Who are you?
That may be a philosophical question but it leads to another, much more practical challenge: How can you prove it?
To prove your identity to British public bodies and companies such as banks, you are often asked for one of the two most widely used forms of photographic ID: a passport or a driving licence. Both are physical documents, pieces of paper or plastic that bear information about you: when you were born, what you look like and so on.
Isn't it a bit curious that we're still using paper and plastic to carry important information like that? Pieces of paper and plastic that can be lost or stolen or copied or tampered with? Especially given that in so many other parts of our lives, information, even the most sensitive and important information, is carried and verified electronically: many of us now do just about everything including delicate financial transactions using a phone that checks our ID by scanning our fingerprint.
At the Social Market Foundation, our chief economist Scott Corfe has studied ID systems and concluded there is a better way for the UK to help people verify their ID.
Making greater use of digital ID verification, we think, could unlock lots of benefits for Britain.
Economically, there's a real chance to help people do more business online - and not just People in Britain either. Smart countries like Estonia are already offering "e-residency" to non-citizens, giving them an ID and an electronic home to use for business worldwide.
We think the UK could do something similar but bigger and better, if the Government puts proper support behind a programme at the Cabinet Office called Verify, which establishes ground rules for private companies to check someone's details and make sure they are who they say they are - online, without the need for paper or plastic.
And let's be clear, paper and plastic aren't just old-fashioned, they're unfair too. That's because they're pretty expensive.
Our research, supported by OT Morpho, shows that in some of the most deprived parts of the country, as many as 40% of adults don't have a UK passport. Many individuals, particularly those on low incomes and in urban areas, also lack driving licences.
Not having accessible ID complicates life in all sorts of ways. It makes dealing with the state - local councils, benefits offices and the rest - even more stressful and time-consuming.
It can also limit your access to vital services: those places where people don't have ID are where you'll find a lot of the 1.5million or so people in Britain who are "unbanked" and don't even have a current account in their own name. That sort of exclusion leaves a person vulnerable to exploitation by employers, landlords and even criminals. It can even make it harder to secure your most basic right, the right to vote.
So we think a cheaper, digital form of ID, the sort of thing you might show on your phone and which could be checked online, would be a real help to a lot of people in Britain who don't get the best deal right now.
It might even lead us to a future where we could pick our leaders the way we choose so many other things: by tapping a screen instead of scribbling on a piece of paper. Electronic voting may still be some way away, but in the meantime, a better ID scheme might mean more people were actually registered to vote.
We also see another possible - and very visible - change in identity systems coming. Instead of producing a battered bit of paper and card when you want to fly, why not use a passport app on your phone? We're already used to electronic boarding cards. Now countries such as the UAE are looking at electronic visas. Britain should be too.
Giving travellers the option of an electronic passport would also help end the symbolic Brexit battle between Leavers and Remainers over whether our passports should be British blue or European red. Because with a passport app, everyone could just choose the colour that suited them.
No more lost documents, a fairer country and an end to silly rows about passport covers: what's not to like? Time for ID to go digital.
James Kirkup is Director of the Social Market Foundation, a non-partisan think-tankSuggest a correction