能提供有效的约因。然而，*哈默诉西德韦案*的法院不同意这种观点，并将约因定义为一方获得的利益，或另一方“给予、遭受或承担的克制、损失或责任”。法院强调，回报承诺不需要是*实际的*损害，而是*法律上的*损害。法律上的损害是指承诺做某件在法律上没有义务做的事情，或者承诺克制做某件在法律上有权做的事情。 本讲详细阐述了约因的“利益-损害”概念，指出给予承诺人（叔叔）利益或给予被承诺人（侄子）法律上的损害就足够了。然而，它承认很难证明承诺人获得了*实际的*利益。它引入了“显示性偏好”的概念，认为叔叔受益仅仅是因为他愿意为侄子的禁欲行为付费。然而，本讲批评了这种论点，认为它证明得太多了，因为它可能验证任何无偿的承诺。 法律损害概念的问题通过乔和简的例子来说明。简提供给乔10,000美元，如果乔承诺在60秒内屏住呼吸，而简违背了承诺。根据利益-损害概念，乔本可以胜诉，因为他有合法权利屏住呼吸。本讲解释了精明的当事人如何操纵这一概念，以创建具有法律约束力的协议，即使没有真正的价值交换。 因此，本讲解释说，普通法用“交易对价”概念取代了约因的利益损害概念。《合同法重述（第二版）》第71条概述的这种方法侧重于当事人进入交易的原因。一项履行或回报承诺构成交易对价，"如果承诺人寻求该项履行或回报承诺以交换其承诺，并且被承诺人给予该项履行或回报承诺以交换该承诺"。交易对价理论不关注利益或损害，而是关注承诺是否诱使了回报承诺。 交易对价的约因概念已在美国占据主导地位，但利益-损害检验仍在许多法院的判决中出现。 最后，本讲从这个角度再次分析*哈默诉西德韦案*。它表明，叔叔承诺为侄子的健康生活支付5000美元，而侄子的承诺诱使了叔叔的承诺。因此，根据交易对价，侄子确实提供了足够的约因与叔叔订立合同。本讲总结了关键要点作为结论：约因对于可强制执行的合同至关重要，利益-损害概念已被交易对价方法基本取代，回报承诺必须诱使承诺人做出其承诺。

This lecture focuses on the legal concept of "consideration" in contract law, using the landmark case of *Hamer v. Sidway* (1891) as its primary example. Consideration is a fundamental requirement for a promise to be legally enforceable as a contract. The lecture contrasts traditional and modern approaches to defining consideration, highlighting the shift from a "benefit-detriment" conception to a "bargain-for" conception. *Hamer v. Sidway* involved an uncle promising his nephew $5,000 if the nephew abstained from drinking, smoking, swearing, and gambling until age 21. The nephew fulfilled his promise, but the uncle's estate later refused to pay, arguing a lack of consideration. The core legal issue was whether the nephew's forbearance from arguably harmful activities constituted sufficient consideration. The lecture explains that consideration is essentially "what's given in return for someone else's promise," the *quid pro quo*. The uncle's estate argued the nephew gave up nothing of value because he was only refraining from harmful behaviors, so this argument failed to provide valid consideration. However, the court in *Hamer v. Sidway* disagreed, defining consideration as either a benefit accruing to one party or a "forbearance, loss, or responsibility given, suffered, or undertaken" by the other. The court emphasized that a return promise doesn't need to be an *actual* detriment but rather a *legal* detriment. A legal detriment is promising to do something one wasn't legally obligated to do or promising to refrain from something one was legally entitled to do. The lecture elaborates on the "benefit-detriment" conception of consideration, stating that either a benefit to the promisor (the uncle) or a legal detriment to the promisee (the nephew) is sufficient. However, it acknowledges the difficulty in proving an *actual* benefit to the promisor. It introduces the idea of "revealed preference" to argue that the uncle benefited simply because he was willing to pay for the nephew's abstinence. However, the lecture critiques this argument, stating that it proves too much because it could validate any gratuitous promise. The problem with the legal detriment conception is illustrated through the example of Joe and Jane. Jane offers Joe $10,000 if he promises to inhale within 60 seconds, and Jane reneges. Under the benefit-detriment conception, Joe could have won since he had a legal right to hold his breath. The lecture explains how astute parties could manipulate this concept to create legally binding agreements even when there is no real exchange of value. Therefore, the lecture explains that common law replaced the benefit detriment conception of consideration with the "bargain-for" conception. This approach, outlined in Section 71 of the Restatement (Second) of Contracts, focuses on the parties' reasons for entering the transaction. A performance or return promise is bargain for "if it is sought by the promisor in exchange for his promise and is given by the promisee in exchange for that promise." Instead of focusing on benefits or detriments, the bargain-for theory looks at whether the promise induced the return promise. The bargain-for conception of consideration has largely won the day in the United States, but the benefit-detriment test still figures into many court's holdings. Finally, the lecture analyzes *Hamer v. Sidway* a second time, from this perspective. It suggests that the uncle promised to pay $5,000 for the nephew's clean living, and the nephew's promise induced the uncle's promise. Thus, according to the bargain-for consideration, the nephew did provide sufficient consideration to form a contract with his uncle. The lecture concludes by summarizing the key takeaways: consideration is essential for enforceable contracts, the benefit-detriment conception has been largely superseded by the bargain-for approach, and the return promise must induce the promisor to make his/her promise.