This is Jörg Heiser on Tino Sehgal

Over the last two years I have encountered three pieces by Sehgal: someone crawling along the floor like a broken robot in Frankfurt; a singing guard at a motley show in Venice; and two kids performing what seemed to be a cycle of instruction pieces in an empty art fair booth in London. Yet a discussion of Sehgal's oeuvre based only on the experience of these works would have felt more like rumour-mongering than art criticism - hence my Grand Tour.

Sehgal studied dance and economics in Essen and Berlin and was interested in performance and conceptual work, but couldn't, as he put it, 'dance his way into the art academy'. His work resides in the time and space it occupies; in the bodies and voices of the performers; in memory and its reception. It does not, he is adamant, inhabit photographs, videos, labels on the wall or even sales contracts - which explains why there are no images with this text. I'm still puzzled that I have been persuaded to accept what might be seen as purist iconoclasm, a footnote to the history of the dematerialization of the art object, as something so different, so appealing.

The scene is the Musée des Beaux-Arts in Nantes, a massive building with polished floors and grand rooms full of paintings of wildly different provenance. In an empty gallery a couple on the floor are kissing, but not passionately, their mouths meeting in the style of a 1930s' film. They wear casual clothes and move slowly, like a pair of sloths practising sexual positions. At one point the man lies beneath the woman, his hands on her buttocks, and she turns her head towards a non-existent camera, recalling Jeff Koons and La Cicciolina in Manet Soft (1991) from the 'Made in Heaven' series. Yet the Koons piece was based less on Manet than on a typical porn pose. Maybe Sehgal's piece needs this playfully vulgar refererence to make you recognize the more subtle art-historic ones - Auguste Rodin's Kiss of 1886 or Constantin Brancusi's Kiss of 1908. The point seems to be they are exhibits in a museum of remembered positions.

The two performers go slowly through their motions as if on a loop. Shortly after someone enters the room the woman exclaims, 'Tino Sehgal', the man says, 'Kiss', and she replies 'deux-milles quatre' (two thousand and four). With the next visitor they reverse the order of who says what. After about an hour another couple arrives and begins mimicking the movements of the first. They move in sync for a minute or so, until the first two performers walk away, leaving their replacements to continue. The piece is conceived to run for the duration of an exhibition - in this case for six weeks, six days a week, eight hours a day. During regular museum hours you came across the piece in the same way you might discover any artwork. It could be described as a 'living sculpture', like Gilbert & George performing the old music-hall number 'Underneath the Arches' for eight hours for their Singing Sculpture (1970).

For Manifesta 4 Sehgal used performers who seemed equally self-absorbed: again, in an otherwise empty gallery, a person on the floor moved like a hydraulic android stuck in a corner. The movements were based on two famous pieces of filmed performance: Bruce Nauman's video Wall-Floor Positions (1968) - the artist probing a set of body positions between wall and floor; and Dan Graham's Roll (1970) - a double film projection, one of which shows the artist rolling through woodland holding a Super-8 camera, while the other consists of the shaky footage actually shot on it. In Sehgal's piece Nauman's and Graham's movements are literally rolled into one, transforming the performers into uncanny puppets. The title Instead of allowing some thing to rise up to your face dancing bruce and dan and other things (2000) brings into play a third piece, Nauman's Elke Allowing the Floor to Rise Up over Her, Face Up (1973), an exercise in endurance: a performer lies still for 40 minutes while, in an act of auto-suggestion, she tries to sink into the floor.

There is a decisive difference between the Manifesta piece and the one shown in Nantes: although they both 'store' historically defined movements in the bodies of the performers, the latter was not accompanied by any label on the wall. The performers announcing the title and date of the piece function like a snake biting its own tail, an integral part of the piece's execution. This became hilariously clear in the recent group show 'Ailleurs, ici' (Elsewhere, Here), organized by the Musée d'Art Moderne de la Ville de Paris. As you entered this off-site exhibition, three guards jumped up from their chairs and hopped around in loose circles, raising their arms and proclaiming 'This is so contemporary, contemporary, contemporary!' One then uttered, 'Tino Sehgal', then all in unison 'This is so contemporary', another '2003', and the third 'courtesy Galerie Mot!', with the last syllable screeched at a ridiculously high pitch, before all of them returned to their seats.

The artist allows the guards some interpretative leeway - they performed slightly differently each time, as if to keep themselves entertained. It felt at times as if they were referring to their own actions as being 'so contemporary', at others as if they were mocking the show, which was spatially dominated by architect Didier Fiuza Faustino's futuristic construction of gold and silver sheets suspended in mid-air. The visitors' reactions ranged from embarrassed amusement to even more embarrassed attempts to ignore what was going on. Either way, people lingered, like nosey parkers transformed from victims into culprits in an instant as they watch others enter.

Sehgal is not the first artist to have used exhibition staff as material - Andrea Fraser and Christian Philipp Müller, for example, have done it to great effect. Yet the clown-like piece in Paris expanded Sehgal's scope, almost calculatedly, from intellectual gravitas to lightweight Pop appeal - a great quality. Beyond the uninteresting prospect of arguments about whether Sehgal's work is superficial or profound, it shows how, as in all good jokes, reality here is stuffed into a tumble dryer and made to revolve around itself. His pieces don't just hoax their way into the institution but establish their own 'institution' along the way, as the performers' status is determined by their own speech - or, as at the Venice Biennale last summer, their singing.

The curatorial concept of the Biennale's 'Utopia Station' section resembled a festival - as if it were less about the show than the gathering. But Sehgal's guard, who strolled around elegantly singing 'This is propaganda' and then proclaiming 'Tino Sehgal, this is propaganda, 2002, courtesy Jan Mot gallery', made you feel as if you had tuned into a short-wave receiver and had stumbled across Johnny Rotten singing 'This Is Not a Love Song', a tune that simultaneously establishes and questions its own status - what kind of propaganda simply states that it's propaganda?

The function of speech in this mind-boggling game of self-reference can be traced back to J.L. Austin's book How To Do Things With Words (1954). Austin distinguished what he calls speech acts that simply say something (constative) from speech acts that do something (performative) - i.e., they accomplish what they say. Thierry de Duve, in his Kant after Duchamp (1996), applies this theory neatly to modern art after Marcel Duchamp: the performative speech act 'this is beautiful' is replaced with 'this is art': 'To say of a snow shovel that it is beautiful (or ugly) doesn't turn it into art.'1 But by simply saying 'this is art' Duchamp famously did the trick.

Judith Butler, in Bodies that Matter (1993), implemented the theory of the performative speech act into her inquiry into the determination of the physical body as 'sexed'. Now even 'mere' descriptions of bodies as being, for example, 'feminine', lose their innocence. In other words, it's not only performative speech acts that do something but also constative ones, as they repeat conventional descriptions and so confirm - or subvert - them. The constative and the performative become intertwined in the endless attempt to determine sex, gender, race; to fix the distinction between what is sealed by fate and what is subject to change.

Sehgal takes these three things - the theory of the performative speech act, its application to the art object and to the body - and throws them together. He commits the sin of applying theory directly to art production - but succeeds because he sticks slavishly to it. 'This is' is the phrase that de Duve awarded so much determining power, and as it is uttered not by a label on the wall but by the performers literally calling the work into being, it is as if they become 'possessed' by art.

In Brussels I thought I knew what to expect: Jan Mot would 'restage' the exhibition at his gallery from a year ago. The empty white cube behind a shop window recalled Yves Klein's Le Vide (The Void, 1958), yet the show's title, 'Le Plein' (Fullness), was in turn pinched from Arman's reaction of 1960. Arman crammed bulky refuse into the same space, so I presumed the performance too would 'fill up' once I entered.

Jan Mot walked into the gallery backwards from his office; he began talking in a sober, self-absorbed way, but it wasn't clear whether he was talking about Sehgal's pieces or actually was one of them. Every time I tried to face Mot he would turn away, continuing to talk, throwing his arms and legs around in circles, and saying, 'Tino Sehgal, This is good, 2001, courtesy the artist'. After a while he turned around, said 'this is a piece by Tino Sehgal entitled this is exchange', shook my hand and invited me to converse with him for five minutes about the notion of market economy.

I rightly suspected this to be a rendition of a Sehgal piece. (It involves visitors being lured into talking to a museum guard about economy - if they do so, they get half of their entrance money reimbursed.) But that was not the end of it. If anyone had entered the gallery at that point, Mot would have been obliged to go back into the office and repeat his awkward entrance. Thankfully no one did, so his gallery assistant entered instead. They embraced, kissed each other and slowly sank to the floor - it was Tino Sehgal, Kiss, 2004, shown for the first time two days earlier, but 'documented' a year before. This could have gone on for ever, and in fact the piece does not stop until the visitor leaves (poor Mot). Its 'fullness' is that of Duchamp's boîte en valise (1936-41) cramming an entire oeuvre into a neat briefcase of meta-art.

Sehgal introduced the referential mode with (Untitled), a 50-minute dance performance of 2001. Naked, Sehgal performs styles of 20th-century dance, from Isadora Duncan through the Ballets Russes, Merce Cunningham, Yvonne Rainer and others. But the point is neither that you recognize all the references nor - though Sehgal is a trained dancer - that you should expect a perfect rendition. Rather, the piece is like a ruthlessly rearranged medley of tunes. Sehgal's body is transformed into a boîte en valise, memorizing a museum of dance.

When Sehgal was invited by The Wrong Gallery (itself a meta-venue) to participate in last year's Frieze Art Fair, he took the basics of a fair - objects sold in booths - and twisted them like a showman turning a balloon into a puppy. As visitors entered the empty booth, two kids acquainted them with Sehgal's oeuvre, in much the same way Mot did with 'Le Plein'. At one point, they 'documented' the piece Tino Sehgal, This is about, 2003, courtesy of Galerie Jan Mot, which involves asking people attending a tour of a private collection in zombie-like voices: 'so what do you think this is about?' Uttered by kids in front of an art crowd, it recalled Linda Blair in The Exorcist (1974) saying, 'your mother sucks cocks in hell'. The evil spirit of the art market was almost tangible.

Sehgal is interested in dance and song as modes of production because they resist transforming ideas into goods: this is work that leads to no product outside itself. In this way it foregoes the ideology that has dominated market theory throughout the 20th century: that of eternal economic growth. Using a deadpan, literal approach, Sehgal spelt this out in a recent theatre production - (Untitled) (1997-2003) - for the Hebbel Theater Berlin: in between the dance ensemble table-dancing to Beyoncé's 'Bootylicious' (2001) Sehgal gave an ultra-dry lecture on market theory, including growth-rate diagrams.

Sehgal's work is driven by one question: are there ways to create something while circumventing the usual cycles of production and consumption? A question as simple as it is impossible to answer, it goes to the heart of 'political art'. The market happily gulps down any discontent, turning it into yet another piquant, bite-sized offer. Frustration with such hungry indifference often leads to escapism, cynicism, or nihilism. Sehgal resorts to none of these. He returns to moving, dancing, talking, singing. And yet his work is less about pious dematerialization than conceptual closure. There are no labels on the wall because there is beauty in an act that, like an orgasm, establishes and erases itself in the same instant. It's the notion of beauty that mathematicians have in mind when they see a simple formula that aptly describes complexity, without the help of brackets.