Case:史蒂克尔诉迈立克案
在1809年史蒂克尔诉迈立克案中，被告(船长)因两个海 员潜逃而向其余海员许诺：如果他们把船开回国去， 将额外分享那两个海员的工资，但后来被告与海员之 间发生纠纷。法庭裁定，由于海员们本应负有将船驶 回国的合同义务，因而被告的许诺并没有对价，故不 具有合同效力。
但也存在例外。
consideration must be sufficient but it need not be adequate.
It means that as long as the promisee’s act or promise satisfies the legal value test, the courts do not ask whether that act or promise was worth what the promisor gave, or promised to give, in return for it.
Moral obligation
Case: White v. Bluett A son promised not to bore his father with complaints
about the father’s distribution of his property among his children was held not to be good consideration for the father’s promise not to sue the son. The opposite case: Hamer v. Sidway: an uncle promised to pay his nephew 5000 if the nephew refrained from drinking liquor, using tobacco, swearing and playing cards untl he was 21. This promise was held to be enforceable because the nephew had a legal right to engage in such activities, and in giving up his rights he had provided consideration for the promise.
2) The orthodox传统的 position is clear: performance of an existing duty imposed by law does not constitute consideration.
3) case: Warபைடு நூலகம் v. Byham.
Preexisting Contractual Duties
Williams v. Roffey Bros &Nicholls Ltd
The defendant contractors entered into a contract to refurbish a block of flats. They sub-contracted the carpentry work to the claimant for price of 2000 pound. The claimant ran into financial difficulties after having completed part of the work. The cause of his difficulties was partly attributable to the fact that he had underpriced the job and partly because of his own inability to supervise his workforce. It was in the interest of the defendants to ensure that the claimant completed the work on time because if, as a result of delay or non-performance by the claimant, the defendants were late in completing the work and they would incur liability to their employers. So the defendants agreed to pay an extra 10300 to ensure that the work was completed on time. The claimant subsequently finished eight more flats but defendant only paid 1500. The claimant sued to the court.
Until recently the rule which English law adopted was that performance of an existing contractual duty owed to a promisor was no consideration for fresh promise given by that promisor.
约因要有价值，一般最常见的是金钱。但也可以是其 他的形式，如权益的放弃，提供服务或消息，修改原 先合约，以后的生意，及可以想象到的“好处”。
Adequacy of Consideration
General Rule: The first rule of the doctrine教条 of consideration is that
Preexisting Legal Duties
1) The question whether performance of a duty which one is already under an obligation to perform can constitute consideration for a promise given in return is currently a very controversial one in English contract law.
What’s consideration
法律上认可的对价（约因）的定义：“ 对受诺人有 损失，或对承诺人有利益 ”[A valuable consideration, in the sense of he law, may consist either in some right, interest, profit or benefit accruing to the one party, or some forbearance, detriment, loss or responsibility given, suffered or undertaken by the other.]
Unit 3 Consideration （约因或对价 ）
约因是英美法中一个重要而且复杂的概念。对确定 一个合约是否有效，包括商贸活动中常有的事后更改 合约(variation)，追加协议（additional agreement), 和解 协议等是否有效，非常重要。
英国法律认为约因是“订约双方愿意去受法律约束 的唯一证据” ( only evidence of the intention of the parties to be bound )