Tesla’s Elon Musk and Google co-founder Larry Page don’t talk anymore. That’s what it says, definitively, in Walter Isaacson’s biography of Musk, published Tuesday. Musk detailed his falling out with Page, who he once considered a very close friend.

Close may be underselling it. The duo was included in Fortune’s 2016 list of “8 business leaders you didn’t know were BFFs.” Musk was known to crash at Page’s house when he visited Silicon Valley. “He’s kind of homeless, which I think is sort of funny,” Page told Ashlee Vance in his 2015 biography Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future. “He’ll email and say, ‘I don’t know where to stay tonight. Can I come over?’ I haven’t given him a key or anything yet.”

In 2014, Page told Charlie Rose that he’d rather give his money to Musk than donate it to charity. “He wants to go to Mars, to back up humanity,” Page said at a TED Talk event. “That’s a worthy goal, but it’s a company, and it’s philanthropical.”

But for the close-knit billionaires, differences over AI proved to be their undoing. The real downfall came when Musk poached one of Google’s top scientists and AI experts, Ilya Sutskever, bringing him to OpenAI. Musk explained to Isaacson that he and Page quickly began to disagree over the direction of AI.

At a 2013 birthday celebration for Musk, per Isaacson’s account, the two fought over whether AI could actually replace humans. Shortly after, Musk approached Demis Hassabis, co-founder of DeepMind, and urged him against letting Google acquire the company. “The future of AI should not be controlled by Larry,” Musk told Hassabis, per Isaacson. It didn’t work, and Google acquired DeepMind in 2014 for $500 million.

Meanwhile, Musk was building OpenAI with Sam Altman. The following year, the two managed to lure one of Page’s most valuable workers, Sutskever, to the other side with a $1.9 million salary and a hefty starting bonus to sign on as co-founder and chief scientist.

That didn’t go over well with Page. “Larry felt betrayed and was really mad at me for personally recruiting Ilya, and he refused to hang out with me anymore,” Musk recalled. “And I was like, ‘Larry, if you just hadn't been so cavalier about AI safety then it wouldn't really be necessary to have some countervailing force.’”

Perhaps the move was already in the making. Despite maintaining what Vance told Vice in 2015 was “a funny relationship,” the two spent a great deal of time together in those early years, owing mostly to Musk regularly crashing at Page’s house. Over time, though, Musk began to believe that Page naively “may very well be working on the end of mankind and some evil AI,” Vance wrote.

Why Musk turned the Page

Musk eventually resigned from OpenAI in 2018, before it went on to create the watershed product ChatGPT. Earlier this year, he revealed in an interview that his decision to walk away from OpenAI made him a “huge idiot.”

At the time, he claimed OpenAI was not being cautious enough with AI’s rapid development—and also that OpenAI’s work presented a conflict of interest with Tesla’s. Musk underestimated OpenAI’s potential profitability, he told CNBC in May. Shortly after he left, the company transitioned from nonprofit to for-profit, a move Musk said he disagrees with, because he doesn’t believe companies should profit off of AI. That’s part of why he and Page fell out.

“I would be constantly urging him to be careful about the danger of AI, and he was really not concerned about the danger of AI,” Musk told CNBC’s David Faber. “He was quite cavalier about it.”

In April 2023, Musk admitted to Tucker Carlson that Page “got very upset” with him over OpenAI. “When OpenAI was created, it did shift things from a unipolar world where Google's DeepMind controlled like three quarters of all AI talent, to where there is now sort of a bipolar world of OpenAI and Google DeepMind,” he said on a Fox News appearance. “Now we're at least seeing OpenAI is maybe ahead.”

Musk’s decision to make OpenAI open source was deliberately meant to rival the DeepMind AI initiative Page was running at Google, Musk told CNBC. “The intent was: ‘What’s the opposite of Google,’ [it] would be an open source nonprofit. Because Google is closed source, for-profit. And that profit motivation can be potentially dangerous.”

It’s a sad state of affairs for two tech billionaires who were once, supposedly, good friends. “The reason OpenAI exists at all is that Larry Page and I used to be close friends and I would stay at his house in Palo Alto and I would talk to him late in the night about AI safety,” Musk told Carlson. “At least my perception was that Larry was not taking AI safety seriously enough.”

Page wanted “digital super-intelligence,” Musk went on, and hoped to “basically [become] a digital god, if you will, as soon as possible.” Musk goes the other way, which is why, he said, the ‘open’ in OpenAI “refers to open source, transparency so people know what’s going on.”

Now if only Musk could patch up his feuds with his ever-expanding list of other enemies, from Mark Zuckerberg to Alexandria Ocasio-Cortez to Jeff Bezos.

This story was originally featured on Fortune.com

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