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Microsoft founder Bill Gates and Tesla and Twitter CEO Elon Musk have never been the best of buddies—in fact they’ve butted heads on some of the biggest issues facing humanity.

The tech tycoons have clashed on subjects like the pandemic and climate change, but now the SpaceX founder has taken a swing at philanthropist Gates’ open letter on artificial intelligence.

Gates published a seven-page essay on his website a week ago about why A.I. is so revolutionary, covering topics such as how the technology will transform health care and education, as well as highlighting some risks and pitfalls creators and the public need to be aware of.

It seems Musk—who recently held the title of richest person on earth, once held by Gates—was unimpressed. Responding to a tweet about Gates’ 3,639-word essay titled “The age of A.I. has begun,” Musk wrote: “I remember the early meetings with Gates. His understanding of A.I. was limited. Still is.”

In his essay, Gates mentions he had been speaking with ChatGPT creator OpenAI since 2016. It’s unclear in what capacity Gates was communicating with CEO Sam Altman’s team, though at the time he was still a board member at Microsoft—which is a partner of OpenAI.

Musk’s relationship with OpenAI began even earlier, in 2015. He was one of the original backers of the business, perhaps explaining why he was in “early meetings” with Gates, but left the organization in 2018 citing possible conflicts of interest because of Tesla’s own A.I. work.

Since then, Musk has been critical of the business he once backed, tweeting in 2020, “I have no control & only very limited insight into OpenAI,” adding his confidence in its safety was “not high.” He then paused OpenAI’s access to the Twitter database for training, saying he needed to know more about the A.I. creator’s “governance structure & revenue plans going forward. OpenAI was started as open-source & non-profit. Neither are still true.”

Conflict of faith

Musk’s attack on Gates may also come from a conflict of faith in A.I. as a model. Despite outlining reservations about the technology, Gates’ take has been described as “bullish,” courtesy of his thoughts such as: “This new technology can help people everywhere improve their lives. At the same time, the world needs to establish the rules of the road so that any downsides of artificial intelligence are far outweighed by its benefits, and so that everyone can enjoy those benefits no matter where they live or how much money they have.”

Meanwhile Musk, although a supporter of the technology, has issued far starker warnings of “scary outcomes” such as a Terminator-like apocalypse. He previously said the technology “scares the hell” out of him.

The Bill & Melinda Gates Foundation did not immediately respond to Fortune’s request for comment.

COVID conflicts

Musk may have taken inspiration from his subject when it comes to making gibes about intelligence. In July 2020, Gates said Musk should scale back his “outrageous comments” on the pandemic, which included saying children were immune to the virus and opposing lockdown. In an interview with CNBC’s Squawk Box, Gates said: “He’s not much involved in vaccines. He makes a great electric car. And his rockets work well. So he’s allowed to say these things. I hope that he doesn’t confuse areas he’s not involved in too much.”

🎶 Billy G is not my lover 🎶
— Elon Musk (@elonmusk) July 30, 2020

Musk responded on Twitter at the time, posting Michael Jackson’s “Billie Jean” lyrics. In a later podcast interview with the New York Times’ Kara Swisher, he called Gates a “knucklehead,” adding: “Gates said something about me not knowing what I was doing. It’s like, ‘Hey, knucklehead, we actually make the vaccine machines for CureVac, that company you’re invested in.’”

Climate wars

Gates has also made a habit of openly complimenting Musk. In 2020, he told technology YouTuber Marques Brownlee that Tesla had been the biggest driver toward electric vehicle adoption, but then revealed he had purchased a Porsche Taycan instead of one of Musk’s models.

Gates similarly showered praise on Musk in the New York Times’ Sway podcast in February 2021, saying “underestimating Elon is not a good idea.” He continued: “It’s important to say that what Elon did with Tesla is one of the greatest contributions to climate change anyone’s ever made,” but countered that by saying Tesla had focused on the “easy stuff” like passenger cars. In the same interview he appeared to take aim at SpaceX, saying he “doesn’t think rockets are the solution” to the climate crisis.

Gates also told French YouTuber HugoDécrypte he “likes” Musk, saying he has “done great work” but added: “I give a lot more to climate change than Elon or anyone else. I give a lot of philanthropic dollars; I back companies—you know, electric cars are about 16% of emissions, so we also need to solve that other 84%.”

The compliments have done little to change Musk’s opinion of the Bill & Melinda Gates Foundation founder. When asked what he thought of a previous Gates essay on electric trucks, Musk responded: “He has no clue,” adding his conversations with the man worth $107.5 billion have always been “underwhelming.”

Musk, reportedly worth $192 billion, has also previously made fun of Gates’ appearance, but last year said he was “moving on” from making fun of Gates.