(FORTUNE Magazine) – "To be honest, I can't believe I lasted this long," says Bill Joy. "It seems like I've been working on the same basic problems since college."

Joy, the co-founder of Sun Microsystems, dropped in to chat the day after announcing that he was leaving Sun. While serving as the company's chief scientist, he's had a remarkable 21-year run of invention, way-outside-the-box thinking, and uncanny technological clairvoyance. Now, he says, "I'm just ready for something new and different."

Anyone who has met Joy can't help but be taken in by his passion for the new and the different. Ask him a question, and he responds like John Coltrane playing a tenor sax solo: He spots a phrase or idea that he likes as it spills from his mouth, and then riffs on its every permutation as he desperately tries to keep up with his racing brain.

In many ways Joy resembles a geek from central casting--his nebula of tousled hair and piercing eyes give him the look of a mad scientist. And he can summon up unworldly concentration when confronting a problem, whether it be some gnarly software code or the recalcitrant guts of a new microprocessor chip.

But what makes Joy especially compelling is the mind-boggling breadth of his curiosity and his playful eccentricities. You never know where time with Joy is going to lead. In the late 1990s, I visited him at his home in Aspen for a profile. On the first day I barely got a question in. Joy wanted us to play checkers--for money--with his then 4-year-old son. We spent the rest of the afternoon discussing everything from cardiac health to the fiction of Wallace Stegner, and tinkering with his incredibly complex home-theater system. And he was just as frenetic away from home. Beyond his day job, Joy has owned a San Francisco gallery specializing in primitive art, advised Bill Clinton on technology policy, dabbled in cattle ranching, and taught himself Spanish.

A voracious reader, Joy likes to open a novel and start reading from the middle, just to see if he can figure out what happened in the earlier pages of the book. And when he decided he needed to lose some weight a few years ago, he started drinking copious amounts of coffee to quell his hunger pangs. He lost over 30 pounds.

In recent years Joy has become better known as a Cassandra of sorts. In 2000 he wrote an article for Wired called "Why the Future Doesn't Need Us," which warned that IT, nanotechnology, and biotechnology are advancing so quickly they could easily run amok like a bad science-fiction movie. He spent two years trying to turn the article into a book, but says that the events of 9/11 have done more to awaken people to the world's perils than his own treatise ever could.

Nevertheless, at heart Joy is an optimist, both about where technology can lead, and about the future of the company he co-founded. "Sun's technologies and strategies are in good shape and in good hands," he says. "And I have a sense that the Internet, despite all the problems with viruses and spam lately, can be made safe. In fact, that might be what I'll work on next."

Let's hope so.

--Brent Schlender