Lee Saedol was one of the world’s top Go players, and his shocking loss to an A.I. opponent was a harbinger of a new, unsettling era. “It may not be a happy ending,” he says.

Lee Saedol was the finest Go player of his generation when he suffered a decisive loss, defeated not by a human opponent but by artificial intelligence.

Mr. Lee was beaten by AlphaGo, an A.I. computer program developed by Google’s DeepMind unit. The stunning upset, in 2016, made headlines around the world and looked like a clear sign that artificial intelligence was entering a new, profoundly unsettling era.

By besting Mr. Lee, an 18-time world champion revered for his intuitive and creative style of play, AlphaGo had solved one of computer science’s greatest challenges: teaching itself the abstract strategy needed to win at Go, widely considered the world’s most complex board game.

“I am very surprised because I have never thought I would lose,” Mr. Lee said at the time in a post-match news conference. “I didn’t know that AlphaGo would play such a perfect Go.”

But the implications of his loss went far beyond the game itself, in which two players compete for territory by placing black and white stones on a gridded board made up of 19 lines by 19 lines. AlphaGo’s victory demonstrated the unbridled potential of A.I. to achieve superhuman mastery of skills once considered too complicated for machines.

Mr. Lee, now 41, retired three years later, convinced that humans could no longer compete with computers at Go. Artificial intelligence, he said, had changed the very nature of a game that originated in China more than 2,500 years ago.

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