CHICAGO -- Talk to anyone who's met Jeff Koons, and they're likely to say pretty much the same thing: "What's with this guy -- is he from Mars?" Maybe he is.

At least, extraterrestrial origins might be the best explanation for his art. It is so compellingly, engrossingly strange that no other account fits. Even though Koons is one of the most famous and popular artists on this planet, familiarity doesn't make his work any easier to get a handle on. His objects, from stainless-steel bunnies to hand-carved wooden cherubs manhandling a sow, absolutely refuse to comply with any of the normal ideas -- normal (begin ital) earthly (end ital) ideas -- about what art should be and how it should work. Where many of even today's best artists risk retreading well-trod ground, Koons takes us somewhere genuinely new. Whether we enjoy being there is almost beside the point.

The Museum of Contemporary Art in Chicago recently opened the first-ever full-scale Koons retrospective -- hard to believe, given that, at 53, he's already been a major art-world figure for about a quarter-century. The Koons show fills two hangar-size spaces. All dividing walls have been removed, so now it's Koons, Koons, Koons, as far as the eye can see.

That's the perfect way to see him. It lets us feel the unified, out-of-this-world sensibility that has governed his entire career.

It's not that the pieces Koons has made over the past three decades look all that much alike. He started out taking store-bought vacuum cleaners and presenting them as sculpture, soon joined by real basketballs suspended in household aquariums. There were also full-size replicas of rubber dinghies and aqualungs, cast in Old Master-ish bronze. In the early 1990s, he showed giant hard-core photos of himself having sex with his wife, the famous Italian porn star known as La Cicciolina ("Chubby Chick"). More recently, Koons has completed simulacra of shiny blow-up toys and Christmas ornaments and gems, enlarged to monumental size in gleaming stainless steel. And in the past few years, he's shown copies of children's inflatable pool toys -- dolphins, lobsters and turtle-headed swimming floats -- cast in aluminum and then painted so they cannot be distinguished from the original objects.

But the thing about all this variety is that it doesn't leave you contemplating each of Koons' very different pieces, the way you might want to stare for hours at each painting in a Cezanne show. Instead, it invites you to take in the whole Koonsian project and admire how he has rewritten all the rules of art -- all the traditions and conventions that usually give art order and meaning -- according to his own eccentric take.

Koons' eccentricity isn't the standard insane-bohemian kind. Quite the opposite: In person and on video, the man comes off more as an attractive, tidily dressed actuary than a raving van Gogh. But there's something slightly off about his manner and his ideas on art and life. He's like a space alien who has spent long years studying how to be the perfect, harmless Earthling, but can't quite get it right. Koons, the Stepford Artist, spouts such a quantity of absurd, Pollyanna cliches -- "the people who are involved in the art world really first and foremost care about people" or "when someone views a work of art ... it's always about them, about their potential" -- that it's hard to remember that he's the guy who posed having sex with his wife. The work Koons makes is similarly skewed. It misunderstands the usual artistic norms. And it's this misunderstanding that makes Koons' work so potent, and so different from anything else that's out there. Many artists (begin ital) act (end ital) like Martians. Very few make art that is so novel it could have come from Mars.

Take those infamous photos of Koons and Cicciolina. They somehow feel like entirely heartfelt celebrations of the newlyweds' love for each other, which is what Koons himself insists they are. (The couple have since gone through a breakup and custody battle that Koons views as one of his life's great tragedies.)

But the pictures are also cheap and nasty porn, as misogynistic as any, with anatomical details Hustler magazine would be proud of. It's as though Koons doesn't know what pornography is, and what its codes and problems are -- or simply refuses to acknowledge them. He just insists on forcing a pornographic photograph to do the work of a Hallmark card, and tough on us if we have a hard time going there with him. After all, why shouldn't slick, explicit images of sex, straight out of a pornographer's studio, also stand for love? -- even if they never have before, and probably won't do that job for anyone except the artist himself.

Koons comes off as a kind of art-world Don Quixote, unable or unwilling to deal with what the world around him, and the images in it, normally mean. In the photographs with Cicciolina, Koons keeps the look of a normal, hairy, naked guy, adrift in the flesh of his shaved and buffed and bronzed porn queen.

That peculiar, productive innocence is present, one way or another, in all of Koons' art. It's there even in the early works, where he buys everyday objects and presents them as sculpture. I think it's wrong to read those pieces as ironic or arch or cynical, in the manner of Marcel Duchamp or even of pop artists such as Andy Warhol and Claes Oldenburg.

Koons doesn't knowingly break down the borders between art and non-art, high and low. Instead, Koons genuinely comes at his objects with fresh eyes, and sees them as worth putting on a pedestal.

"Through my work, I tell people to ... embrace who they are," Koons says. "I have embraced my past and I appreciate the beauty in it." And that includes the "beauty" and pleasure to be found in consumerism and consumer products.

Koons honestly admires basketballs and fish tanks, even if he refuses to be limited by what they normally are used for; he refuses to segregate them into separate categories. He doesn't see why he shouldn't use his artistic license to bring them together -- in order, he claims, to talk about our consumer society's pursuit of perfection. (The floating balls, flawlessly suspended in the middle of the tanks, are supposed to stand for that perfection.)

Let's go back to Mars for a minute.

Imagine a kind of Martian cargo cult, where ordinary pictures and objects from Earth have reached that planet, and then been used -- or rather, misused -- as sacred art. So on Mars, pornography becomes a vehicle for romantic, almost Platonic sentiment. An ephemeral blow-up lobster seems so precious that it gets immaculately reproduced in everlasting cast aluminum. (Maybe the fact that pool toys need to float gets lost on a planet where all water is frozen.) And then there's the blow-up Mylar bunny that gets cast in mirror-finished stainless steel: As reworked by the Martian Koons, "Bunny" has lost all the good humor of the original toy and now feels like a totem for some sinister rabbit ritual.

The sheer labor and expense that goes into making each of these objects give a sense of the heartfelt importance attached to them. They have an almost votive quality, as though that labor has to serve a higher end than simply making functionless art. The famous "Balloon Dog," Koons' 12-foot-tall replica of a knotted balloon animal, feels as though it's meant to be worshipped, not just contemplated.

With Koons, it's as though we're seeing objects from our own everyday world transported to a distant place where they have been transformed and reused to vastly different ends, then brought back down to us again without a key to their repurposing, leaving us with no choice but to use them as art. No wonder this show can leave a viewer reeling.

Almost every object in it works like a Duchampian ready-made, but at many unearthly removes from its original function. It's as though Duchamp's urinal-become-fountain-become-sculpture were uncovered eons from now, and reused yet again to house a sacred relic. Then buried. Then re-rediscovered and presented as superb ancient art. The object's artistic aura might have been preserved, even increased, with time and its reuses, but its meanings would have become so layered and remote that they could never be deciphered.

It's said that art can take you outside yourself. Koons makes art that transports you 100 million miles.

Jeff Koons is at the Museum of Contemporary Art in Chicago through Sept. 21, and will not tour. Call 312-280-2660 or visit www.mcachicago.org. See more photos on Koons' work at washingtonpost.com/style.