在英语里，约因和对价都是consideration,那么两者有什么区别呢？先让我们来看看consideration的英文解释：
consideration
n. 1) payment or money. 2) a vital element in the law of contracts, consideration is a benefit which must be bargained for between the parties, and is the essential reason for a party entering into a contract. Consideration must be of value (at least to the parties), and is exchanged for the performance or promise of performance by the other party (such performance itself is consideration). In a contract, one consideration (thing given) is exchanged for another consideration. Not doing an act (forbearance) can be consideration, such as "I will pay you $1,000 not to build a road next to my fence." Sometimes consideration is "nominal," meaning it is stated for form only, such as "$10 as consideration for conveyance of title," which is used to hide the true amount being paid. Contracts may become unenforceable or rescindable (undone by rescission) for "failure of consideration" when the intended consideration is found to be worth less than expected, is damaged or destroyed, or performance is not made properly (as when the mechanic does not make the car run properly). Acts which are illegal or so immoral that they are against established public policy cannot serve as consideration for enforceable contracts. Examples: prostitution, gambling where outlawed, hiring someone to break a skater's knee or inducing someone to breach an agreement (talk someone into backing out of a promise).

总的来说，要形成一个法律上“有意义”的买卖合同，需要具备若干条件，如当事人具备行为能办，意思表示一致且真实，内容合法或合乎公序良俗，，符合法定形式等等。此外，英美法还要求有对价（ consideration），法国还要求有约因（ cause）。以德国法为代表的大陆法系把合同与法律行为理论联系起来。依他们的主张，合同的成立是一个事实判断问题，其着眼点在于“某一法律行为是否已经存在，行为人从事的某一具体行为是否属于他的表示行为”。而合同生效与否则是一个法律价值判断问题，法律行为“只有成立后，才谈得上进一步衡量其是否有效的问题”，着眼于“行为人从事的某一法律行为是否符合法律的精神和规定，因而能否取得法律所认许的效力”。
在英美法上，并不着意区分合同的成立与生效，这中间涉及到英美法对合同（contract）的理解问题。理论和法律对于该允诺的违反给予救济，而在一定意义上承认允诺的履行为一种责任。据此，只要合同成立就具有合法性，因为不符合法律的合同不能产生强制执行力（enforceqbilitv），自然也就是要以“对价”（consideration）为基础。所谓对价，指的是当事人一方得到某种利益、权利或好处，另一方损失某种利益、权利或好处，从而使双方都收到好处。换言之，对价就是当事人间的一种“我给你是为了你给了我的关系”。
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