Microsoft co-founder Bill Gates speaks while his wife, Melinda, looks on during an interview in New York.

It's an enjoyable challenge For the Gates, the endeavor is also a pleasure. "We have fun doing it. Both of us love digging into the science behind our work," writes Bill. Whether helping to eradicate polio or breed high-yield "super cows," often, deciding where to allocate money requires they study biology, chemistry or agronomy, he says in the letter. "I'll spend hours talking to a crop researcher or an HIV expert, and then I'll go home, dying to tell Melinda what I've learned. "It's rare to have a job where you get to have both a big impact and a lot of fun. I had it with Microsoft, and I have it with the foundation," writes Bill. "I can't imagine a better way to spend the bulk of my time." There is a specific need In the letter, the Gates acknowledge the extreme wealth inequality their fortune represents. "No, it's not fair that we have so much wealth when billions of others have so little," says Melinda. "And it's not fair that our wealth opens doors that are closed to most people." So, "If we think it's unfair that we have so much wealth, why don't we give it all to the government?" asks Bill. "The answer is that we think there's always going to be a unique role for foundations."

Philanthropic foundations can "take a global view to find the greatest needs, take a long-term approach to solving problems, and manage high-risk projects that governments can't take on and corporations won't. If a government tries an idea that fails, someone wasn't doing their job. Whereas if we don't try some ideas that fail, we're not doing our jobs," says Bill. It's how they were raised "We both come from families that believed in leaving the world better than you found it," writes Melinda. "My parents made sure my siblings and I took the social justice teachings of the Catholic Church to heart. Bill's mom was known, and his dad still is known, for showing up to advocate for a dizzying number of important causes and support more local organizations than you can count. "Our goal is to do what our parents taught us and do our part to make the world better," says Melinda. After working for the foundation full-time for 18 years, "By now the foundation's work has become inseparable from who we are. We do the work because it's our life," says Melinda. See also: Bill and Melinda Gates: 'No, it's not fair that we have so much wealth' Billionaire Bill Gates on the impact of A.I.: 'Certainly' we can look forward to longer vacations Billionaires Warren Buffett and Bill Gates are driven by the same character trait