The radio discussion would explore, the producer said, how the medium of transmission alters the message transmitted. Take a controversial subject: contrast poetic and journalistic responses. Consider how art might react to daily news. And the performance poem: confine it to the judgment of the silent page. My task, he said, was to provide a conventional literary critical appraisal of Levi Tafari's performance piece 'Plastic Fantastic'. The issues raised would be discussed with a schoolteacher-novelist and a poet who is also a radio and television producer.

'Plastic Fantastic' is RAP. RAP, the author said, is an acronym for Rhythmically Articulated Poetry. Surely, then, all poetry apart from visual and pre-stress concrete can be described as RAP? No: the category is not open in that way, there is a further discriminating element. RAP works primarily aloud, often with music. RAP is performed, and the voice of the author, along with the reactions of the audience, sometimes as chorus, sometimes as laughter and applause, are part of an experience which differs with each audience, each performance. If RAP aspires to be text on a page (the author is assembling a print anthology to be called RAP in Paper) does it live as poetry, or is it largely a prompt sheet? If RAP excludes other kinds of poetry, does poetry include RAP? Just as it includes ballads and some song lyrics, so it includes some RAP; the RAP it excludes, however, may be good as RAP but not as poetry.

Having been asked to appraise the written text of 'Plastic Fantastic', I indicated elements in it which made it clear that a textual approach was irrelevant to this kind of language. The syntax was short, static, uninventive. In prosodic terms the performer's voice might impose regularity of accent, but cold-hearted scansion showed norms violated in every line, to no particular effect. The satirical counters were vague; verbal and aural cliches abounded. It was familiar, off the peg language, and thematically one-size-fits-all. This harsh seeming appraisal, I said, was inappropriate for 'Plastic Fantastic' which had a different kind of existence.

'Thank you, Professor Schmidt,' said the presenter with corrective irony, as though I had perpetrated an act of gross discourtesy in providing what I had been asked to provide. Because I was invited to apply inappropriate terms to a work (like appraising a Calder mobile according to aerodynamic principles, or a formaldehyde sheep by the slaughterhouse rule book), even though I indicated their inappropriateness, I represented the high culture which is (a journalistic axiom) inimical to the popular, the multi-cultural, etc. The schoolteacher-novelist announced that members of one of her A-Level groups reading Hopkins's poetry preferred 'Plastic Fantastic' to the dead Jesuit's work. (She had previously commented I now understood why on how badly poetry was generally taught in schools.)

I recount this event because it underlines how vexed communication can become when categories are blurred. A work from one genre can affect work in another genre without being required to 'cross over' or undefine itself. Blurring does occur, however, and it takes two principal forms. A dead sheep in formaldehyde can occupy space beside 'The Burghers of Calais'; but can it usefully demand to be 'read' as in some way existing within the same plastic forms and conventions as 'The Burghers'? 'The Burghers' can occupy a space beside a great Aztec sculpture, product of a rigidly formal, ceremonial culture; but, again, can it usefully demand to be 'read' as existing within the same forms and conventions? Even so, debates within the visual arts do not appear to the outsider to be quite so vehemently politicised as debates within the literary arts, where ideas of dominance and emancipation frame much academic and popular discourse.

Levi Tafari performs 'Plastic Fantastic' to microphone and we, his studio audience, are required to provide the chorus. It certainly sounds differently from how it read on the page, even when one voiced it aloud in private. His rich voice imposes a rhythmic norm. He over-rides syntactical units or makes a staccato of their brevity. He performs from memory, with variations on the text here and there (it is not quite stable).

Assuming we understood the metrical pattern and the syntax, you and I would read a poem by Tony Harrison, or by Hopkins, in roughly the same way: the formal properties of the original are sufficiently strong to communicate themselves through our voices, however differently we speak. If you and I and the author were to record 'Plastic Fantastic', it would sound and be quite different not the difference between three interpretations of a play script, but something deeper. The work exists in and for this RAP artist's own voice. It is non-transferable, the ultimate in copyright protection. Not the right words in the right order, but the right voice imposing order on the words. This is a vigorous and fascinating genre, self-righting, self-affirming, property of a particular self.

We come back to the issue of RAP on paper, its value as poetry. Why does it aspire to be valued as poetry traditionally is, anyway? Why does it seek validation as text? There is tape, there is the CD at the service of its radical invention of voice and audience. There is no sense in which RAP has been marginalised within contemporary British culture, despite the rhetoric of exclusion: it is infinitely more popular than print poetry, infinitely more profitable, in its fruitful marriage with music. Yet it desires a kind of cultural authentication which is irrelevant to it, and if that authentication cannot be unpatronisingly provided, the rhetoric invariably becomes political.

There is much to be learned from RAP, and RAP has learned, or could learn, much from conventional poetry from Hopkins, Kipling, Belloc, Cummings, MacNeice Hughes, Harrison, just as Duffy, Armitage and others have learned from RAP. There are RAP works which stand the test of paper and are anthologised, just as the intrinsic power of certain song lyrics and ballads makes them central to the tradition. Such poems earn their place on the page: they are there not by virtue of cultural accommodation; editors selecting them have not been patronising a new form.

Multiculturalism is not a one-way street. What most imperils it is the blurring of definitions and a dilution or abdication of judgement in order to accommodate, often within the wrong categories, work which belongs in new or other categories. This kind of assimilative tolerance becomes a form of repression, applying familiar templates to what is unfamiliar, appropriating and neutralising the new. Multiculturalism should provide us with a variety of compasses; the danger is that, politicised, it will provide no compass at all. The exercise of reading 'Plastic Fantastic' as a text was, I thought, to demonstrate that it was not a text in the traditional sense. The exercise was used to discredit textual reading tout court, and by extension to devalue the kinds of work that respond to it.