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Prime Minister Theresa May recently told her party conference: "If you believe you are a citizen of the world, you are a citizen of nowhere." This year's Reith Lecturer, the philosopher Kwame Anthony Appiah, argues the case for the cosmopolitans.

"If you believe you are a citizen of the world, you are a citizen of nowhere. You don't understand what citizenship means." Prime Minister May's assertion was both blunt and bold.

She was taking to task the rootless managers of British businesses who behaved "as though they have more in common with international elites than with the people down the road".

For others, too, cosmopolitans have become objects of suspicion. The term conjures up those smug global business-class travellers, cutting the queue with their platinum frequent flyer cards, their baggage stamped for Davos or Doha.

Image copyright Getty Images Image caption Davos: the spiritual home of the global elite

Yet there's an obvious and urgent irony.

We live in an age of planetary challenges and interconnection between countries - from global warming to the refugee crisis. The need has never been greater for a sense of a shared human fate.

So why have political leaders in many nations chosen this moment to attack the spirit of cosmopolitanism? How has cosmopolitanism come to signify a narrowed, rather than a broadened, compass of concern?

On a literal reading Mrs May was absolutely right. Citizenship is a relationship between a person and a state.

Given the absence of a world state, there is, strictly speaking, nowhere for a citizen of the world to be a citizen of. But we citizens of the world always knew we were speaking figuratively.

Indeed, when, two and a half millennia ago, Diogenes the Cynic first called himself a kosmou polites (a citizen of the world, in Greek) he gave us both an etymology for the word "cosmopolitan" and a long cultural project - the interpretation of that metaphor.

Image copyright Getty/Hulton Archive Image caption Diogenes: The first cosmopolitan?

Diogenes himself was born a citizen of the Greek town of Sinope, in what is now coastal Turkey.

But there was no contradiction, in those days, between identifying with the people of your polis (city in Greek) and identifying with all the other Greeks.

Similar realities can guide us now. All of us experience narrower identities embedded in more encompassing ones.

Mrs May herself presumably wants the Scots to understand that they can be members both of the political community of Scotland and of an encompassing political community of Britain.

And, whatever your views on Brexit, you can be British and also be European, without belonging to a European state.

That encompassing European identity is political. It can guide decisions about which policies to support when it comes to migration and trade and a great deal else.

Most of those policies will be carried out by your national government, in interaction with others. So Europeans can have a sense of a common European fate while understanding that their literal, national citizenship is the primary means through which they will be able to get things done.

Image copyright NYU Photo Bureau: Slezak Image caption This year's Reith Lectures delivered by Professor Appiah tackle questions of identity

Nothing stops Europeans from being patriots of their own country.

"If you believe you are a citizen of the world": the Prime Minister spoke of this belief as if it were a regrettable hallucination.

Yet what's true of Europe is true of a global identity.

Transnational institutions can stand alongside national ones in helping us deal with our human problems.

You can feel a profound loyalty to a particular community and to humanity. There is not a fixed stock of loyalty to divide between your nation and the rest of the world.

To suppose otherwise is like thinking that being loyal to your friends requires a diminished loyalty to your spouse. And these are issues for ordinary citizens, not just for elites.

To be sure, people are understandably wary about technocratic establishments that enjoy more power than accountability.

International institutions can be poorly structured and managed. Nationalists seize on these sources of discontent. But those are breaches of the cosmopolitan ideal, not examples of them.

The fault doesn't lie with the cosmopolitan impulse and the solution is not the solace of bulwark nationalism.

Image copyright Getty Images Image caption The Ebola virus spread rapidly around the world from West Africa

Real cosmopolitanism is not a privilege; it is an obligation. It does not belong to the rarefied circles of some frequent-flyer upper class.

It belongs to anyone who cares about global justice, about the environment, about the alleviation of strife and carnage beyond our immediate national borders.

A disease that starts, unnoticed, in an African forest can devastate a Manchester family; CO2 emissions from India can derange the weather around the Gulf Stream; an ideological pathology that incubates in schools halfway around the world can bring down jets and skyscrapers.

We can be tempted to imagine - like children who think they can hide by closing their eyes - that our human concerns can stop neatly at the border, with a wider world kept forever at bay.

But that is the unaffordable luxury.

If cosmopolitanism involves a simple recognition that our lives are interrelated in ways that transcend boundaries and that our human concerns must, too, it has brute reality on its side.

A citizen of the world? Better believe it.

The Reith Lectures 'Mistaken Identities' continue on BBC Radio 4, and can be heard on the BBC Radio iPlayer.