When, if ever, is it permissible for one part of a functioning democracy to vote to split away and establish its own independent state?

The Catalan parliament tried to do just that in October 2017, but was forcibly rebuffed by the Spanish government. The government appealed to the wording of the Spanish constitution, which spoke of the “indissoluble unity of the Spanish nation”. In Spain, therefore, no region is permitted to hold its own independence referendum.

But elsewhere the situation is quite different. Both the British and the Canadian governments have permitted independence referendums to go ahead in Scotland and Quebec respectively, and have conceded that if there is a clear majority in favour of a split, divorce proceedings must begin immediately.

Here we see a clash of political philosophies. On one view, once a democratic state has been created, it can only be broken up by agreement of the whole population: the Spanish people would collectively need to change the constitution in order to permit Catalonia to leave. On the other view, a part of the state can withdraw its consent, and by a democratic vote of its members, decide to depart. According to this view it would be wrong for, say, the British or Canadian governments to try to prevent Scotland or Quebec from leaving.

Which view is correct? International law isn’t much help here. Generally, it doesn’t recognise a right of secession, except in the case of colonies, who are entitled to demand independence from their imperial masters. In other cases, it supports the territorial integrity of established states. But once a region secedes and a new state is successfully created, international law will defend the new state’s right of self-determination against attempts by the original state to recapture it.

Democratic principles also fail to yield a straightforward answer. The problem is that secessionist movements are a case of democracy pitted against democracy; the majority will of the people in the region that wishes to secede pitted versus the majority will of the people in the state as a whole.

Of course, if the two majorities agree that secession should proceed, then in principle there should be no problem. In fact, there have been relatively consensual break-ups in the past, such as that between Norway and Sweden in 1905. But if the regional and national majorities do not agree, then we need to go back to first principles and ask how states gain their legitimacy to settle the issue.

Some philosophers think states are like clubs, which exist in virtue of the individual consent of their members. In the case of a club, if some members are dissatisfied and wish to leave to set up a club of their own, then it would obviously be wrong for the remaining members to try to stop them.

Similarly, if regions like Catalonia, Scotland, and Quebec are unhappy and want to leave, then it would be wrong for Spain, the UK, and Canada to try and force their succeeding regions to stay. So, on the club analogy it is ultimately permissible for part of a functioning democracy to split away and establish its own state.

But the analogy between states and clubs is faulty. Unlike clubs, states are territorial entities. To understand what this difference amounts to, it is worth reflecting on the fact that the territories governed by states contain both people and physical features that cannot be moved easily, if at all.

When a state breaks up, the seceding region almost always contains a large number of people who voted against the secession, who clearly do not consent to their change of status, and who would undoubtedly be reluctant to give up their houses, jobs, and friends to move to a different part of the country.

In Quebec, for instance, the Anglophone population voted overwhelmingly to remain in Canada in 1995. And had the secessionist referendum succeeded, they surely would have been reluctant to give up their Canadian citizenship or leave their lives in Quebec. Yet these consent and residency concerns do not apply to the break-up of a club, since the quitters only take with them the people who want to quit and leave the others to carry on with their business.

Secession also involves the division of territory, and state jurisdictions invariably contain physical features that people value: landscapes, monuments, religious sites and so forth. Secession inevitably means losing control over some of these features. When the Serbs resisted the loss of Kosovo, they did so partly because the territory being taken from them included the battlefield where in 1389 Duke Lazar fought the invading Ottomans, an emblematic moment in Serbian history. Here again the club analogy breaks down. When members leave a club, they may be able to ask for their subscriptions back, but they can’t expect to carry off part of the clubhouse and its equipment.

Given the territorial nature of states, it is beginning to look as though secessionists cannot choose to leave willy-nilly but must have very good reasons to justify their claim to secede. In fact, some philosophers argue that secessionists have to prove they are victims of serious injustice at the hands of the state – a loss of basic rights, for example – before they can permissibly split a functioning democracy and establish their own country.

Such a test would rule out virtually any secession from democracies as we see them today. The Catalans, Scots or Quebecois might portray themselves as victims, but it would be a stretch to suggest that they are suffering from deep injustices that only independence could remedy. And if the injustice account is correct, then it would vindicate the hard line taken by the Spanish government against the Catalans.

However, the injustice approach seems to miss what is really driving secessionist demands in the world as we know it: identity. It is no accident that the people calling for independence are always and correctly described as nationalists. Their central claim is that they represent a culturally and politically distinct people whose right of self-determination can only be recognised by having a state of their own.

There are surely cases in which identity-based secessionist demands can be justified. Among the many things wrong with colonialism, one is the indignity of being subject to alien rule – rule by outsiders who do not share your culture and values. A people’s collective identity matters to the legitimacy of its government. So, in principle a nation contained inside a larger democratic state can assert its separate identity and seek independence.

Yet since identities are flexible and evolve over time, the challenge for the indignity approach to secession is to prove the charge of alien rule. Can it be shown that identities have become polarised in such a way that they cannot be accommodated by political changes that fall short of secession? For instance, would partial self-determination through devolved government of the kind practised in Catalonia, Scotland, and Quebec be enough to address the indignity concern and therefore undercut the rationale for secession?

This question points to a problem with referendums as a secessionist device. Voting only tells us that a certain proportion of people favour independence. By themselves, referendums fail to explain the reasons that lie behind the preference being expressed. We accordingly have no way of telling whether the voters’ concerns could be addressed by solutions that fall short of secession. Equally, we cannot tell from a referendum whether an identity gulf has emerged that would prohibit a group of people from living together in a unified state. So even if identity-based democratic secession is justified in principle, it faces serious practical limitations.

These limitations are apparent in the Brexit vote. Since the EU is not a state and has no territorial jurisdiction over the UK, the UK’s vote to leave is not technically a form of secession and thus the rationale behind Brexit may be different. But the Brexit vote does share the same defect as a secessionist vote: it displays a majority preference without telling us much, if anything, about the reasons behind it. And like most secessionist movements, Brexit reminds us that breaking up is hard to do.

David Miller is professor of Political Theory at Nuffield College, Oxford. He is the author of National Responsibility and Global Justice and Strangers in Our Midst.

This article is part of the Agora series, a collaboration between the New Statesman and Aaron James Wendland, assistant professor of Philosophy at the Higher School of Economics. He tweets @ajwendland.