For reasons that we don’t need to explore*, I’ve found myself in my day job obliged to reinvestigate what I learned as a law student on the concept of estoppel. Not only is estoppel a quaint word but it’s a peculiar principle that arises in a number of different forms. It is another creation of equity, that odd and wonderful portion of the legal system that cares less about a strict interpretation of the law and rather (as the name implies) seeks to find what is the fair resolution in all the circumstances.
To simplify, estoppel enables someone who has acted to their detriment in reliance on another’s representation, or even just a course of dealing, to hold them to that representation (or to the representation that is implied by that course of dealing). The person making the representation is bound by the fact that the other has relied on their indicated intent, and will lose out if that intent isn’t delivered on. The key three elements of the doctrine of estoppel are representation, reliance and detriment.
A word that comes up often in equity case law is ‘unconscionable’ – someone is estopped from holding strictly to the letter of the law if their previous conduct would make that unconscionable. Instead, the court intervenes to ensure fairness. As law professor Alastair Hudson sets out in his guide to equity:
“estoppel is truly an example of equity at its purest, in that the court can do almost anything it wishes to prevent an unconscionable benefit being taken by the defendant or uncompensated detriment suffered by the claimant”
Again reflecting that heritage in equity, the courts have proven very open-minded in deciding on the appropriate remedy for such unconscionable behaviour. As Hudson explains: “the court’s intention is … to provide a remedy that prevents the claimant from suffering detriment”.
It is of course not quite as simple as this (the law, particularly equity, never is). “Estoppel in all its forms is based on a variety of underlying conceptions, varying from ‘honesty’ to ‘common sense’ to ‘common fairness’,” says Hudson. Indeed, Lord Sumption in a key recent Supreme Court case went even further, in saying that the “various doctrines of estoppel” offer a “safeguard against injustice”. Yet these broad words are not always fulfilled in practice: the English courts are clear that the various forms of estoppel – though they are sometimes referred to collectively, often as equitable estoppel – are separate and distinct, and not part of any great unified theory (courts elsewhere in the Commonwealth tradition have suggested otherwise). Though in his guide to equity and trusts, Hudson calls a chapter ‘Equitable estoppel’, he starts by pointing out the inaccuracy of the term, not least as some estoppels arise in common law, and not only in equity. In another of his books, he argues that the lack of clarity in defining and separating the different categories of estoppel is a necessary aspect of equity, leaving the courts free to reach the fair decision, and the fair remedy, according to the specific individual circumstances.
This peculiar artefact of fairness in the law is not only interesting in itself. Estoppel also helps illuminate some ongoing philosophical debates with regard to fairness more broadly. Indeed, the great fairness philosopher John Rawls talked about something that sounds very like estoppel within broad societal relations: “a person is [morally] required to do his part as defined by the rules of an institution when two conditions are met: first, the institution is just (or fair) … and second, one has voluntarily accepted the benefits of the arrangement or taken advantage of the opportunities it offers to further one’s interests”. In this, Rawls acknowledges his debt to earlier legal positivist philosopher Herbert (usually HLA) Hart, and especially Hart’s principle of mutual restriction. Rawls goes on: “The main idea is that when a number of persons engage in a mutually advantageous cooperative venture according to rules, and thus restrict their liberty in ways necessary to yield advantages for all, those who have submitted to these restrictions have a right to a similar acquiescence on the part of those who have benefited from their submission.”
In effect, Rawls argues, the whole rest of the societal group has acted to their own detriment, by themselves submitting to the social requirements, in reliance on the expectation that all who take the benefits of their shared endeavour should also submit. Free-riding on that goodwill and seeking to personally avoid the detriment is in many circumstances a breach of a representation given to society (not always explicitly, but perhaps by the course of dealing of enjoying the benefit) and ought to be remedied in some equitable fashion.
In a thorough and thoughtful article on free-riding, Australian philosopher (no, this is not an oxymoron) Garrett Cullity comes to talk about the unfairness of free-riders as being the act of objectionably granting themselves preferential treatment. Through his analysis, Cullity develops the thinking of Rawls and produces the following Principle of Fairness in relation to these kinds of challenges:
“If a person receives benefits from a scheme that satisfies the following conditions, it is unfair for her not to meet the requirements it makes of her in respect of her enjoyment of those benefits.
(i) The practice of participation in the scheme represents a net benefit for her.
(ii) It is not the case that practically everyone would be made worse off by the practice of participation in the recognition as obligatory of those further requirements that must in fairness be regarded as obligatory if the requirements in question are regarded as obligatory.
(iii) She is not raising a legitimate moral objection to the scheme.”
This begins to seem very like the way in which common resource problems are in practice effectively managed, according to the analysis of the great Elinor Ostrom, with mutual sharing of the burdens so that the benefits can also be shared, and often with scope to punish or ostracise those who seek to free-ride. And indeed, Cullity’s ‘recalcitrant fisherman’ example, with which he tests various versions of a possible rule against free-riding, seems very close to an explicit common resource problem and how it might be managed effectively.
In a later article further developing this thinking, Cullity directly references Ostrom. His articulation of general principles in this latter article is striking. I especially like one that is highly relevant to current considerations: “Relationships of trust involve an expressed commitment to reciprocal self-constraint for the sake of cooperation.” This has particular resonance given that, as I explain in my book, trust is the reciprocal of fairness. Cullity argues that failures of appropriate impartiality are at the core of all unacceptable unfair behaviour. He explains that any free-rider granting themselves preferential treatment by exempting themselves from contributing to a common good is exactly such a failure of appropriate impartiality.
Fairness in various forms – including the legal concepts of equity, philosophical approaches to fairness principles, and practical solutions to challenges of managing common resources – reach similar conclusions: there need to be mechanisms and controls over free-riding or reneging on a shared endeavour from which all participants benefit. People undertake a fair set of obligations through their participation in human societies; estoppel is just one way in which they are held to those obligations.
See also: Fairness piercing the veil, and filling gaps in the law
Fairly sharing the commons, sharing the future
I am happy to confirm as ever that the Sense of Fairness blog is a purely personal endeavour
* I don’t think I should inflict on readers a discussion of the current definition of a constructive obligation under IAS 37 accounting standard, nor proposals under consideration for building upon that definition
Understanding Equity and Trusts (7th ed), Alastair Hudson. Routledge, 2022
Rock Advertising v MWB Business Exchange Centres [2018] UKSC 24
Great Debates in Equity and Trusts, Alastair Hudson. Macmillan, 2014
A Theory of Justice, John Rawls. Harvard University Press, 1971
Are There Any Natural Rights? HLA Hart. Philosophical Review, 64, 1955
Moral Free Riding, Garrett Cullity. Philosophy & Public Affairs, Vol. 24, No. 1 (Winter, 1995)
Governing the Commons: The Evolution of Institutions for Collective Action, Elinor Ostrom. Cambridge University Press, 1990
Public Goods and Fairness, Garrett Cullity. Australasian Journal of Philosophy, Vol. 86, No. 1, March 2008
Fairness in Business and Investment, Paul Lee. Routledge, 2026







