First; they are separate defences because they operate on different levels. Defences to restitutionary claims are of three kinds: (1) those which deny an essential element of the principle against unjust enrichment (as change of position, if it is enrichment-related, denies enrichment; or as bona fide purchase or ministerial receipt deny enrichment); (2) those which preclude a claim even if the terms of the principle are fulfilled (as counter-restitution impossible operates; or, as change of position does if it responds to an element of injustice); and (3) those which operate as defences to claims generally, and which therefore can operate as defences to claims in restitution (limitation, res judicata). Change of position is a defence of the first kind (or, in some jurisdictions, of the second kind); estoppel is a defence of the third kind. They operate at different levels, and the mere fact that a more specific defence is available should not oust the more general one.

Second; they are separate defences because they focus on different things. The essentials of estoppel require a statement, reliance thereupon, and consequential detriment. That detriment may flow directly from the statement itself, or - much more likely - from the consequences of falsifying the statement. I tell you: "here is £x; it's yours". Your spending it demonstrates reliance upon my statement. The detriment would consist in your having to return it if you cannot keep me to my statement Since the statement referred to the entire of the £x, the estoppel relates to the entire of the £x. On the other hand, the essentials of change of position require a receipt, and reliance thereupon by consequential expenditure. If you have spent £y (a sum less than or equal to £x) the defence relates only to the £y consequentially expended and not to the entire of the £x (if y<x). Though both defences can be cast in terms of reliance, the essential focus of each is very different: the focus of estoppel is upon the effect of the prior statement; the focus of change of position is upon the subsequent spending. The fact that there may be an overlap on some fact situations between their functions does not alter the fact they focus upon very different things.

Third, since they operate on very different levels focussing upon very different things, there is no justification, as a matter of principle, for merging one into the other, or ousting one in favour of the other. Indeed, when the similarities between change of position and other defences have been judicially noticed, they have always been followed by a strong statement insisting on their necessary separation as achieving different functions. For example, Lord Goff in Lipkin Gorman, insisted on the separation between change of position and bona fide purchase, notwithstanding that Prof Birks has wondered in print whether the former should not subsume the latter within it. Again, Millett LJ as he then was in Portman BS v Hamlyn Taylor Neck (A Firm), insisted on the separation between change of position and ministerial receipt, notwithstanding that the latter has often been described as a specialised application of the former (on this, see Stoljar on Quasi-Contract, and I think there might be something similar in early editions of Goff & Jones). If it does not subsume these defences, change of position should not oust or subsume other defences such as estoppel.

Sorry for going on for so long; I hope some of this is helpful / makes some sense.

Best

Eoin.

EOIN O'DELL BCL(NUI) BCL(Oxon)
Editor, Dublin University Law Journal.
Barrister, Lecturer in Law, Trinity College, Dublin 2, Ireland.
(353/0 1) 608 1178 (w) 677 0449 (fx); (353/0 86) 286 0739 (m)
(All opinions are personal. No legal responsibility whatsoever is accepted.)