What causes famines? In 1981, Amartya Sen - India's first Nobel laureate in economics - offered a radical answer: not food scarcity, but inequality in food distribution.
"Starvation is the characteristic of some people not having enough food to eat. It is not the characteristic of there being not enough food to eat."
It was a childhood encounter with famine victims in his native Bengal more than 80 years ago that led the Indian economist and philosopher Amartya Kumar Sen to this startlingly simple realisation.
Now 91, he has devoted his distinguished academic career - and more than two dozen books - to explaining this seminal insight and examining its many implications for tackling poverty and enabling sustainable growth.
Sen's work has received worldwide recognition. In 1998, he became the first Indian to be awarded the Nobel Prize in Economics for his contributions to welfare economics and the social choices of the poor. His influence on development economics and on how we measure social and economic progress can hardly be overestimated.
Childhood in the shadow of famine - Sen's path to economics began early.
Sen's work has achieved global recognition. In 1998, he became the first Indian to win a Nobel prize in economics for his contributions to welfare economics and the social choices available to the poor. His influence on development economics and on the way that we measure social and economic progress has been equally notable.
Sen reasons that since a healthy and well-educated person is better able to make informed economic decisions, human development should be viewed as an effort to advance real individual freedoms, rather than through the lens of metrics like income-per-capita.
Co-developed with the political philosopher Martha Nussbaum and others, this "capabilities" approach is incorporated in the UN's Human Development Index (HDI), which transcends traditional indicators to include such capabilities as access to health and education.
Sen's focus on the poorest people on the planet and how policy can improve their lives has expanded the scope of his ideas well beyond the universities where he has spent most of his life.
Born in 1933 in Santiniketan, just north of Kolkata, he grew up on the campus of a small, progressive school and college founded by the renowned writer, philosopher, and poet, Rabindranath Tagore, a close friend of his maternal grandfather.
While attending the school, Sen had direct, personal experience of the 1943 Bengal famine. "One day a chap came wandering in, very obviously deranged," he told The Guardian newspaper in a 2001 interview. "I got chatting to the man and it became quite clear he hadn't eaten for about 40 days."
As more and more starving people crossed the small campus in a desperate quest for food, Sen began to appreciate the awful scale of the disaster, which was ultimately responsible for the deaths of more than three million individuals.
These early experiences plainly influenced his decision to study economics.
Sen earned his first BA in the subject (with a minor in mathematics) in 1953 from Presidency College, the same branch of the University of Kolkata from which his friend and fellow economist Abhijit Banerjee would graduate some three decades later.
While at Presidency, Sen was diagnosed with oral cancer and given a 15% chance of living for another five years; a prognosis that thankfully proved false.
It wasn't a lack of food - it was a lack of purchasing power.
In 1955, he earned another first-class BA in economics, this time from Trinity College, Cambridge, in the UK, where he later studied for a PhD, advised by the legendary British development economist Joan Robinson.
Sen was the Master (head) of Trinity College from 1998 to 2004. And it was there that he turned his youthful interest in philosophy into serious study and began to develop the philosophical ideas for which he has also become famous.
Sen's scholarly interests range from gender to justice, but it is his groundbreaking work on famine and what he has called its "class-dependent" causes that underpins almost all of them.
In 1981, he published "Poverty and Famines: An Essay on Entitlement and Deprivation", in which he argued that famines are the result of inequalities built into food distribution mechanisms.
His investigations revealed, for example, that the Bengal famine of 1943 was caused not by a drop in food production, which had remained stable, but by the failure of farm labourers' wages to keep up with inflation.
Sen has claimed that "no famine has ever taken place in a functioning democracy," - a conviction influenced by another shocking childhood experience: the inter-communal violence between Muslims and Hindus that he witnessed before the partition of India in 1947.
Sen has recalled the day that a Muslim daily labourer stumbled through the rear gate of his family home, bleeding from a knife wound in his back. Driven by extreme poverty, the man had come to Sen's primarily Hindu neighbourhood searching for work, knowing that he risked death by doing so. Indeed, the price of what Sen has called this man's economic "unfreedom" was his death.
Sen's influential book "Development as Freedom", first published in 1999, explored the notion of unfreedom, both economic and political. Ten years later, "The Idea of Justice" reasoned that justice itself can only be achieved if people enjoy real freedoms and opportunities
Sen still enjoys enormous respect in academic circles. He is currently the Thomas W. Lamont University Professor at Harvard - a rare tribute and testimony to his exceptional distinction and influence - as well as a professor of economics and philosophy at the university.
Even so, his work has attracted occasional criticism.
Some experts on famine have argued that the principal cause of famine is epidemic disease. Sen has also been called out for using "ballpark" figures in an examination of gender inequality. And critics on the left have accused him of too cautious and mainstream a view when it comes to the shortcomings of globalisation.
Sen nevertheless retains his carefully balanced, pragmatic approach to the foundational questions of human freedom.
His intellectual influences include Karl Marx, as well as Adam Smith and J.S. Mill, but while plainly left of centre he rarely takes overtly political positions. As he told The Guardian back in 2001, "I could not develop enough enthusiasm to join any political party."
Sen's moral engagement is undeniable, however, and may be his most enduring legacy. In India he is still known as the "Mother Theresa of Economics," thanks to his Nobel prize. Amartya, moreover, means "immortal" in Sanskrit.
