When the Tate Modern opens in May, will visitors like the radical grouping of art into 'Nude' or 'Object', or simply get lost looking for the masterpieces? asks Karl Sabbagh. Right, the academics' views.

On January 6, The End of the Twentieth Century, an artwork by the German expressionist Joseph Beuys, was the first piece to be moved into Tate Modern, the huge new modern art gallery on Bankside in London. It is actually 31 pieces, large chunks of basalt, each weighing a ton or more. It was completed six months later than planned, and initiated a frenetic process of art installation in 65 rooms that will have to be completed by May in time for the Queen to open the museum.

But one of the first reactions of academics when Tate Modern opens in six weeks is likely to be surprise. The museum's staff have come up with a way of installing artworks that is both ground-breaking and controversial. You might expect Beuys's piece, constructed towards the end of the 20th century, to be installed somewhere near the end of a journey that begins with, say, the impressionists. But, in fact, the work is in a suite called Landscape/Matter/Environment, one of four suites that between them are intended to cover the span of 20th century artists. The other three are called Still Life/Object/Real Life, Nude/Action/Body and History/Memory/Society. The decision to arrange the Tate Modern collection in this way was taken after a year-long brain-storming process in 1998 and 1999.

We are used to taxonomies in science - the "cutting at the joints" that divides plants, molecules or insects into distinct classes. The best taxonomies reflect something about the origins of the objects and about real (rather than observed) relationships between them. In linguistics, too, although languages are a cognitive rather than a biological creation, they can be classified in a system so that connections can be inferred between them. But the further away you get from an objective relationship between one element and another, the more difficult it is to devise a "correct" taxonomy, and, in fact, the easier it is to devise spurious ones.

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When the Tate came to devise a fresh way of organising its art in the new gallery, it was faced with a hugely diverse range of "objects" - the products of modern artists - and one often-used principle the Tate wanted to avoid. Nicholas Serota, director of the Tate galleries, describes the traditional approach employed by many museums: "You walk in, turn left and, in the case of a 20th-century museum, you find yourself in 1900. And you walk through the galleries in an enfilade basis, you come out in the year 2000, and you exit. We're not going to produce that kind of a tunnel."

But why not? "I think it's essential," Serota says, "for the credibility and future of the museum that there should be an intellectual underpinning to what we are doing. Because even if a casual visitor who has simply come to look at the building doesn't necessarily take away all the messages that we are trying to convey, something will stick, a germ that sits in their mind and generates a seed."

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When the Tate team set about trying to come up with a plan, a small room at the old gallery at Millbank was kept off limits to other staff, as Frances Morris and Iwona Blazwick, two senior curators, covered the walls with flip charts and Post-It notes setting out ideas. Morris is a small, short-haired woman in her 30s, and her questioning way of speaking conceals firm opinions and a comprehensive knowledge of 20th-century European art. The "writing on the wall" was an amazing display of erudition (along with a few spelling mistakes - "Zietgiest", for example) and displayed attempts to categorise the world of modern art in a dozen different ways.

Headings included "Strategies of making", "Geographies", "Zeitgeist of ideas" and "Decades and defining moments". The hope was that one of these systems would provide an organising principle for the entire collection. Over weeks the curators invited colleagues to their small room and sought their help in fitting 20th-century artists and artworks into one or other of the schemes.

One of the headings on the wall, the one eventually adopted, was "Genres of art". Morris explains what this meant: "We began by thinking of the traditional 19th-century genres in art - portraiture and the nude, landscape, still life and narrative history painting - and how they have moved in the 20th century to a multiplicity of subjects and themes arising from them. Then we made connections between the 20th-century subjects and the earlier genres, and extrapolated subjects around the body, subjects around the environment, subjects around the everyday, and subjects around society, as broad areas of interest that can be traced back to the earlier genres. Thus there is an art-historical thread behind all these four sections, and yet the sections themselves deal with subjects that exist in art and life."

Morris sees each of the new Tate's suites as telling a story about modern art. "The first story," she says, "is a number of complicated stories about what happens to the human body in art and society. It has to do with the human figure, and the human mind, and with the present body and the absent body. Then there is a second story about objects in the real world. That comes from still life and deals with the domestic realm and how we interact with vessels. It deals with the private and the public, with consumerism, and with the object in industry over the 20th century.

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"There's a third story, which is to do with the wider environment, the landscape we see, but also the landscape that we threaten, that we dig up, that endangers us. And there is a fourth story that we'd like to tell, to do with history and politics and ideology and with gender, and with issues surrounding race and class, and third world/first world. Each suite addresses one of these stories."

The "stories" were tested by seeing whether the Tate's collection could sit comfortably in the four suites. It was a bit like a jigsaw puzzle where you try to avoid having any pieces left over or any holes in the jigsaw. In fact, because more than 80 per cent of the Tate's collection was in storage and only 15 per cent on display, it was not too difficult to find enough works to cover the four themes, although some gaps still had to be filled.

Right up to the end of last year, the curatorial team had still not made final decisions about some of the spaces.

Some of the delay came because their system of classification meant that some works could fit into more than one suite. There was also the problem that the Tate was relaunching its gallery on Millbank as Tate Britain, and the works of some British artists could fit in either Tate Britain or Tate Modern. "Where does Francis Bacon fit in during the next year?" Morris asks, "Who should premiere Spencer? We've had a lot of discussion."

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One has to admire both the desire to break the mould and the talent that has gone into applying the new system of classification. But Serota sees this as not before time. He feels there is already a movement in this direction by other museums and he wants to be first. At the time the "Genres of art" idea first came up, he warned his colleagues: "We're beginning to get to an area of difficulty in terms of how we speak about all this, both within and outside the gallery, because we're getting into an area of intellectual property. There are a lot of other museums that are thinking about this question at the moment. We want to make sure that the first place it happens is the Tate. We don't want to see it four months earlier at the Pompidou Centre."

The risk is that when the museum opens on May 12, visitors used to the comfortable old chronological method will be confused and disoriented. Many people - known as "masterpiece hunters" to museum staff - go to museums to see one particular work. How will they fare in Tate Modern? Well, it could introduce an element of guesswork and fun into the proceedings. Matisse's Snail was one of the most popular works at Millbank. Now that it is moved to Bankside, where might you start looking for it? In the landscape suite, perhaps? And sure enough, that is where it is.

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Karl Sabbagh is the author of Power into Art (Allen Lane), published on April 6, about the six years of design and construction of Tate Modern. He has also produced a four-part series about the project, to be shown on Channel 4 on Saturday nights, from April 1.

Art books, pages 30-31