Thought Experiments in Law: Practice and Theory

Maks Del Mar is Reader in Legal Theory at Queen Mary University of London. He is working on a book onImagination in Legal Thought (for Hart / Bloomsbury), which examines the role and value of four devices in legal thought: fiction, metaphor, personification and hypothetical narrative. For a taste of his work on imagination, see also ‘The Legal Imagination’ (

Maks Del Mar is Reader in Legal Theory at Queen Mary University of London. He is working on a book onImagination in Legal Thought (for Hart / Bloomsbury), which examines the role and value of four devices in legal thought: fiction, metaphor, personification and hypothetical narrative. For a taste of his work on imagination, see also ‘The Legal Imagination’ (

A Post by Maks Del Mar.

Recent years have witnessed a surge in studies of the role and value of thought experiments (TEs) in a range of fields, especially in philosophy, history, economics and the natural sciences. Within this literature, however, very little acknowledgement is made of the pervasiveness of TEs in legal practice.[i] This is a great pity, for TEs – in the form of hypothetical variations on existing facts or new and sometimes fanciful hypothetical narratives – are a key mode of legal thought and an important engine of legal change. Whether one looks at legal education (and the Socratic, case-based method), exchanges between Bar and Bench, or the reports of case judgements, one cannot miss just how much the law is full of them. Studying their varieties, and their role and value, in legal thought can thus bring a whole swag of new examples to the interdisciplinary study of uses of the imagination, while of course also illuminating the practice of legal thought itself.

Let me unpack this in two stages – first, giving you an example of TEs in one case, and second, offering some possible avenues of further research into the role and value of TEs in legal thought.

An example: TEs in legal practice

On 26 June 1957, the sales manager of a garage business visited the offices of an advertiser and agreed to a contract for advertising their business on council bins. Having done so, he returned proudly to the garage, only to be met by the furious owner of the business, who thought it a bad agreement. The owner promptly called the advertiser to call off the contract. The advertiser refused to call off the agreement. Instead, they began advertising the garage business and continued to do so for the next 3 years (as per the terms). Upon fulfilment of their side of the deal, the advertiser demanded the sum due under the contract. The garage business refused to pay. The advertiser sued.

Following a series of decisions in lower courts, the case finally reached the House of Lords.[ii] The House was split 3 to 2, with the majority (led by Lord Reid) deciding in favour of the advertiser. According to the majority, when the garage business owner tried to get out of the contract (in legal jargon, a ‘repudiation’), the advertiser had two options to choose from: accept that repudiation and sue for damages, or continue with the contract and then sue for the sum due. The only restraint on the freedom of the advertiser to choose was if the performance of the contract required the active participation of the garage business owner (which it did not).

The case involved two thought experiments (TEs): one introduced by counsel for the garage business owner, hoping to persuade the judges that finding in favour of the advertiser would lead to unjust results (as displayed in the TE); and the second mentioned by one of the judges, and designed to inquire into what may be normatively relevant to the issue being decided. These two modes of engaging in TE might be called: 1) the mode of persuasion, and 2) the mode of inquiry, respectively. These are not mutually exclusive categories, but rather constitute different motivations behind the use of TEs as well as different ways of processing them.

As paraphrased by Lord Reid, the TE in the mode of persuasion went as follows:

A company might engage an expert to go abroad and prepare an elaborate report and then repudiate the contract before anything was done. To allow such an expert then to waste thousands of pounds in preparing a report cannot be right if a much smaller sum of damages would give him full compensation for his loss. It would merely enable the expert to extort a settlement giving him far more than reasonable compensation (per Lord Reid, at 1182).

As mentioned above, the advocate for the garage business owner had proposed this TE in the course of oral argument. If they were to find in favour of the advertiser, the judges who formed the majority had to find an answer to this TE. And they did so: Lord Reid said that in the TE, unlike in the case, the expert would have no legitimate or substantial interest, but only a financial interest, in pursuing the report. Lord Reid was thus able to distinguish the TE, and reject its attempt at a reductio ad absurdum of the arguments being proposed on behalf of the advertiser. 

The second TE – proposed and processed in (arguably) the mode of inquiry – appeared in Lord Keith’s judgement. Lord Keith was in the minority, and thus found in favour of the garage business owner. The TE imagined a ‘servant’ being engaged for 3 months. Before the servant was due to begin, the employer refuses to honour the contract. The servant – reasoned Lord Keith – can’t simply impose himself on the employer. He or she must seek to minimise their loss by seeking other employment – and suing for damages for breach of the contract.

Part of the function of this second TE is also about persuasion. In line with Lord Keith finding in favour of the garage business owner, it seeks to emphasise the importance of the (already well-known) principle of mitigating (i.e. minimising) one’s losses, and thus not wasting resources (as arguably would be the case if allowing the advertiser to have gone on to perform in the way he did). However, the TE might have been proposed, and may also be processed, in the mode of inquiry. In other words, it could assist in a search for what else might be normatively relevant in this issue – here, perhaps, the potential relevance of imposing an unwanted contract on some party, and the circumstances in which this might be unfair or unjust.

This second TE might have been better constructed – getting closer to some as-yet unrecognised principle of non-imposition (rather than the well-known principle of mitigating losses), but also to the facts of the case. Thus, for example, one might imagine a politician contracting with a painter to paint her portrait. Prior to the first sitting, the politician gets cold feet and decides not to go ahead. Because the politician is famous, and often in the media, the painter could nevertheless go ahead and paint them. Should the painter be allowed to do so? This gets closer to the idea that there may be something unjust or unfair about imposing an unwanted contract on a party – and forcing them to accept damages rather than seeking to perform – but in a way that captures the reputational reasons at stake (relevant also in the case, given it was an advertising contract). This second TE, then, is used not so much (or not only) to persuade, and reduce another party’s arguments to some absurd result, but instead to inquire into a potentially normatively relevant factor (in this case, an extra, and not-yet recognised principled reason for limiting the freedom of the innocent party being faced with someone who wants to get out of a contract).

Future research: TEs in legal theory

We have some examples – but what about the theory? How might we articulate the role and value of thought experiments in law? Although I cannot develop it here, one might propose further study along the following four dimensions:

  • first, the cognitive processing of TE, especially the epistemic attitude involved in both constructing and reading hypothetical narratives;
  • second, the formal features of TE, including their narrativity and their degrees of plausibility;
  • third, the social dynamics they enable, especially the practice of collective, interactive, and often humorous play of variation; and
  • fourth, their diachronic dimension, i.e. how they exist, persist and change as resources over time.

In broad brushstrokes, one might argue that TEs in the mode of inquiry are experimental and communicative resources, which enable collective, interactive simulation into or rehearsal of normative relevance across time. They are able to function as such resources thanks to a combination of factors, signalled in the above four dimensions. Thus, their role and value may in part explicable by:

  • the epistemic attitude of supposition, which is involved in the process of simulation or rehearsal, thereby freeing persons constructing or contemplating them from the normativity of belief or action (taking them epistemically off-line as it were);
  • their form as narratives, involving more or less implausible or fanciful facts that allow for the testing of emotional reactions to narrative scenes. Thus, a crucial aspect of TEs – which attention to form brings out – is how they involve emotional judgment;
  • the kind of collectivity and interactivity they enable – one can think of them as a particular, institutional kind of pretend play, and thus as inherently dialogical or multi-voiced (carnivalesque), for their role and value depends (in part) on multiple agents participating in exploring variations of them or contrasts to them; and
  • their resourcefulness over time. TEs in legal thought often appear in the margins or as indirect, by-the-way remarks (obiter dicta) – as a result, they may or may not be picked up by later advocates and judges. In a way, and perhaps somewhat paradoxically, because they do not advertise their own normative weight (unlike, for instance, the ratio decidendi of a case), they often play crucial roles in later cases. Their normative influence is full of potential. They have, in other words, a unique diachronic role and value in legal thought: they are particular kinds of carriers of potential normative relevance across time. 

This is just one possible way forward – but however we go about researching them, it would surely be amiss of philosophers interested in the imagination to ignore them altogether. The legal archive of thought experiments is simply too rich – and too important, with significant implications for parties in a case and the state of law generally.

[i] To cite but one recent example: The Routledge Companion to Thought Experiments (2017) includes chapters in many different fields of practice, but not law. 

[ii] White & Carter v McGregor [1961] 3 All ER 1178.