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This chapter argues that there are two different kinds of enrichment: pecuniary value and assignable rights. While most enrichments are valuable rights, enrichment can consist of value without rights or rights without value. When enrichment is an assignable right, its value is irrelevant to the claim for restitution. It has been assumed that there are two different kinds of restitution, either of the enrichment itself or its value in money. However, if there are two kinds of enrichment, then there is really only one kind of restitution, which always consists of giving up the actual enrichment received, which is either value or rights. The difficulty is not in deciding which kind of restitution is available, but in deciding which kind of unjust enrichment has occurred. This problem affects mistaken payments and failure of consideration in particular. Also, if change of position is concerned with disenrichment, then what counts as a relevant change of position will depend on whether the enrichment is value or rights.

This chapter examines the danger that, without proper appreciation of the underlying philosophical foundations of the subject, courts are apt to use the action in unjust enrichment to serve individualized notions of ‘justice’. It begins with an explanation of the principle of corrective justice that generally is thought to underlie the law of unjust enrichment. It then examines the extent to which courts in different jurisdictions have adhered to those foundations. First, because English courts have remained close to the principle of corrective justice, they seldom have resolved restitutionary claims by reference to intuitive notions of fairness or justice. Second, because Canadian courts occasionally have lost sight of the principle of corrective justice, they occasionally view cases of unjust enrichment as opportunities for achieving ‘equitable’ results on the basis of judicial discretion. Third, because of the effects of the American realist movement, American courts frequently have used the principle of unjust enrichment as a means of securing largely political goals of ‘social justice’.

This chapter provides an overview of the chapters in this volume. Not only does it provide a brief summary of the main issues raised in each chapter, it pursues those issues and considers the questions they prompt, and the avenues for further inquiry they suggest. The chapter follows the organization of the book, which groups chapters under the headings ‘normative foundations’, ‘enrichment’, ‘unjust enrichment and property’, and ‘reasons for restitution’.

It is widely accepted that a substantial chunk of restitutionary claims arise out of so-called defective transfers. An asset previously held by the claimant passes into the hands of the defendant, and the claimant's consent to its passing is either defective or wholly absent. But why exactly is a claim triggered on these facts? Unjust enrichment does a poor job of explaining why the law should require such gains to be given up. Appeals to corrective justice aren't much better. Instead, this chapter argues that these claims arise as a means of protecting and effectuating a claimant's interest in exclusively determining the disposition of his assets. In other words, it is because, at the outset — it was the claimant to whom the law had exclusively reserved the power to determine how and by whom that asset be used and enjoyed, and he did not (properly) consent to it being used and enjoyed by the defendant — that we recognise the defendant as liable to make restitution to the claimant. Such ‘proprietary’ theories have been given remarkably short shrift by a majority of restitution theorists. The chapter shows why their criticisms miss the mark, before concluding with an indication of how this understanding of defective transfer claims can offer principled solutions to perceived problem areas, such as indirect receipt, insolvency, and claims for services.

This chapter traces some strands in the evolution of Peter's thought regarding unjust enrichment and the law of tracing. English law allows a claimant to assert rights in the traceable proceeds of an unauthorized disposition of the claimant's asset; Peter consistently argued that such claims arise to reverse unjust enrichment. It is argued that his understanding of tracing shaped his understanding of much of the law of unjust enrichment. The chapter begins by setting out some of the new and controversial arguments made by Peter in Unjust Enrichment. It then argues that all of these arguments were, to a greater or lesser degree, influenced by his understanding of the nature of claims to traceable proceeds.

This chapter examines the nature and origins of legal responsibility for gain in private law. Contrary to recent suggestions, it argues that such responsibility is consistent with our traditional, liberal premises and stems from the relationship between a defendant's gain and some harm to the plaintiff (conceived of as a set-back of her entitlement, not just a factual loss). Legal responsibility for unjust enrichment arises where a defendant has caused harm; where such harm is in prospect and needs to be deterred; or where the plaintiff has suffered harm for which he was not morally responsible. These three different models of responsibility span the line regularly drawn between cases of unjust enrichment ‘by subtraction’ and ‘wrongdoing’, raising significant questions about its normative importance. The chapter explains why unjust enrichment law remains an important and necessary private law category, despite its seemingly paradoxical dependence on the concept of harm. It also criticizes the assumption that cases of un-induced mistaken payment are descriptively paradigmatic of unjust enrichment law. Most types of restitutionary liability, it argues, entail the defendant causing harm.

This chapter argues that resulting trusts always arise to effect restitution of unjust enrichment. It then explains how that understanding of the resulting trust in its traditional categories can provide the paradigm for understanding all property rights to restitution of unjust enrichment.

This chapter discusses the law of unjust enrichment. A claimant must show three things to make out a claim in unjust enrichment: that the defendant was enriched, that his enrichment was gained at the claimant's expense, and that his enrichment at the claimant's expense was unjust. If these requirements are satisfied, then the questions arise, whether there is any defence to the claim or any legal ground for the defendant's enrichment. If not, then it must be asked what remedy should be awarded.

This chapter elucidates the normative basis of unjust enrichment. It considers whether unjust enrichment fits within the formalist framework that treats the free will as the sole aspect of the human being commanding respect. It argues that this framework explains quasi-contractual obligation but that it cannot explain liability for unjust enrichment based on plaintiff’s having conferred a benefit on defendant in reasonable reliance on defendant’s assurance that she was part owner of assets legally vested in the defendant. Nor can formal right explain liability for unjust enrichment arising from mistaken payments. To understand why plaintiff succeeds in these cases, we need a conception of freedom more robust than that which animates formal right. Specifically, the courts’ concern in those contexts for the frustration of the plaintiff’s ends make sense only if we understand freedom richly as autonomy and transactional law as encompassing entitlements vis-à-vis courts to the protection of autonomy against the rigours of formalist property. The chapter argues that a significant part of unjust enrichment law vindicates, not the plaintiff’s private right against wrongdoing by the defendant, but her public-law entitlement to the court’s subordinating private property to the common good of autonomy where property and the good collide. This entitlement is correlative to the court’s duty to preserve its public character.

Peter Birks was one of the most influential legal scholars of his generation. He owed that influence to the admiration in which his rigorous and innovative thinking was held by lawyers and judges, not only in this country, but throughout the Commonwealth and in Europe. Birks was most widely known through his writings, but in Oxford, in particular, his reputation also rested on his teaching, especially in the famous restitution seminars that he conducted with various colleagues over three decades. He had an enormous impact on the law of restitution/unjust enrichment both in the universities and in the courts. The ‘Birksian school of thought’ has pursued, and will continue to pursue, rational transparency and elegant coherence in legal reasoning, not only in the law of restitution, but across English private law generally.

This chapter begins with a discussion of the performance concept. It shows that the Leistungskondition provides a simple solution for the majority of all unjustified enrichment cases, but does not cover situations which have to be covered by the ‘enrichment in another way’ provision. The chapter also looks at where, in English law terminology, another ‘unjust factor’ must be found. The chapter then covers the enrichment of the defendant and claimant.

In the law of unjust enrichment, the question when a plaintiff should have access to proprietary remedies is one of the most controversial issues. This chapter attempts to expose the philosophical and historical foundations of the most important category of proprietary remedies; namely, trusts arising by operation of law, including trusts over traceable proceeds. In so doing, it also explores the historical and philosophical foundations of all trusts, and emphasises the conceptual unity between constructive trusts and express trusts.

The chapter explores the nature and meaning of enrichment in the law of unjust enrichment. It seeks symmetry in several aspects of the law. First, enrichment or gain should be the mirror of loss. In the law of wrongdoing, a gain made at the moment of the infringement of a claimant's rights should be assessed in the same way as a loss incurred at the moment of infringement of a claimant's rights. Second, the same approach to the basic measure of enrichment, or gain, received should be taken in the law of wrongs as the approach taken to it in the law of unjust enrichment. The chapter explains that just as an immediate loss to the claimant is an undesired consequence, the value of which is objectively assessed, so too an immediate gain or enrichment transferred from the claimant is a desired consequence, the value of which is objectively assessed. Different functional considerations govern the award of subsequent losses and the disgorgement of subsequent profits, the latter of which is properly treated as a different remedy.

The mapping metaphor was first used by William Blackstone in 1756, when he said that the duty of the ‘academical expounder of the laws’ was to make clear how the various parts of the law fitted together. This chapter is concerned with three maps which together show where unjust enrichment belongs and how it is itself divided. The first of the three maps fixes its relation to other categories of the same kind. The second relates those event-based categories, and unjust enrichment in particular, to the law of obligations and the law of property. These are not categories of causative event but of responses to events. The third map raises the level of magnification so as to expose the layout of unjust enrichment itself. This chapter gives a more complete picture of the classification of rights by reference to their causative event and looks at four categories of civil wrongs: torts, equitable wrongs, breaches of statutory duty not amounting to a tort, and breaches of contract.

This chapter examines the new approach adopted by Peter in Unjust Enrichment. In Unjust Enrichment, Peter abandoned unjust factors in favour of an approach which generally calls for the reversal of enrichments which lack a legal basis. If this approach were to be adopted in English law, it would significantly change the way in which unjust enrichment interacts with other areas of law, and contract law in particular. German law points out some of the issues which English law would need to address. The chapter considers three situations: enrichment through execution of non-contractual agreements; enrichment through execution of non-enforceable contracts; and enrichment through execution of ill-founded disputed claims.

This chapter begins in the tenements of ancient Rome to show how medieval and later lawyers tackled the problem of the enrichment of a landlord which was liable to happen if, on the termination of the lease, a tenant was not allowed to remove the improvements which he had made to the landlord's property. It then brings the story right up to the present day with an examination of recent changes in practice and in the Dutch Civil Code, which have sought to meet the legitimate claims of tenants without forcing landlords to pay for improvements which they do not want.

The law of restitution is concerned with the award of a generic group of remedies which arise by operation of law and which have one common function, namely, to deprive the defendant of a gain rather than to compensate the claimant for loss suffered. These are called the restitutionary remedies. Because there is a group of remedies having a common function of depriving defendants of gains, it can be assumed that there is an independent body of law which can be called the law of restitution. To understand what these remedies are, how they operate, and when they are available requires examination of a complex body of law. This book identifies and analyses the principles which underlie the principles and rules forming the law of restitution. Concepts such as unjust enrichment, vindication of property rights, restitutionary claims, implied contract theory, and rescission are explored.

Although the unjust enrichment principle often operates positively, in the sense that the claimant must establish that the defendant has been unjustly enriched before restitution is available, sometimes the principle may operate negatively. This is because restitutionary relief may be awarded to prevent the defendant from becoming unjustly enriched. Even though the unjust enrichment principle has been clearly recognised at the highest level, the place of that principle within the law of obligations and the function of the principle still require careful justification. A number of commentators have suggested that restitutionary liability should be based on some other theoretical foundation, including proprietary theory, no unifying principle of unjust enrichment, and unconscientious retention. Possible functions of the unjust enrichment principle are discussed, namely, the formulaic function and the normative function. The approach adopted by England with respect to the function of the unjust enrichment principle is also considered.