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English unjust enrichment law is part of the English law of obligations, alongside contract and tort, and property. A claim in unjust enrichment requires benefits that have been obtained by someone to be given up if it would be "unjust" to retain them. The enrichment must be "unjust" if no valid legal transaction is present, such as a contract, trust, gift or estoppel. "Restitution", or restoration of the unjust gain, to the party to whom the enrichment came from is the main right that follows from an unjust enrichment. English courts have recognised that to found a claim there are four steps: (1) someone has to be enriched, (2) at the expense of someone else, (3) the enrichment must be unjust, and (4) there must be no defence, such as the defendant changing its position on the strength of the enrichment. Around 10 major "unjust factors" are typically recognised in English law, many of which are typically understood in contract as "vitiating factors". If someone receives an enrichment at another's expense, and this is a mistake, it happens with the claimant's ignorance of the transfer, after a failure of consideration, under duress, under undue influence or exploitation, through legal compulsion, out of necessity, when the transaction is illegal, or the claimant lacks capacity or acts ''ultra vires'', then this will found a claim, so long as no defence operates. Unjust enrichment is an action based on strict liability to return the enrichment, and may frequently work concurrently with a claim in tort. For example, if someone is forced to make a contract to transfer property, the "unjust factor" of duress will vitiate the contract. The claimant will be entitled to have their property returned, and will also have a claim in tort against the one who made the threat. The law of unjust enrichment is among the most recent and unsettled areas of English law, because only in 1991, in ''Lipkin Gorman v Karpnale Ltd'',〔() (UKHL 12 ), () 2 AC 548〕 did the senior judiciary endorse unjust enrichment's existence. The principles of enrichment and expense are more stable in cases of transfers of goods or money, but contested in cases where labour and benefits in kind are passed. The scope of unjust factors raises controversy, as does the very approach of English law to require an "unjust factor" be identified, rather than presume there is a claim if facts of transfer are made out (as in a "resulting trust") but no positive act of consent or other legal ground is found. The law's remedial response to unjust enrichment is also contested, particularly regarding the availability of proprietary claims. It is established that proprietary restitution is available for tracing, and some mistakes, although it has not been acknowledged for other unjust factors. One view is that a proprietary response should always be available. A second view is that claims should only generate personal remedies, while an and unrelated to property law. A third view is that proprietary restitution ought generally to be available, unless the law would regard the claimant as having taken on the risk of the defendant's insolvency.〔''Westdeutsche Landesbank Girozentrale v Islington LBC'' () AC 669, per Lord Goff〕 The categories of defences are also controversial: "change of position" is a general defence, although the law recognises estoppel, the ''bona fide'' purchaser defence, and others as alternatives. ==History== The modern law of unjust enrichment, at least in English law, was only expressly recognised by a majority of the House of Lords in 1991 in ''Lipkin Gorman v Karpnale Ltd''.〔 Nevertheless, many judges had previously acknowledged the basis of private law actions as founded up the principle of unjust enrichment before.〔eg ''Fibrosa Spolka Akcyjna v Fairbairn Lawson Combe Barbour Ltd'' () AC 32, 61-4, per Lord Wright〕 Where they had not, cases that reflected the same idea had been described as being based on concepts such as "quasi-contract", resulting trusts, multiple statutes, specific actions like money had and received, or claims for rescission of consent based obligations. Through the 20th century, common lawyers followed reformers in the civil law, particularly the German Civil Code of 1900, to fashion a law of unjust enrichment. The American Law Institute drafted a ''Restatement of the Law of Restitution'' in 1937, and the first major text by Robert Goff and Gareth Jones appeared in 1966.〔R Goff and G Jones, ''The Law of Restitution'' (1966)〕 Lord Goff gave the leading judgment in ''Lipkin Gorman v Karpnale Ltd'' 22 years later. Peter Birks was also instrumental in promoting the autonomy of unjust enrichment within the law of obligations, in his ''Introduction to the Law of Restitution'' and through his teaching. *Theophilus, Paraphrase 3.27.3, 5 *Gaius, ''Institutes'' *Hugo Grotius, ''Introduction to Dutch Jurisprudence'' (1634) §§III *Hugo Grotius, ''De jure belli ac pacis'' or ''The law of war and peace'' (1625) Book II, Ch X, §1 *William Blackstone, ''Commentaries'' (1765) *''Moses v Macferlan'' (1760) 2 Burr 1005 *''Kelly v Solari'' (1841) 9 M&W 54 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「English unjust enrichment law」の詳細全文を読む スポンサード リンク
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