Private Law Taxonomy and Unjust Enrichment: Of Grids, Trees and Oranges

This is the second in a series of Inaugural Lectures being held to mark the appointment of a number of new Chairs in the School of Law at the University of Leeds.


High Court judges do not get to say “I don’t know the answer.” Yet there is a vast amount of law – statutes and cases – for a judge to wade through. The importance of private law (or public law for that matter) taxonomy is that it helps the judge or lawyer to organise and sift through material effectively and separate those characteristics of the case that matter from those that do not, a vital task in hard cases where it is – by definition – not obvious what the law is and what the answer to the case before the judge ought to be. It has recently become an area of fervent debate and discussion and two broad camps have developed. I refer to them as the interpretivists, who insist on relatively timeless, unchanging categories with no overlaps and the pre-eminence of principle over policy, and often (but not always) defend a category and principle of unjust enrichment, and the contextualists, who insist on the importance of policy, context, and the possibility of overlapping categories and frequently downplay the importance of principle, and many of whom do not recognise unjust enrichment as a useful category.

In this lecture I will map out the contours of the debate between these two camps and show that neither is completely correct. Private law cannot be captured by purely two-dimensional grids such as those put forward by Peter Birks, or two-dimensional trees such as that advanced by Jacob Weinrib. While clearly unjust enrichment or any other private (or public) law category-concept must have some content, to try to identify one single principle from which everything follows is doomed to failure. Yet at the same time general principles are unavoidable, and the key is in understanding how they relate both to other principles and to the particular context in which they are applied. I will sketch out a future agenda by showing that whatever unifying features we adopt for the law of unjust enrichment – and I will suggest some - must be normative to some extent and try to show how we might start to think about the dynamic dimension of principles required to allow legal development to occur.

Duncan Sheehan is Professor of Business Law at the University of Leeds School of Law. He joined the School in 2016 having previously been professor of Commercial Law at the University of East Anglia. He is a member of the Society of Legal Scholars Executive Committee, a member of the Secured Transactions Law Reform Project, and an academic member of the Chancery Bar Association. He also sits on Advisory Groups to respectively the Law Commission’s Bills of Sale Project and to the City of London Law Society’s Secured Transactions Code Project, and was a Distinguished Visiting Fellow at the TC Beirne School of Law, University of Queensland in March and April 2014.