(B) THE PRINCIPLES UNDERLYING THE GENERAL DEFENCES
Two fundamental principles can be identified to justify the recognition of the general defences.
(i) Justice Favours the Defendant Retaining the Benefit
The principle which justifies the recognition of the general defences in particular is that, because of the particular circumstances of the case, the justice of the defendant retaining a benefit outweighs the justice of the claimant recovering it. This was recognized by Lord Mansfield in Moses v Macferlan when he said that ‘[the defendant] may defend himself by every thing which shews that the plaintiff, ex aequo et bono, is not entitled to the whole of the demand, or to any part of it’. It is as a result of this principle, for example, that the restitutionary claim will be defeated to the extent that the defendant’s position has changed in reliance on the validity of the receipt of the benefit. Similarly, the claim may be defeated if the claimant had participated in an illegal transaction, simply because such participation weakens the claimant’s claim that the defendant ought to make restitution of any benefit.
This focus on justice can be related specifically to the principle of corrective justice. As Grantham and Rickett have recognized, ‘[t]o be consistent with the correlative structure of corrective justice, equal respect must be accorded to the defendant’s interest in selfdetermination and his entitlement not to be deprived of his wealth except voluntarily’.
(ii) Security of Receipt
A second principle can also be identified which can be used to justify the recognition of the general defences, namely that the defendant’s receipt of a benefit should be secure. What this means is that, when a defendant receives a benefit, he or she should not be subjected to any unnecessary insecurity when deciding what to do with it because he or she may have to make restitution to the claimant. One of the prime functions of the general defences is to identify those circumstances where it is reasonable for the defendant to treat the benefit as his or her own to do with as he or she wishes. So, for example, where the claimant has voluntarily transferred a benefit to the defendant, it is entirely appropriate that the defendant should be entitled to believe that the receipt of the benefit is secure so that he or she will not be obliged to make restitution to the claimant.
-  See Baylis v Bishop of London  1 Ch 127, 140 (Hamilton LJ).
-  (1760) 2 Burr 1005, 1010; 97 ER 676, 679. 3 See Chapter 25.
-  11 See Chapter 27. 5 See p 4, above.
-  13 RB Grantham and CEF Rickett, ‘A Normative Account of Defences to Restitutionary Liability’ (2008)
-  CLJ 92, 101.
-  14 Benedetti v Sawiris  UKSC 50,  AC 938,  (Lord Reed).
-  15 See further p 37, above.
-  16 PBH Birks, ‘The Fourth Part of the Principle’ in PBH Birks, Restitution—The Future (Sydney: The
-  Federation Press, 1992), 123.
-  See p 149, above.