As we recently sat in a Port-au-Prince park, which housed thousands in tents after the earthquake, Farmer told me he believed the earthquake had awakened a "latent altruism" in human beings.

A big driver of this altruism is "humans, young or old, are pretty good at recognizing injustice when they see it," he added, and they reflexively want to fix it. His comment really captured my imagination and set me on an exploration for the origins of altruism.

Altruism, in its purest form, is hard to define and a source of surprising debate. In the animal kingdom, the ridiculously cute prairie dog, which lets out a bark to warn others of predators, is also calling attention to itself. It is an act of self sacrifice on behalf of others. Vervet monkeys do the same, and vampire bats will share their food with hungry members of their group, even if it jeopardizes their own health. All of these are seemingly instinctively altruistic behaviors -- but with humans, decoding altruism gets trickier.

As Farmer noted, human beings are capable of considerable generosity. By the numbers, Americans gave an estimated $373 billion in 2015 . As a percentage of our income, we also give more now than we did in decades past. Families making between $100,000 and $200,000 gave about 4.2% of their discretionary income; those making more than $200,000 gave around 4%; and those families making between $50,000 and $100,000 gave around 6%, according to the Chronicle of Philanthropy . And it's not just money. We also give our time: More than 60 million people volunteered last year to help a charity.

Of course, generosity and altruism are not the same thing. Generosity is an act, while altruism is a way of life.

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Many biologists believe we evolved this way and point out that altruism "feels good," even though it requires personal sacrifice. According to evolutionary biologists, Darwin's theory of natural selection -- which spawned the phrase "survival of the fittest" -- gave rise to kin selection, group selection, and direct or indirect reciprocity, as explanations for our altruistic behavior.

Kin selection is exactly what it sounds like. It is a behavior that may decrease the chance of survival or success for the individual while increasing that of their kin. Parents will understand it reflexively -- loving your children and the will to sacrifice yourself because of it.

Richard Dawkins has referred to it as the " selfish gene ," because we behave particularly altruistic toward those who are likely to spread our genes. As Dawkins implies, while we humans may be altruistic, our genes -- the blueprint of us -- are programmed to act selfishly.

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Group selection is similar to kin selection, except the generosity is targeted toward a group with which the donor identifies. After our family, the next circle of altruism is extended to friends and coworkers, for example. The belief is that a group may benefit from the aggregate qualities of the individuals more so than the individuals can alone. While genetics does not play a primary role in group selection, not everyone is invited or permitted to join the group, and individuals can be excluded as well.

The last category encapsulates the other theories: reciprocity. Are we humans selfish at our core, acting altruistically only when it may benefit us directly or indirectly? If so, it is then a sort of reciprocal altruism, charity with the expectation of something -- money, good will, a favor or recognition -- in return? That doesn't sound like altruism at all. More like a transaction. With reciprocal altruism, scientists say, our true motivations may be tainted, and hidden even from ourselves.

As I learned, however, on my many trips to Haiti, there are organizations and individuals who don't fit neatly into any of these categories. Their altruism seems pure, just like the prairie dog.

Farmer, who is also one of the founders of Partners in Health , has been working to be a " Partner to the Poor " for 30 years, without any discernible gain to himself. It is incredible work, under the harshest of living conditions and tremendous sacrifice. In its mission statement , Partners in Health aspires to bring modern medical science to those most in need around the world and to serve as an antidote for despair. Farmer told me, "the mission is both medical and moral."

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In early May, I traveled into the central plateau region of Haiti, to a town called Mirebalais, to visit the hospital that Farmer and Partners in Health built in 2013. The poorest region of the poorest country in this hemisphere of the world now has a 300-bed hospital because of their efforts.

It is a remarkable structure, shimmering white with lots of glass, built in an area that was nothing but dirt and dust. Very few believed it could be done, and even fewer believed it should be done. Farmer, though, wanted to show what was possible and, as he says, "shake the world out of complacency."

There are intensive care units for both adults and infants, a large emergency ward and an oncology unit for cancer patients, because Farmer also functions under the belief that the "location and circumstances of your birth should never dictate the quality of your health care."

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There are six operating rooms at University Hospital , one of which Farmer set aside for me to volunteer my time as a neurosurgeon. Caring for patients in Haiti has been among the most meaningful experiences I have had.

After spending so much time with Farmer over the last few decades, we have become close friends, but I am still not entirely sure what motivates his altruism.

When Farmer first came to Haiti in his early 20s, he couldn't precisely identify his motivations. He said he felt "the desire to help people, especially people living in poverty," and that feeling has stayed with him.

"I think I grew into the motivations I thought I had," he told me.

Beyond his early efforts, it could be the urgent desire he feels to use his education and talent to help as many as possible, as quickly as possible, before he can no longer do so.

Photos: Inside life-altering University Hospital in Haiti In 1985, Dr. Paul Farmer met Ferle Jean Sauvener -- aka Baby Bobby -- while treating his mother for cerebral malaria during her pregnancy. It was the beginning of a rewarding partnership that yielded a lifetime of benefits. Hide Caption 1 of 15 Photos: Inside life-altering University Hospital in Haiti More than two decades later, Sauvener has grown up to become a doctor, joining Farmer in treating patients at University Hospital in Mirebalais, Haiti. Hide Caption 2 of 15 Photos: Inside life-altering University Hospital in Haiti Sauvener checks on Sonya Auguste. University Hospital is a public teaching hospital aimed at increasing Haiti's community of health care professionals. Hide Caption 3 of 15 Photos: Inside life-altering University Hospital in Haiti The hospital offers care to about 185,000 people in Mirebalais and the surrounding area. Hide Caption 4 of 15 Photos: Inside life-altering University Hospital in Haiti The 205,000-square-foot, 300-bed hospital is fully powered by 1,800 solar panels. It opened in March 2013, only a few years after the devastating 2010 earthquake that left hundreds of thousands of Haitians dead or injured. Hide Caption 5 of 15 Photos: Inside life-altering University Hospital in Haiti In December, Farmer helped treat 10-year-old Jamson Louis, who had HIV and tuberculosis and was failing to improve. After Jamson arrived at the hospital, Farmer prescribed a new medication regimen. Hide Caption 6 of 15 Photos: Inside life-altering University Hospital in Haiti A month later, Jamson showed significant improvement -- gaining 6 pounds. Hide Caption 7 of 15 Photos: Inside life-altering University Hospital in Haiti In addition to treating patients, Farmer is chief strategist and co-founder of Partners In Health. Hide Caption 8 of 15 Photos: Inside life-altering University Hospital in Haiti Nurse Thamar Julmiste cares for newborns at Hospital St. Therese in Hinche, Haiti. She's shown here with a boy who was born a day earlier, weighing only 1.7 pounds and not breathing. Julmiste helped resuscitate the infant. Hide Caption 9 of 15 Photos: Inside life-altering University Hospital in Haiti Partners in Health includes a team of caregivers who treat victims of malnutrition such as Jeanette Desperance, 38, here in Boucan Carre, Haiti, holding her 14-month-old granddaughter, Maylove Louis. Hide Caption 10 of 15 Photos: Inside life-altering University Hospital in Haiti An 11-month-old sleeps through her pediatric heart screening. Hide Caption 11 of 15 Photos: Inside life-altering University Hospital in Haiti Drs. Jennifer Rutledge and Jeanty Elise speak with an 11-year-old girl who is ill with congenital heart disease. Hide Caption 12 of 15 Photos: Inside life-altering University Hospital in Haiti Patients wait during the morning rush at the women's clinic. Hide Caption 13 of 15 Photos: Inside life-altering University Hospital in Haiti Steve Meuse, 6, is a first grader who lives in the orphanage at the Partners in Health campus in Cange, Haiti. Four years ago, he was treated for malnutrition. Hide Caption 14 of 15 Photos: Inside life-altering University Hospital in Haiti Sauvener shares a moment with his mother, Alourdes Gracia. Their decades-long connection with Farmer and the hospital is a reminder of how their lives have come full circle -- as "Baby" and his colleagues use their skills to help the community. Hide Caption 15 of 15

Perhaps, it is the enormous pressure he may feel to live up to being the title figure in the book " Mountains Beyond Mountains ," in which author Tracy Kidder calls on Farmer to be "a man who would cure the world." I suspect it may have to do with his own theory of being moved to correct the injustices he has seen and experienced his whole life.

It could also be Farmer was influenced by a powerful figure in his life who ignited his transformation. In essence, he may have been inspired, just as he inspired me.

My personal attitudes toward charity and altruism, in part, have been shaped by wanting to live up to the ideals Farmer has shown me, because if pure altruism really does exist in humans, it probably looks a lot like him.