What do lighthouses, the control of infectious diseases among livestock and the metric system of weights and measures have in common? Most economists, I imagine, would say that these are all examples of public goods, in the specialised sense of goods and services which, if they are provided at all, are (or might as well be) provided on a universal basis, because excluding anyone from benefiting is impossible (or very costly) and the benefits to one do not reduce the benefits to others (or do so only minimally). Any vessel within telescope range can benefit from the lighthouse with no loss to anyone else, farmers can only protect their herds from foot-and-mouth disease if all herds are protected and mandated systems of weights and measures underpin just about every aspect of everyday life.
The features of 'non-excludability' and 'non-rivalry' between beneficiaries, taken together, mean that public goods are not generally provided by markets -there's no way of making money out of them- and typically depend on political authority (or occasionally philanthropy). Not every service provided by the state is a public good- far from it- but whatever else they may do, states are essentially in the business of providing public goods, both physical (such as lighthouses) and intangible (such as the regulation of weights and measures).
Public goods and territorial boundaries
So far so uncontentious. What is sometimes overlooked however is that public goods can be, and routinely are, shared between states. What is involved in the sharing of public goods across territorial boundaries has implications for how we think about such matters as borders and sovereignty.
The three examples mentioned above all exemplify the phenomenon in different ways. Lighthouses in Ireland, both north and south of the border, are the responsibility of a single all-island authority (the Commissioners of Irish Lights) which is in turn part of single structure of provision and pooled resources encompassing the islands of Great Britain and Ireland. The control of infectious diseases in livestock in Ireland is a matter of EU regulation, under which the entire island north and south forms a ‘single epidemiological unit’ . The regulation of weights and measures, for any country using the International System of Units of which the metric system is part (i.e. almost every country in the world, although not all use it exclusively) is subject to an international system of governance involving non-trivial obligations on participating countries, regarding for example the calibration of instruments in commercial or scientific use. So in these two islands and these two jurisdictions we have examples of public goods shared across territorial boundaries based on arrangements that are, respectively, bilateral, European and global.
Why does this matter? Well, for a start it allows us to put the so-called Irish border problem into a broader context. One of the functions of borders is to protect public goods within national territories (when we say that public goods are 'non-excludable' this is typically relative to a particular territory or space). The livestock example illustrates this point particularly well: if you want a national herd to be protected from infectious diseases, you generally need to be able to isolate it from cattle that are outside the reach of your regulatory authority. But the point applies to virtually any regulation of product markets in the context of international trade or cross-border supply chains. The possibility of territorial exclusion of non-compliant goods (or infected cattle) is an essential component in providing this sort of regulatory public good, and the absence of (visible) borders between jurisdictions therefore implies the presence of shared public goods. The border question is thus in part a question about how to continue to provide certain shared public goods across the UK/Ireland territorial boundary once one country has left the political structure that to date has enabled that provision. (Let's not get distracted by the notion of 'alternative' technical and administrative arrangements, which simply serves to evade this essentially political question.)
You can't waltz on your own
Secondly, a focus on shared public goods, particularly in the context of the border, throws a different light on the vexed question of sovereignty. One of the unchallenged received ideas in the Brexit debate has been the assumption that national sovereignty equates to unilateral decisiveness over all matters of domestic policy at all times, and that any international arrangement that reduces the range of policy over which a country is decisive represents a loss of sovereignty. Thus some opponents of Brexit will refer to a trade-off between sovereignty and trade: their difference of opinion with Brexiteers is on the scale of the trade-off and whether it is worth it, not on whether it exists.
This is to overlook the fact that for governments as for individuals, there are courses of action that depend on not being unilaterally decisive. Some actions just have more than one agent. Nobody thinks of a waltz or a performance of a string quartet as involving a trade-off between individual autonomy and other desirable things: autonomy obviously includes the power to choose to engage in joint actions over which none of the agents will be individually decisive. The same holds for the constitutional counterpart to individual-level autonomy, sovereignty. The Brexit slogan of 'take back control' gets its force from a tacit assumption that if a government is not 'in control', somebody else must be 'in control', as infantile a way of thinking about relations between states as it would be about interpersonal relations*.
The sharing of public goods between jurisdictions is matter of joint action in this sense. On the island of Ireland, two sovereign states were faced with the seemingly intractable problem of a border that divided communities and provided a focal point for violent contention. Getting rid of the border was not something either or even both states could do through actions over which they were individually decisive: the solution had to involve sharing between the two jurisdictions of those public goods that the border served to protect, and thus joint action on the part of both governments. As it happened, the solution emerged as a by-product of joint action on the part of all the EU member states (and it is very unlikely it would have emerged on any other basis, at least at the time it did).
We can contrast this perspective with what now seems to be the firmest commitment the UK government is currently prepared to offer on the border: “We have been clear that we are happy to accept a legally binding commitment not to put infrastructure, controls or checks at the border. We hope the EU do likewise.” (See here.) The logical problem with this statement is that it presents the absence of border infrastructure as achievable through a concatenation of separate actions (or refrainings from action) on the part of the EU and the UK. If you think that joint action to address the reasons borders exist in the first place is needed to ensure the survival of an invisible border, this is no commitment at all.
Taking sovereignty seriously
Finally it is worth bearing in mind that the construction of an invisible border (and it was a construction) contributed to putting UK sovereignty in Northern Ireland on a firmer footing than it had had for nearly thirty years, a reminder that not all threats to sovereignty are external. There is sometimes a tendency to exaggerate the role of ‘constructive ambiguity’ in the peace process, as if the achieved settlement rested on some sort of devious Celtic hocus-pocus rather than hard-edged institution-building. There is no ambiguity whatsoever about the question of sovereignty in the Good Friday Agreement, or in the 19th amendment to the Irish constitution. The remarkable thing about the invisible, frictionless border and the intangible infrastructure that underpins it was that all this was achieved, as it had to be, without impinging on UK sovereignty.
A theory of sovereignty that can only accommodate the sharing of public goods between jurisdictions by positing a dilution of sovereignty itself is not going to be adequate here (in the modern world it is unlikely to be adequate anywhere). No wonder the promoters of a no deal Brexit prefer to stake out their position in terms of Big Words (sovereignty, the will of the people) rather than concrete practicalities. The gap between the Big Words of political contention and the dull, meticulous, fundamentally uncontentious business of just getting things to work in a complex environment was the space in which Anglo- Irish politics escaped from its inherited constitutional impasse. The single market showed how government functions could be unbundled from each other and meshed together across territorial boundaries: public goods could be shared, the border could disappear and sovereignty could take care of itself. These are the achievements that any future arrangement to preserve the 1998 settlement will have to replicate. The sloganistic deployment of Big Words has nothing to contribute to that task.
*Whether the sharing of public goods between jurisdictions involves costs to sovereignty is not something that can be decided a priori. It depends on whether the underlying institutional arrangements are voluntary for all countries involved, on the extent to which decision-making rules serve to neutralise potential power asymmetries, on whether delegated powers remain under the collective control of governments and so on. For a comprehensive survey of the relevant issues see this paper by Oona Hathaway .