We know a lot about Elvis Presley. But the most significant of the many things we don't know about him is how, when and why he came to believe that, as he vowed to himself when he was a skinny, poor, boy in the poorest state of the union, one day things would be different.

For an intellectual bigot like Albert Goldman, Elvis' success is a fluke. For Peter Guralnick, it was testament to his extraordinary hope, optimism and openness to the vibrant, varied cultures - musical, social and racial - that surrounded him.

One thing's for sure, Elvis meant to be famous. Millions of children dream of transforming their - and their parents' lives - with a clich�d rags to riches story. But his dream came true. And no matter how you explain that, it's clear that Elvis did his utmost to realise his dreams in ways that contradict the lazy stereotype of him as a shy, stupid, country boy tied to - or almost strangled by - his momma's apron strings.

He was, as Sam Phillips noted, "more afraid of being hurt than anyone I ever met". But he confronted that fear in remarkable ways. When he was eight, he hitched rides to the WELO radio station to see his idol Mississippi Slim and, through sheer childish persistence, played on the show. When he was ten, he sang Old Shep at the Mississippi-Alabama Fair, insisting "I'd set my heart on singing and nothing in the world could have stopped me."

In Memphis, the orthodox version of his discovery makes him sound like a passive accomplice. But even before strolling into Sun Studios in the summer of 1953, he performed (Johnny Burnette says) at the local fire station, may have played with Burnette's group, often sat in his truck pondering Sun Studios and behaved, not like the idiot savant of myth, but like a talented, ambitious, young man calculating how best to ensure his gifts were discovered.

He famously said in a speech in 1971 that every dream he ever dreamed came true a hundred times. But that fulfilment was marred by tragedy - the untimely death of Gladys, the end of his marriage and the emotional and physical cost, to himself, of living up to a title - The King - he regarded as sacrilegious.

But if Elvis Presley had never dreamed, we might never have heard of him. That, for me, is what makes his performance of If I Can Dream in the 1968 comeback special so revelatory. That song - right down to its vaguely expressed, yet sincere, yearnings for a better world - is his manifesto. Watching him rehearsing the song on DVD makes the final performance even more moving. He waits, obediently in his white suit, sounding as respectful as a very well-trained waiter. And then the song takes hold of him.

In those takes, we witness the transformation that electrified friends, acquaintances and musicians - the shift from Elvis the person to Elvis the star, a legendary figure of myth, mystery and magnitude. At the end, he spreads his arms out, in a pose vaguely reminiscent of the crucifixion and mumbles an unsure, apologetic, "thank you and goodnight".

Although he later often sang My Way, Paul Anka's song never feels, to me, as if Elvis believes it. He'll sing it as well as he can but the song's strange mix of fake humility and self-dramatising bombast isn't him. He bares his soul in If I Can Dream - it is his mini-autobiography. Nothing in My Way matches the inspiring grandeur of the line: "As long as a man has the strength to dream, he can redeem his soul". Listen to the passion and belief with which he nails that line and it's hard to believe he ever sang fluff like A Dog's Life.

Director Sidney Lumet once said that, watching Elvis, he felt in the presence of a restless spirit, doomed never to feel at home, tragically unaware of his own separation from the rest of the human race. Lumet was haunted by that impression - echoed by Priscilla who suggested that, like his dad, Elvis was prone to terrible bouts of loneliness when he seemed utterly adrift from humanity. Maybe he was unaware of that separation or maybe he dreamt that, once things were different, it might disappear. Either way, his dreams changed our lives and his. And If I Can Dream gets closer than anything to revealing how a poor country boy apparently destined for anonymous oblivion became Elvis Presley. That's why, for me, it is the one indispensable Elvis song.

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Also by Paul Simpson :

 The enigma of Suppose
 Elvis the Singer
 The Shock of Elvis
 Passion and Soul: A Little Less Conversation, a Lot More Fans
 The King of Pain!
 The Elvis Mystery
 Elvis: a star is torn by Paul Simpson

Paul Simpson is the author of The Rough Guide to Elvis.