If you're younger than 35, you'll probably live long enough to put David Levy's prediction to the test. Levy says that by 2050 we'll be creating robots so lifelike, so imbued with human-seeming intelligence and emotions, as to be nearly indistinguishable from real people. And we'll have sex with these robots. Some of us will even marry them. And it will all be good.

Levy lays out his vision of a Brave New Carnal World in Love and Sex With Robots: The Evolution of Human-Robot Relationships, which, despite its extended riffs on sex toys through the ages, is a snigger-free book. Levy's no Al Goldstein. Rather he's a 62-year-old British chess master turned artificial-intelligence expert persuaded that robot sex can brighten the lives of many, many unhappy people. "Great sex on tap for everyone, 24/7,'' he writes on the final page of the book. What's not to like?

"Chess'' and "sex'' aren't words that normally share the same sentence, but in Levy's case, the one led to the other. A keen chessman since boyhood, by the time he got to St. Andrews University he played at the international level. At the university he got interested in computers and the challenge of programming machines to play chess. Eventually he earned international recognition for his work on chess-playing computers and natural-language software, and in the mid '90s headed a team that won the Loebner Prize, widely regarded as the world championship of conversational software. Today he owns a firm that develops electronic hand-held brain games.

Designing computers that talk like humans naturally led to the larger question of how humans interact with robots, which are nothing more than computers with arms and legs and a head. The Japanese have taken the lead in developing "partner robots,'' machines that, for example, might do household tasks for elderly people. But if you could invent a robot that serves cocktails, could you not invent a robot that would make a superior bedmate?

It sounds like a mighty tall order. A machine with skin that feels like ours? With our physical dexterity? And, most important, with a mind like ours - imperfectly rational, sometimes emotionally intelligent, sometimes emotionally dumb?

"I think it's a reasonable assumption,'' Levy said in a telephone interview from his home in London. He lays out his case in a voice that's calm, rational, almost flat, more geeky than goatish.

"If one looks at the advances in technology in the last, say, 40 or 50 years, they've been immense, and the more we learn about the science and the technology, the quicker it will be to discover even more within that science."

Smart money never bets against technological advances, but it helps if you stack the deck. "The automaton simulates man when man has been defined in an automaton's way," literary critic Hugh Kenner wrote. Is that what Levy does?

"I take a pragmatic point of view," he said, "partly because in my original field, computer chess, that was how the problem was solved." Not by making machines that thought like chess masters but by making machines that beat chess masters. Similarly, Levy thinks, robots need only "simulate" human intelligence and emotions "to the point that they are absolutely convincing." If you can't tell whether the thing is man or machine, what difference does it make? You'll treat it as if it were alive. The rest is philosophical hairsplitting.

So who will avail themselves of 21st-century sexbots?

Sad cases, for one, people so physically unattractive or anti-social or isolated or emotionally crippled that they have trouble finding human romance. People who love their computers more than their fellows. Hey, they're out there already.

"They're lonely; they're miserable," Levy said. "I think society will be a much better place when they have an alternative that satisfies them without doing any harm to other people."

Add in those who have a satisfying sexual relationship but are simply curious and somewhere between 20 percent and 50 percent of the population will experience man-machine mating at least occasionally, Levy predicts.

He respects the fact that plenty of people, out of moral or religious conviction, will contemplate this with horror.

"But by and large," he said, "it will be very good for society, very beneficial, and I think that will be the majority view within a relatively short space of time."

Sexbots may put prostitutes out of business, he notes.

Near the end of the book Levy alludes to a set of vexing questions. If robots become utterly humanlike, must we not treat them as more than machines? So if you marry a robot, can it inherit your estate? If you catch it boffing the mail carrier, can you toss it out with heavy trash? If your robot pops your neighbor in the mouth, who does your neighbor sue?

Levy admits he doesn't know the answers.

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