Election 2018 San Francisco Business Tax Homelessness

FILE - In this Feb. 23, 2016 file photo, a man stands outside his tent on Division Street in San Francisco. (AP Photo/Eric Risberg, File)

What does it mean to be poor? There are two basic ways to define poverty. To get a better measure of who needs help — and a better sense of how to provide it — society needs a third definition.

The first definition is absolute poverty — essentially, material destitution. Human beings need food, water and shelter, and if we can’t afford these things, life is pretty miserable. In the U.S., the federal government has poverty guidelines that are based on food consumption: If you make less than about three times the minimum amount people need to spend on food each year, you’re poor.

A single adult living on $12,140 or less is considered poor as of 2018. For a family of four, the figure is $25,100. There is also a Supplemental Poverty Measure that includes not just food but clothing, shelter and utilities. Thanks in part to increased government assistance, U.S. poverty according to this measure has fallen, especially for children.

Critics of the federal poverty guidelines argue that these numbers are too low, thanks to growing inequality — in the 1960s, the federal poverty level was about half of the median income, but is now well below that. Moreover, as a country grows richer, hunger becomes less common, so using it as measure of poverty becomes less useful.

This is where the second measure — relative poverty — comes in. The Organization for Economic Cooperation and Development defines poverty this way: If you earn less than half of the median income, you’re poor. By this measure, the U.S. is doing worse than other rich countries.

But this, too, feels unsatisfying. Imagine a future U.S. in which the median American is fabulously wealthy — with flying cars, robot servants and multiple overseas vacations every year. Should someone with half as many flying cars, robot servants and overseas vacations be considered poor? That seems like a stretch. Intuitively, then, it seems that a third definition of poverty is necessary — one that measures more than just material well-being but also takes into account economic growth.

Luckily, there is just such a concept: It’s called material security. Psychologist Abraham Maslow believed that safety ranked second only to food and shelter as a basic human need. Someone who has food and a roof over their head today, but doesn’t know whether they will tomorrow, should be considered poor.

Imagine a 55-year-old single woman with diabetes working a part-time job making close to minimum wage. Thanks to government assistance, her total income is $15,000 a year. But if she loses her job or has a medical emergency, she will probably become homeless. That in turn will make it very hard to get a new job, or to pay for her future health-care needs. In short, her situation is very precarious.

As Maslow would predict, this kind of insecurity causes extreme stress. And this precariousness exists along several dimensions — housing, health care, income, the risk of violence — which makes it hard to capture in a single measure. For example, the U.S. Department of Agriculture tracks food insecurity, a survey-based measure of how worried people are that their food will run out. This kind of risk has been on the rise in the U.S.

The risk of eviction, meanwhile, can be roughly measured by the percentage of people’s incomes that they spend on shelter each month. As of 2015, 17 percent of Americans spent half or more of their incomes on rent.

A reasonable, common-sense definition of poverty should also strive to measure how secure people feel — in their homes, their health, and their jobs. This measure might well show that poverty in the U.S. is worse than the current statistics say. But an accurate view of a problem is the first step toward addressing it. And eliminating poverty should be a priority of any wealthy society.

Noah Smith is a Bloomberg Opinion columnist.