Offer and acceptance does not always need to be expressed orally or in writing. An implied contract is one in which some of the terms are not expressed in words. This can take two forms. A contract which is implied in fact is one in which the circumstances imply that parties have reached an agreement even though they have not done so expressly. For example， by going to a doctor for a checkup， a patientagrees that he will pay a fair price for the service. If one refuses to pay after being examined， the patient has breached a contract implied in fact. A contract which is implied in law is also called a quasi-contract， because it is not in fact a contract； rather， it is a means for the courts to remedy situations in which one party would be unjustly enriched were he or she not required to compensate the other. For example， a plumber accidentally installs a sprinkler system in the lawn of the wrong house. The owner of the house had learned the previous day that his neighbor was getting new sprinklers. That morning， he sees the plumber installing them in his lawn. Pleased at the mistake， he says nothing， and then refuses to pay when the plumber delivers the bill. Will the man be held liable for payment？ Yes， if it could be proven that the man knew that the sprinklers were being installed mistakenly， the court would make him pay because of a quasi-contract. If that knowledge could not be proven， he would notbe liable. Such a claim is also referred to as“quantum meruit”.
Consideration and estoppel
Consideration is known as“the price of a promise”and is a controversial requirement for contracts under common law. It is not necessary in some common law or civil law systems， and is considered by some to be unnecessary as the requirement of intention to create legal relations by both parties meets the same requirement under contract. The reason that both exist in common law jurisdictions is thought by leading scholars to be the result of the combining by 19th century judges of two distinct threads： firstly， the consideration requirement was at the heart of the action of assumpsit， which had grown up in the Middle Ages and remained the normal action for breach of a simple contract in England &；；Wales until 1884， when the old forms of action were abolished； secondly， the notion of agreement between two or more parties as being the essential legal and moral foundation of contractin all legal systems， promoted by the18th century French writer Pothier in his Traite des Obligations， much read （ especially after translation into English in 1805） by English judges and jurists. The latter chimed well with the fashionable will theories of the time， especially John Stuart Mill s influential ideas on free will， and got grafted on to the traditional common law requirement for consideration to ground an action in assumpsit.
The idea is that both parties to a contract must bring something to the bargain， that both parties must confer some benefit or detriment （ for example， money， however in some cases money will not suffice as consideration—e.g. when one party agrees to make part payment of a debt in exchange for being released from the full amount） . This can be either conferring an advantage on theother party， or incurring some kind of detriment or inconvenience towards oneself. Three rules govern consideration.
Consideration must be real， but need not be adequate. For instance， agreeing to buy a car for a penny may constitute a binding contract. While consideration need not be adequate， contracts in which the consideration of one party greatly exceeds that of another may nevertheless be held invalid for lack of real consideration. In such cases， the fact that the consideration is exceedingly inadequate can be evidence that there was no consideration at all. Such contracts may also be held invalid for other reasons such as fraud， duress， or being contrary to public policy. In some situations， a collateral contract may exist， whereby the existence of one contract provides consideration for another. Critics say consideration can be so small as to make the requirement of any consideration meaningless.
Intention to be legally bound
There is a presumption for commercial agreements that parties intend to be legally bound （ unless the parties expressly state that they do not want to be bound， like in heads of agreement） .On the other hand，many kinds of domestic and social agreements are unenforceable on the basis of public policy， for instance， between children and parents. One early example is found in Balfour vs. Balfour. Using contractlike terms， Mr. Balfour had agreed to give his wife￡30 a month as maintenance while he was living in Ceylon （ Sri Lanka） . Once he left， they separated and Mr. Balfour stopped payments. Mrs. Balfour broughtan action to enforce the payments. At the Court of Appeal， the Court held that there was no enforceable agreement as there was not enough evidence to suggest thatthey were intending to be legally bound by the promise.
The case is often cited in conjunction with Merritt vs. Merritt. Here the court distinguished the case from Balfour vs. Balfour because Mr. and Mrs. Merritt， although married again， were estranged at the time the agreement was made. Therefore any agreement between them was made with the intention to create legal relations.
Third parties
refrain /ri frein/vi.抑制；克制；戒除
enforceable /in f s bl/adj.可强行的，可强迫的，可实施的
binding / baindi /adj.（书面材料）有约束力的；应履行的
remedy / remidi/n.补救办法，补偿
trifling / traifli /adj.微不足道的，不重要的
indeterminate / indi t minit/adj.不明确的，不确定的，模糊的
consensual /k n sen u l/adj.经双方同意的，一致同意的
anthropological /n r p l d ik l /adj.人类学上的
jurisdiction / d u ris dikn/n.司法权；裁判权；管辖权
opt-out / ptaut/选择性排除；不参加；选择退出
estoppel /is t p l/n.禁止反言
estrange /is treind /vt.使疏远
French Civil Code 《法国民事诉讼法典》
pacta sunt servanda有约必守
l i ngua franca通用语；共通语
l aw juri sdi cti on法律管辖
choi ce of l aw法律选择；准据法
l egalcapaci ty法律行为能力