Earlier this month, Bill Gates took the stage at the Recode conference to talk about philanthropy with his wife, Melinda. They discussed mobile payments, contraception, and billionaires giving away their fortunes. Then the conversation turned to artificial intelligence, and Gates grinned and swiveled in his giant red leather chair. "Certainly, it's the most exciting thing going on," he said. "It's the Holy Grail. It's the big dream that anybody who's ever been in computer science has been thinking about."