New York

ALPHAGO, the artificial intelligence system built by the Google subsidiary DeepMind, has just defeated the human champion, Lee Sedol, four games to one in the tournament of the strategy game of Go. Why does this matter? After all, computers surpassed humans in chess in 1997, when IBM's Deep Blue beat Garry Kasparov. So why is AlphaGo's victory significant?

Like chess, Go is a hugely complex strategy game in which chance and luck play no role. Two players take turns placing white or black stones on a 19 by 19 inch (48 cm by 48 cm) grid; when stones are surrounded on all four sides by those of the other colour, they are removed from the board and the player with more territory at the game's end wins.