(C) CLASSIFICATION OF DEFENCES AS ‘ENRICHMENT-RELATED’ AND ‘UNJUST-RELATED’

The general defences have sometimes been classified as either ‘enrichment-related’ or ‘unjust-related’.^[1] The purpose of this classification is to show that the general defences have different functions. The so-called ‘enrichment-related’ defences operate where the defendant is not considered to be enriched as result of events occurring after the receipt of a benefit, whereas the ‘unjust-related’ defences operate where it is not considered to be just that the defendant should make restitution to the claimant. But nothing is to be gained from such artificial classification, particularly because restitutionary claims are not confined to the reversal of the defendant’s unjust enrichment but are also founded on the commission of a wrong and the vindication of property rights for which the language of ‘enrichment’ and ‘unjust’ is inappropriate. A further objection is that such classification of the defences artificially restricts their analysis, principally because a number of them can be analysed as operating both to show that the defendant is no longer enriched and that it is not just for the defendant to make restitution to the claimant.^{[2] [3]} Ultimately, as Lord Mansfield recognized in Moses v Macferlan, all defences are related to the justice of the defendant making restitution and this should be the key issue with which we should be

• [1] PBH Birks, Unjust Enrichment (2nd edn, Oxford: Oxford University Press, 2005), chs 9 and 10.
• [2] See in particular the defence of change of position examined in Chapter 25.
• [3] See Birks, Unjust Enrichment (2nd edn), 209. In that book Birks classified defences by reference towhether they were enrichment-related, which he called ‘disenrichment’ defences, or expense-related, which hecalled ‘disimpoverishment’ defences. This essentially equates to defences which are concerned with changes inthe defendant’s change of position, which are considered in Chapter 25, and changes in the claimant’scircumstances, which are considered in Chapter 26.