"One wet cold night many years ago at the age of 22, a newly qualified doctor, I went to attend my first confinement. Very nervous I arrived with my new black bag. My knock was answered by the young husband, pallid and shabby, with the familiar signs of long unemployment upon him. He took me upstairs to a room stripped of all but the bare necessities of life. There lay the patient on a mattress covered by a threadbare blanket – a girl of my own age – in labour with her second child.

"By the bed stood a cot and standing grasping the wooden bars was a child with bulging forehead and crooked legs, the classic picture of rickets, a disease of undernourishment. The young mother clutched my hand with her own moist bony fingers on which she wore a greenish brass wedding ring twisted round with cotton to prevent it falling off. In that room that night I became a socialist and I joined in the fight – not against a class but a system – a system which refused to accept responsibility for the welfare of the most helpless among us."

– Dr Edith Summerskill, BBC, 1946

These unforgettable words from Labour MP, Dr Edith Summerskill, were part of the barrage of argument and sentiment unleashed by the post-war Labour government to gain support for the introduction of the National Health Service. Summerskill depicts a family that has fallen below any reasonable poverty line. But what is it that touches you most about the depiction? The bareness of the room? The malnutrition of the child? Or the ill-fitting brass wedding ring?

In one sense, the definition of poverty is easy. It is lacking the financial resources to meet your most basic needs. But it very soon gets more difficult; for what counts as a “basic need”? Summerskill, and the family she visited, presumably agreed that a wedding ring for a married woman in mid-twentieth century England was a basic need. A brass ring was probably shameful, but no ring at all would have been scandalous. Yet for some there may be a lurking feeling: why waste any money on a ring if it means you can’t afford to get the right vitamins for your infant child?

Today that scepticism is likely to take on a different form: how can someone be poor if they have a smartphone? Anyone who asks this question is likely to think that if you have money for such luxuries, you must have enough money to meet your basic needs, if only you spent wisely. Any shortage of necessities is your own selfish fault: a result of imprudence or desires above your station.

Not long ago, the satellite dish filled the role the smartphone plays today. Decades before, in 1937, the poverty research pioneer Benjamin Seebohm Rowntree confronted the same complaint. How can people be poor, the middle classes said, if they can afford to go to the cinema? Rowntree responds:

"[W]orking people are just as human as those with more money. They cannot live just on a “fodder basis”. They crave for relaxation and recreation just as the rest of us do. But… they can only get these things by going short of something which is essential to physical fitness, and so they go short …. They pay dearly for their pleasures!"

The critical factor for Rowntree is not how people spend their money, but what they have to give up in order to have what they regard as essential to a normal human life. The middle-class commentators did not have to choose between a wedding ring and vitamin C for their newborns. They did not have to choose between going to the cinema and eating lunch the next day. On this view, to understand whether someone is in poverty, the best test is not what they buy but the choices they face and the sacrifices they have to make to do what is taken for granted by others.

Notice that Rowntree talks not about people in poverty but working people. If you have a wage, isn’t that enough to bring you out of poverty? This fallacy has been with us a long time, and it is what motivated Rowntree’s first empirical investigation of poverty. Since there was virtually full employment in York at the end of the 19th century, the argument was made that there could be very little poverty. Yet Rowntree found a large percentage of people living without the financial means to provide them with healthy living conditions. Even today, a full-time job is far from a guarantee against poverty. According to the Social Metrics Commission as many as 26 per cent of single-parent families in full-time work are in poverty. If part-time work is included, there are more people in poverty in working families than there are in workless families.

Poverty can be measured in different ways, but the driving insight behind contemporary measures is the notion of relative poverty formulated by Peter Townsend in 1979. The idea here is that being poor means not having enough to do what is normally expected or encouraged in the society in which you live. Being poor is being excluded from normal activities: you can’t send your children on a school trip or you turn down a wedding invitation because you can’t afford a gift. Although going without food is a common sacrifice that parents in poverty make for their children, poverty is not the same as malnutrition, or homelessness, or sleeping rough. Rather, it is a grinding struggle to do ordinary things. It is having to make tough, basic choices that those with sufficient wealth do not have to make.

Poverty researchers and activists have to battle against an ideological tide which insists that if you are in poverty it is either your own fault for not trying hard enough to get a job, or for failing to acquire the skills that would qualify you for a better-paid job. And if it is no one’s fault but your own, you have no justified claims against others or the state. At most, we might give assistance out of the goodness of our hearts, but there are no claims of right or justice.

What makes this argument so stubborn is that it contains a tiny grain of truth. For some people, if they had made different choices in the past, they might be faced with better prospects now. If they showed more determination, perhaps they would have a job now. Norman Tebbit famously told his audience that his father got on his bike to look for a job in the 1931 depression. But these bicycle expeditions did not create new jobs, and someone else’s father would suffer the misery of unemployment if Tebbit senior did manage to edge ahead in the ruthless struggle for work.

As international and historical comparisons demonstrate, structural factors - most notably the state of the economy, government policy regarding minimum wage, unemployment benefits, housing support, income support, and the provision of public services – are the primary determinants of the level of poverty in any country. Of those vulnerable to poverty, individual grit, determination and resilience can sometimes make a difference. But it is also becoming more widely accepted that poverty grinds down people’s ability to plan and make effective choices.

And here we return to our starting point. Poverty is not so much about the choices people make, but rather what they find themselves forced to choose between. Economists love to teach us about the concept of “opportunity cost”: what you have to give up in order to pursue a particular option. Poverty can be understood in these terms. If you are rich, your choices have very little opportunity cost; if you are poor, the opportunity costs can be immense. A key role of government should be to ensure conditions in which everyone can do what is normally expected in their society without suffering the dire consequences of making tough choices that involve extreme sacrifices. No one should be forced to choose between feeding their children and buying a wedding ring.

Jonathan Wolff is Blavatnik Chair in Public Policy and Governing Body Fellow at Wolfson College, Oxford. He is the author of Ethics and Public Policy: A Philosophical Inquiry.

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