Warren Buffett, the legendary financier and Berkshire Hathaway chairman, says he reached a crucial milestone: He's officially given away half of his company shares to charitable causes.

On Wednesday, Buffett earmarked a $4.1 billion distribution for five foundations, bringing him closer to his 2006 pledge to give away all of his Berkshire stock, or 99% of his net worth at the time. He has donated about $41 billion so far.

In a lengthy note published early Wednesday morning, he also said he was resigning from the Bill & Melinda Gates Foundation, noting that he has been an "inactive trustee" for years. He did not provide a reason for his decision or mention the pending divorce of its namesake founders, which has raised questions about the Seattle-based philanthropy's future.

But he said his philanthropic goals are "100 percent in sync with those of the foundation," adding that his participation is "in no way needed" to achieve those goals.

"For years I have been a trustee - an inactive trustee at that - of only one recipient of my funds, the Bill and Melinda Gates Foundation (BMG)," Buffett wrote. "I am now resigning from that post, just as I have done at all corporate boards other than Berkshire's."

In his letter, Buffett said his late wife, Susan Thompson Buffett, had wanted to donate more money when they were young, but he that held off. It was only after her death 15 years ago that he "stepped on the accelerator," as he put it.

"Over many decades I have accumulated an almost incomprehensible sum simply by doing what I love to do," Buffett wrote. "Compound interest, a long runway, wonderful associates and our incredible country have simply worked their magic. Society has a use for my money; I don't."

He said his remaining 238,624 class A shares are worth about $100 billion.

He encouraged other superwealthy people to give away the bulk of their fortunes, writing "the easiest deed in the world is to give away money that will never be of any real use to you or your family."

Buffett's wealth and philanthropy play into a broader debate over wealth in America. The Biden administration wants to increase taxes on corporations as well as heirs to fund an expensive infrastructure plan. And a 2011 Obama administration proposal dubbed the "Buffett rule" would have required millionaires and billionaires to pay the same tax rate as low-income families and other working people.

Buffett himself currently pays very little in taxes, in large part because almost all of his wealth is tied up in Berkshire Hathaway. A trove of leaked Internal Revenue Service documents published by the investigative news organization ProPublica showed that Buffett himself paid a true tax rate of 0.1%.

He addressed the issue indirectly in his letter Wednesday.

"I have relatively little income," he wrote. "My wealth remains almost entirely deployed in tax-paying businesses that I own through my Berkshire stockholdings, and Berkshire regularly reinvests earnings to further grow its output, employment and earnings."

Buffett's philanthropic giving consists of 16 annual donations to five foundations, which then distribute funds to various nonprofits. According to public records reviewed by the trade publication Inside Philanthropy, they include the Bill & Melinda Gates Foundation, three foundations run by his children, and the Susan Thompson Buffett Foundation, which was founded in 1964 and later renamed after his late wife. The Gates Foundation is the largest recipient.

The eventual sources of Buffett's giving is less understood, with donations often made anonymously. Much of the Susan Thompson Buffett's Foundation's giving has gone to various efforts to protect abortion rights, as well as women's health organizations such as Planned Parenthood.

According to Inside Philanthropy, the smaller foundations run by Buffett's family include the NoVo Foundation, which is focused on ending violence against women, the Howard G. Buffett Foundation, which contributes to environmental causes including food and water security, and the education-focused Sherwood Foundation.