You young kids with your so-called "violent" lyrics. I bet you've never shot a man in Reno, just to watch him die.

Generally speaking, music-making is a pretty egalitarian business. If you've created a song, it doesn't matter much who actually performs it, so long as they're talented enough. Content isn't much of a barrier - Bob Dylan takes on the persona of a middle-aged miner's wife in "North Country Blues", for example, and because his lyrics are so strong no-one really bats an eyelid. Similarly, cover versions of songs tend to work because they don't "belong" to particular artists. Indeed, cover versions sometimes enter the public consciousness far more than the originals - it took me quite a while to realise that The Who weren't the original writers of "Summertime Blues", because they rock out so well when performing it.

Sometimes, though, particular artists put their stamp so very firmly onto a song that it's impossible to imagine anyone other than that artist performing it. Jeff Buckley's "Hallelujah" gets pretty near, having been played so much that it's almost entirely eclipsed the original Leonard Cohen version. An even better example, though - probably the epitome of this phenomenon - is Johnny Cash's cover version of Nine Inch Nails' "Hurt".

The album on which Cash's version of "Hurt" appears - American IV - The Man Comes Around - is one of the last he recorded, aged 70, before his death in 2003. Like the others in his American series, it's a fairly quiet album, full of reflective songs. It's a very simple, stripped-down sound, of the kind which can only possibly work if the voice of the person singing has enough character to carry it off. Well, Cash had that character when he was 25, and it only grew over the years. By the time of the American series recordings, although clearly the same deep, rich voice that had boomed out throughout his career, it was weaker, more vulnerable.

"Hurt" was never a happy song. The lyrics are, in fact, unremittingly bleak, containing a horribly detailed description of the song's protagonist mainlining heroin, a remembrance of a life full of lies and disappointments, crushing loneliness, and a feeling that life produces nothing of value (an "empire of dirt", as the chorus says). Indeed, Nine Inch Nails' frontman Trent Reznor wasn't too thrilled with the idea of allowing his song to be covered by the man who had built most of his songs around a "boom-chick-a-boom" rhythm, considering the idea of the cover "a bit gimmicky". However, the combination of Cash's elderly voice and some of the most depressing lyrics in musical history works incredibly well.

"Hurt" received extensive airplay when it was released in 2002, so I'm not about to describe it again here. I will say, though, that it is a fantastic rendition. Cash conveyed the loneliness of the lyrics perfectly, and sadness comes from every word without ever spilling into sentimentality. This, in itself, wouldn't make it indelibly a Cash song - there are several singers who can pull off the same kind of performance. The key is that Cash was a unique man, who had done things that no-one alive can conceivably claim to have done.

Consider, for a minute, the fact that Cash toured with Elvis in his younger days. He was right there at the birth of rock'n'roll. You think your Beatles collection is old? Cash's first album came out six years before Please Please Me. The Beatles released 24 albums and EPs; Cash released 83. All the bands that you know and love, except for those that began in the last six years, were founded during his recording career, and you can bet that they were influenced by him in some way. He had unmatched credibility among the American working class. Try to imagine almost any modern rap star performing in a prison without getting stabbed. Now go and listen to At Folsom Prison, with its background of clanks and rattles as the business of the prison goes on around the band, and listen to the raucous and enthusiastic reception that Cash gets.

That credibility was due, in large part, to Cash's imprisonment history. Despite the fact that he clearly milked it for all it was worth, it's still true that he was held by the cops at least seven times, and his addiction to painkillers and amphetamines is well known. Cash's image as a hellraiser was pretty accurate, and he remains the only man who was able to wear all black - just because he liked it - and not look like at least a bit of a poser.

In short, Cash's life meant that he was uniquely placed to sing "Hurt". Those years of drug abuse, of practically destroying his life, come through in every wobbly syllable of the song. When he protests that getting close to him will result in pain, it's not a vicious warning; he conveys a weary realisation, borne out by long experience.

If anyone else were to record "Hurt" now, not only could it never measure up, it would instantly be seen as a shameless attempt to cash in, and would be buried without trace. There is simply no-one alive with the experience or the character who could do justice to it. This means that there is only one man left alive who can sing the song at all, and that's Trent Reznor (who gets away with it solely because he wrote the damn thing). Even he considers Cash's version better, saying in an interview "I just lost my girlfriend, because that song isn't mine anymore."

Is that a bad thing? Should we treat all songs equally, regardless of prior performances? In this case, no. Cash achieved something extremely rare - not only did he make a cover version that improved on the original, he got so near to perfection with it that other attempts are doomed to fail. That is an incredible cap to an unsurpassed legacy, and I think it can be left there, a fitting epitaph to an amazing man.