In Miller, the Supreme Court touched on the difficult divide between ‘law’ and ‘politics’. Applicants may be politically motivated when bringing an action for judicial review. Decisions of the court will frequently have political consequences – here the need for legislation, with parliamentary debate and possible amendments conditioning the exercise of executive power, rather than an executive action merely subject to parliamentary accountability. But the court asks what the law is. It determines legal questions ‘relating to the constitutional arrangements of the United Kingdom’ and not the ‘political issues’ of the ‘wisdom of the decision to withdraw’, or the content of any future agreement with the EU. When examining conventions, judges are ‘neither the parents nor the guardians of political conventions; they are merely observers’. Courts do not oversee, create or enforce political rules. But they can recognise them as facts, when doing so is relevant to deciding the law. This may seem overly simplistic. But, it is key to understanding the complex inter-institutional interactions on which the UK’s uncodified, evolving constitution is based.

Diverse understandings of the nature of the constitution, and the relative powers of governmental institutions, did influence the different conclusions reached in Miller. But this does not mean that the judiciary has too much power. The constitution is legal as well as political. If Parliament had provided a specific legal duty or power in the 2015 European Referendum Act, the courts would have enforced it. Nor should we downplay the role of politics. Whilst the Sewel convention is only observed and not enforced by the judiciary, that does not make it unimportant. The observation that legislative consent motions have been used for legislation altering the powers of the devolved legislatures or administrations is important. It should inform the political enforcement of the Sewel convention.

If we fail to recognise the difference between political and legal motivations, questions, reasoning and enforcement, we run the risk of undermining the delicate checks and balances on which the UK constitution is based. We may undermine the extent to which the UK constitution balances parliamentary sovereignty and the rule of law enforced by an independent judiciary.

Alison Young is Professor of Public Law at the University of Oxford and a Fellow of Hertford College.

Click here for further analysis of the Supreme Court’s Miller judgment.