English unjust enrichment law is a developing area of law in unjust enrichment. Traditionally, work on unjust enrichment has been dealt with under the title of "restitution". Restitution is a gain-based remedy, the opposite of compensation, as a loss-based remedy. But the event it responds to is the "unjust enrichment" of one person at the expense of another.
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A core example (whatever the theoretical analysis) is a mistaken payment. If we have a contract for a book, and the deal is it costs £10, but when you give me the book I mistakenly give you a £20 note, you have been unjustly enriched. If it was the case that you realised I gave you £10 too much, then you have committed the tort of conversion. Unjust enrichment, and the duty to reverse unjust enrichment, operates regardless of awareness. You are strictly liable to reverse it. However, unlike in conversion, there is always a "change of position" defence available. If you (unaware that you now have £10 too much in your wallet) go and consume £10 worth of chocolate, which you would not have done had you not had that extra £10, your position has changed. If I were to demand my £10 back, you could legitimately say, "but then I would be worse off, and that is unfair". This is the change of position defence.
‘It is evident that in these situations the strict liability from which we recoil is actually the only acceptable regime. The reason is that the [demand for a repayment of windfall gain] does not aim to make you bear a loss or to inflict a deterrent punishment on you. Strong facts are needed to justify unpleasant outcomes of that kind which will leave you worse off. The [enrichor] is not trying to make you worse off. He seeks only that you should give up the gain obtained at the shop’s expense.’ (Birks, 7)
Another available defence is ministerial receipt, i.e. the recipient defendant receives the assets as agent for another.
It means that good value is given for receipt of assets without notice of breach of trust. It is a complete defence to any knowing receipt claim.
Other than mistakes, a variety of categories of "unjust factors" are said to generate unjust enrichment situations. Undue influence, duress, incapacity and illegality are examples of vitiating factors in contract. Contract law's analysis has been used to explain why courts do not uphold contracts in these situations. These are cases of unjust enrichment. The following eleven categories are examples of "unjust factor" (or what Peter Birks argued could be unified under one principle of a basis of a right being absent) which may ground a claim of restitution for unjust enrichment.[1]
Unjust enrichment is a developed and coherent field in continental civil law systems. Continental lawyers say someone is unjustly enriched when there is no basis for their possession or title to some right or property. A more correct way of saying it is that someone has been "unjustifiedly enriched". In German, the term is Ungerechtfertigte Bereicherung (§812 BGB) and in France the term is Enrichissement sans cause. English lawyers, however, have been accustomed to identify an "unjust factor". The difference between "unjust factors" and "absence of basis" as a unifying principle has generated a lot of debate, particularly since Peter Birks changed his mind in his second edition of Unjust Enrichment (2005) in the Clarendon Law Series, and argued that the continentals had got it right.
The two leading theorists that have revived unjust enrichment were Lord Goff, who produced Goff and Jones on Restitution and Professor Peter Birks.