As we watch the Brian of 1966 playing with his daughters in his Beverly Hills garden, we hear the Brian of 1976 – at the nadir of his drugged-out retreat from reality – tunelessly croaking ‘Still I Dream Of It’ at the piano. The juxtaposition of the images of Brian in his prime with the sound of the man at his most regressed is painfully poignant. Most pathetic of all is the lyric: “Time for supper now /Day’s been hard and I’m so tired, I feel like eating now / Smell the kitchen now, hear the maid whistle a tune, my thoughts are fleeting now…”

Yet this song is precisely about the way Brian is still somehow connected to a redemptive realm of beauty. “Still I dream of it“, he sings with a pang of desperation, “and it haunts me so /Like a dream that’s somehow linked to all the stars above…” It’s as if he is pining for the lost muse that inspired him to write the sixties masterpieces he described as “symphonies to God” – songs like ‘Surf’s Up’, ‘God Only Knows’, and ‘Don’t Talk, Put Your Head On My Shoulder’ … not forgetting ‘I Just Wasn’t Made For These Times’.

For Don Was, who has included the ‘Still I Dream Of It’ demo on the soundtrack album released to coincide with I Just Wasn’t Made For These Times, the song’s pathos is irresistible. As a life member of The Brian Wilson Appreciation Society, he is fully committed to the legend of the man’s genius. He knows, too, that pop is almost as haunted by the achievement of Brian Wilson as Brian is haunted by “it” – by whatever it was precisely that his genius allowed him to apprehend or intuit at the height of his melodic powers.

The phrase “Brian Wilson Is A Genius” was first magicked up one afternoon in the summer of 1966 by Derek Taylor, the former Beatles press officer who’d moved to Los Angeles in 1965. Pet Sounds had just been released, and everyone knew that Brian had crossed some invisible line between pop ephemerality and genuine musical brilliance. Taylor was simply canny enough to make it official, and to validate pop music in the process. Within days, “Brian Wilson is A Genius” was a buzz phrase in Swinging London, where the Beatles received Pet Sounds with something akin to awe.

Hard-core Beach Boys fans will know that Pet Sounds failed to reach the top ten in America, and that Brian never managed to complete Smile, its intended follow-up. (A three-CD box set of Smile tracks and fragments, The Smile Era, is due for release this autumn). Smile was to have been Wilson’s ultimate “symphony to God”, but a heady combination of lysergic acid and inherent mental instability led to his virtual breakdown, and eventually to a total retreat from the music industry. This breakdown – the failed promise of Smile, that Holy Grail of pop – is central to the obsession many people have with his lost greatness.

“Genius” is actually a rare commodity in pop music; it’s not a word bandied about idly. We don’t call Hendrix a genius, or even Dylan. Genius has less to do with rock heroes than with pop solipsists, mavericks who invent their own sonic worlds to live in. Pop geniuses, we feel, are baffling talents who could have lived in any era. It is remarkable how many of them, in our minds, are hunched over keyboards rather than letting rip on electric guitars. The image of the adolescent Brian Wilson, pouring out his feelings alone at the upright in his bedroom, remains a potent one.

Wilson provides the link between pop and the undisputed greatness of Tin Pan Alley songwriters like George Gershwin, whom he idolised. The miracle of Brian’s melodic gifts, like those of Lennon and McCartney, even prompts comparisons with Mozart and Schubert. As Tom Petty puts it in the documentary, “I don’t know if he’s a genius or not, but I know that that music is probably as good a music as you can make … as you can write.” The particular appeal of his genius lies in the fact that the Beach Boys were the very obverse of hip – the unlikeliness of these songs growing out of disposable surf pop – and in the singular naivety and ingenuousness of his personality.

If there was a pop genius before Wilson, it was Phil Spector, another of the Beach Boys’ heroes (though note that Ray Charles had been referred to – on several of his own album titles! – as a genius.) Spector had the necessary megalomania and flamboyance to imprint himself on his productions, and to envision pop as something heroic and Wagnerian. The failure of his mightiest creation, Ike and Tina Turner’s ‘River Deep Mountain High’, resulted in a retreat from reality similar to Brian’s. It is ironic, given that he was barely capable of writing a song, that Spector is lauded as a genius more often than even John Lennon or Paul McCartney.

When Julian Cope compiled an album called The Godlike Genius of Scott Walker in 1981, he was investing heavily in the enigma of the teen heart-throb turned existentialist recluse. Pop stars weren’t meant to sing Jacques Brel songs or seek inspiration in Ingmar Bergman films. As with Brian Wilson, there was something kitsch about the doomed introspection that lay behind Walker’s appeal, but his subsequent stretches of silence, broken only by music of impenetrable spookiness, have confirmed his status as pop’s premier cult hero.

The figure of the doomed troubadour is particularly susceptible to having the mantle of Genius laid over his fevered brow. Singers as different as Nick Drake, Tim Buckley, Arthur Lee and Alex Chilton – the first two dead, the latter pair all but spent forces – have been hailed as geniuses and become the objects of cult worship. In all four cases, the element of tragedy and failed promise is a crucial factor. Genius must be accompanied by torment, we feel, or the very least by major eccentricity. In the realm of black music, both Stevie Wonder and Prince meet at least two of the requirements of genius: they’re both mad and they’re both multi-instrumentalists. As for female stars, perhaps only Kate Bush qualifies as a genius in rock’s obstacle race. Once again, it is significant that one thinks of her first and foremost as a composer at the keyboard.

Casualties of the criteria sketched above include the many composers (Burt Bacharach, for example) who have worked as part of a team: the essential solitariness of a writer like Jimmy Webb seems more alluring to pop theorists and dreamers. True, Brian Wilson worked with several lyricists: his chief accomplice on the Smile songs was that other cult “genius” Van Dyke Parks, whose playfully cryptic couplets so unsettled the other Beach Boys that they effectively put an end to the partnership. But Wilson was always so lost inside his own musical universe that the word “collaboration” seems inaccurate. “I know that I’m not a genius, and it’s a great embarrassment to me to be thought of as one,” says Parks, whose own new album Orange Crate Art features vocals by Brian Wilson. “I’m not a natural, unlike Brian. He sits down and plays something in the same key in which I played it for him thirty years ago. This guy is absolutely brilliant. There’s not enough I can say about his abilities.”

In Thomas Mann’s novel Doctor Faustus, musical genius is a pact with mental torment. In the story of Brian Wilson, it is almost as if the man’s supernatural talents were the direct result of the damage he suffered as a child. Let’s leave the last word to Tony Asher, another of Wilson’s lyricists. “Brian Wilson is a genius musician,” Asher pronounced tersely after working on Pet Sounds, “but he is an amateur human being.”