Brexiteers tell their fellow citizens that they should “take back control” of their borders, as an increasing number of illegal migrants take the Western Mediterranean route into Europe via Spain. In the US, the recent government shutdown arose from Donald Trump’s demand for nearly $6bn to build a wall on the Mexican border. There are few major global political issues that don’t involve borders. However, we humans are deeply territorial animals. And we rarely stop to think about how borders arose or what good is achieved by marking off a specific territory.

Rights over borders are, or are closely analogous to, standard property rights, and the justification of such rights in our philosophical tradition is found in John Locke’s Second Treatise on Government (1689). According to Locke:

“[E]very man has a property in his own person: [to] this no-body has any right to but himself. The labour of his body, and the work of his hands, we may say, are properly his. Whatsoever then he removes out of the state that nature hath provided, and left it in, he hath mixed his labour with, and joined to it something that is his own, and thereby makes it his property.”

Locke’s “state of the nature” is the world before it is owned by anyone, and, as the late Jerry Cohen put it, Locke is suggesting that justified “self-ownership” can lead to justified “world-ownership”.

To get a sense of what is at stake here, remember the story of the Little Red Hen. The hen owns her body, and therefore the capacities of that body. If she chooses to clear the field, plant seeds, harvest grain, and make bread, while the other animals lounge around in the farmyard, she has a right to that bread, and – as long as there is land left for the other animals – a right to fence the field she’s ploughed, ready for next season.

But the idea of “mixing” one’s labour with the world can only be a metaphor, and the view it seeks to express has many problems. Why not assume that the world is jointly owned? In this instance, if the hen had to ask permission of the other animals before fencing a bit of it off for herself, she’d almost certainly have to agree to give them some of her bread. And even if she’s entitled to the bread, why is she permitted to fence her fields, claiming an eternal right that she will be able to bequeath to her chicks, which might turn out to be no less lazy than the rest of the current cohort of other animals?

And what if one of the other animals would love to help the hen, but is too sick to do so? And what if, at some distance, there is another farm, with animals who work as tirelessly as the Little Red Hen but where the land is much less productive? They’ve laboured no less hard, so shouldn’t she share her bread with them?

Locke’s account of how property is acquired seems intuitively plausible at first, but that plausibility soon disappears when its basic unfairness is recognized. The only situation Locke’s theory is suited for is one in which everyone has equal capabilities and equal opportunities. In the real world, that is never the case.

Yet for the sake of argument, let’s assume Locke’s theory does explain how property is acquired in the first place. Having ownership over something involves the right to pass that thing on to whomever one chooses. So, the hen would now be permitted to transfer her territory to her chicks, and they to their chicks, and so on, for ever. These are all morally just transfers. But imagine that at some point some stronger animals come and take the land by force. That would be quite unjust, and of course the history of the world has been one such robbery after another.

Consider, for example, the English colonisation of America which Locke himself was seeking to justify. Maybe if property can be justly acquired, not all property has been theft; but current legal property rights have emerged out of a history of theft which makes them morally indefensible on the Lockean theory.

When there has been an unjust transfer, justice requires rectification – putting things right. On a small scale, that’s pretty straightforward. If your child has grabbed another’s toy, you make them give it back, perhaps giving them a chocolate as compensation.

But in the world at large, it’s impossible to unravel what’s happened, especially when we take into account that every single one of the people who now exists would not have existed without the unjust transfers of the past (imagine the Vikings had stayed at home). So, there’s little hope of providing a Lockean justification for current national borders.

But perhaps there are arguments for borders not based on property rights. Take the view that nations are entitled to control their borders to sustain social cohesion and national culture. The most plausible way to understand the importance of these goods is individualistic: the citizens of the nation in question benefit from cohesion and cultural identity, and those benefits may be under threat from immigration.

Yet the argument for “social cohesion” is doubtful. If immigrants are respectful, one might expect them not to damage indigenous culture but to increase the overall cultural capital of the nation in question.

The argument about social cohesion also looks suspiciously like pleading for a special case. If those being kept out are refugees, for example, why should their need for cohesion and culture matter less than that of the already privileged citizens of the host nation?

Given these weaknesses in the Lockean and social cohesion theories, it is beginning to look as if the justification for national borders is much less clear than most people believe.

Of course, conservatives might see the view that borders have no ultimate justification as an excessively abstract idea – and might be suspicious of demands for a grand moral theory to justify borders.

But the worries above arise not through applying some general, highly contentious moral principle, but from basic views about fairness which are central to the common-sense morality of most of us, including conservatives.

Others may raise concerns over the radical practical implications that might seem to arise when we recognize the weakness in our territorial claims. Am I, for example, suggesting that we in the UK open our borders immediately to all comers? No; opening our borders right now to everybody would undoubtedly lead to great harm to both citizens and immigrants. What one should do politically always depends on what is feasible.

Still, British citizens ought to recognize that the case for excluding potential immigrants, especially those in dire need, is questionable. We should begin to move in the direction of allowing significantly more refugees into our country. And, of course, all citizens of all countries should do the same. That would be the right way to “take back control” of their borders from those who unjustly seek to enforce them. Doing this might result in a world, as imagined by John Lennon, without closed borders, without war, and without the suffering and injustice to which borders continue to lead.

Roger Crisp is Professor of Moral Philosophy at St. Anne’s College, Oxford. He is the author of Mill on Utilitarianism, Reasons and the Good, and The Cosmos of Duty: Henry Sidgwick’s Methods of Ethics.

This article is part of the Agora series, a collaboration between the New Statesman and Aaron James Wendland. Aaron is Assistant Professor of Philosophy at the Higher School of Economics and the co-editor of Wittgenstein and Heidegger and Heidegger on Technology. Follow him on Twitter: @ajwendland.