One of the major differences between the English and Continental law of unjust enrichment seems to be the justification for the claim in restitution. Whereas German law founds the claim on the lack of a legal ground (Rechtsgrund), English claims in restitution are said to rest on a specific ‘unjust factor’, such as mistake, compulsion or failure of consideration. This chapter concentrates on the role of unjust factors and legal grounds in a specific area of unjust enrichment, namely where the claimant willingly conferred a benefit – in particular, money – on the defendant. It does not deal with cases of encroachment, payment of another's debt, improvement of another's property or restitution for wrongs.

Restitution for mistake and the condictio indebiti

Liability mistake and condictio indebiti

The Roman unjustified enrichment claim that attracts the greatest interest today is the condictio indebiti. It required that the claimant conferred a benefit on the defendant in order to discharge a liability that, however, did not exist. The action did not lie when the claimant knew that the liability did not exist. Whether there was also a requirement that the claimant had to be mistaken is disputed. It may be that in classical law a mistake by the claimant was presumed if he performed in terms of a non-existent liability, and that the defendant had to rebut this presumption by showing that the claimant knew that the liability did not exist.