Philip Hensher: A building to extend your mind

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The unveiling of the plans for the extension to Tate Modern had the air of a project assured of success from the start. Herzog and de Meuron's original work on the old power station, though much admired, has been criticised by architects as being an adaptation of an already great building, rather than a completely original project. Nobody could level that accusation against this proposed 11-storey glass pile - spectacular even in the plans.

Tate Modern has been one of the biggest public successes for decades, attracting 25 million visitors in six years. The extension, which adds 60 per cent gallery space, still comes nowhere near exhausting the majority of the collection in storage. There seems no doubt that the £165m needed will be raised from the Lottery and other sources - already, the Tate is the most splendid flagship for the nation's cultural life. If it opens, as planned, in 2012, the coincidence of that - the Queen's Diamond Jubilee celebrations and the London Olympics - will make for an unforgettable summer; one people will be writing books about in 100 years.

Exciting as the prospect is, this is a good moment for the Tate to start thinking hard about what it does. There are four Tate galleries. Tate Britain, on Millbank, collects British art - classic to the present day. Tate Modern collects classic modernism from across the world, though mostly Western art, and new art. Tate St Ives has a special connection to English modernism, especially that which emanated from St Ives. Tate Liverpool has a sort of evangelical role, seeming to borrow from the central collection and introducing what it can of modern art to the North-West.

There is an enormous amount of overlapping. The classic British collection in Millbank can sometimes be squeezed out by Gilbert and George, who are also showing in Bankside; or the historic collection of classic modernism in Bankside makes the work of interesting new artists fight for space. Some great artists - say Ben Nicholson - are eligible, in effect, for four galleries; others, still greater, like Constable, are only eligible for one.

Tate has lived with these tensions and has conducted an effective campaign for the heart of British cultural life around the problems imposed by its own structure. But, surely, the biggest tensions arise from the attempts to include the avant-garde and living artists alongside classic, historic collections. I doubt that the question was considered when Tate Modern was mooted - we were living in the 20th century. But increasingly, it seems to be doing two separate things with one budget: it is a museum of classic 20th-century art, from Cubism to post-modernism; and it tries to sample the zeitgeist with purchases from, and exhibits of, artists still making their way.

These tensions became painfully apparent with the Chris Ofili scandal, which ended with the gallery being castigated by the Charities Commission. Ofili, a first-rate painter, was a trustee, and the gallery found itself in deep water when it purchased his 12 Apostles.

The problem was, really, that a system designed to deal with the acquisition of classic art was being asked to take on the knottier one of acquiring the work of living artists. A museum of the avant-garde shouldn't be prevented from buying an Ofili - they were magnificent - but would find it necessary to have elaborate systems in place by which such purchases would prove accountable. It might, too, find it necessary to examine the range of sources from which it acquired artists' work, something which, it has been suggested, is far too narrow. Tate's critics say that it buys from the same few dealers, whose clients routinely turn into trustees of the gallery. I don't think that the world out there is as inexhaustibly full of undiscovered geniuses as this seems to suggest. But Tate's procedures ought to be transparent, accountable and capable of defending itself against these accusations.

All of which is to say that Tate Britain is going to have to become a museum of Dead British Artists, and Tate Modern a museum - probably the first in the world - of Twentieth Century Art. What would make this possible would be a new Tate, clearly understood to be a Museum of the Avant-Garde, which would collect art as it emerged, confidently entrusted with a buccaneering role and the right to get it occasionally wrong as talent-spotters. Where better for such a museum than this new extension and what better time than this moment of expansion? All it would need is a separate entrance and a different name.