Why these billionaires are pledging away their wealth

FORTUNE -- On Wednesday, the Giving Pledge -- a movement spearheaded by Warren Buffett and Bill and Melinda Gates -- revealed the names of 40 individuals and families who are taking up the challenge to give away at least half of their net worth during their lifetime or at death. Along with a commitment, the philanthropists all posted a letter explaining why they were taking the plunge. Analyzing their remarks offers a rare glimpse into the minds of the richest of the rich: how they view their fortunes and where they think their money should go after they die.

The most common philosophy: That a life of monetary excess can be an empty life if the wealth does nothing to advance a greater cause.

Buffett says he and the Gates have pitched the idea to about 70 to 80 people on the Forbes 400 list so far. While Buffett is pleased that half of those contacted so far have risen to the occasion, many of them, including Ted Turner, Larry Ellison, and Thomas Monaghan, had already made a commitment to give away much of their wealth.

Still, Buffett says there are many more people to recruit, and he plans to urge others to join the giving campaign through dinners similar to the first hosted in May 2009. "We're off to a terrific start," he says.

The cast of billionaires came from far and wide, including a sizeable number from California. For some, such as Monaghan, who founded Domino's Pizza, (DPZ) joining the Buffett-Gates mission was almost like answering a religious calling. Bill Gates contacted the 73-year-old businessman and philanthropist, who has made it part of his mission to promote Catholic higher education. "I came into the world penniless and as a Catholic Christian, I know that I cannot take any of it with me, so it has long been my desire to use the material resources that I have been blessed to help others in the most meaningful ways possible," Monaghan writes in his pledge letter.

Leaving a legacy of philanthropy

Buffett stresses that The Giving Pledge is free of any issues-driven agenda. The idea is simply to encourage the richest to give up their wealth to any philanthropic pursuits that inspire them. New York City Mayor Michael Bloomberg -- who joined Buffett and the billionaire husband-and-wife team of Tom Steyer and Kat Taylor for a conference call about the effort -- says the flow of private donations can support innovation. For instance, where public dollars might be better suited to pay for time-tested teaching methods and materials, private donations could fund innovative ways of teaching or new measurements of success.

In his pledge letter, the founder of Bloomberg LP recalls being delighted to hear that one of his senior managers at the financial news and data company uses philanthropy to recruit new hires. The manager told Bloomberg his pitch: "What other company can you work for where the owner gives nearly all the profits to charity?" Bloomberg writes. The mayor adds, "Nothing has ever made me prouder of my company than that one story."

While Bloomberg found inspiration from business, Barron Hilton's pledge letter reads like a tribute to his father, the hotel magnate Conrad Hilton. The younger Hilton writes: "Throughout his life, he embraced the power of prayer, and felt it was our God-given responsibility to alleviate the suffering of the most disadvantaged among us."

Steyer, who founded the San Francisco hedge fund Farallon Capital Management, and his wife thought of their children, saying they wanted to leave their kids "a different kind of inheritance, an example of at least trying to lead a worthy life."

Philanthropists Eli and Edythe Broad gained a fortune after the company they founded, SunAmerica, merged with American International Group (AIG, Fortune 500) in 1999. The couple has promised to give away 75% of their wealth during or after their lifetimes. For the past 10 years, the Broads have devoted their lives to philanthropy, supporting mostly projects related to education reform, scientific and medical research and the arts.

The missing billionaires

The level of giving varies -- Oracle's (ORCL, Fortune 500) Ellison pledges to give away "at least 95%" of his wealth, while Ron Perelman leaves it a bit more vague, pledging "up to half my assets ... after my family and children have been provided for." The Giving Pledge did not return telephone calls and e-mails seeking clarity on the 50% minimum and Perelman's pledge.

Buffett says the campaign to get more of America's richest to give up their wealth continues, though not always smoothly. Some billionaires approached by the country's two richest men have already rejected the idea. Buffett wouldn't name names, but says that general dissatisfaction with the government played a role for some. (He says he frequently got a political lecture from those billionaires after making his request.) Others believed in keeping the money in the family, what Buffett calls "a fairly dynastic view of wealth." And some "just had a plane to catch."