A few weeks ago, I wrote an article about Pioneer 10, the 35-year-old space probe that is now heading out across the great empty spaces between our Solar System and the red star Aldebaran, the eye of Taurus, 68 light years away.

I mentioned that, as is the case with all earth's spaceships that travel beyond the Solar System, Pioneer 10 carries a plaque bolted onto its main frame that was designed by the late Dr. Carl Sagan and his colleague, Dr. Frank Drake. This plaque contains some easily decipherable information about the earth and about us, just in case some alien form of intelligent life should discover Pioneer and be curious about where it came from. What I did not mention was this: Just in case our curious alien friend might have the means to play it, the scientists at NASA also popped in a recording of a Johann Sebastian Bach fugue, played by Glenn Gould.

Some very wise person (sadly, I can never remember who it was) said we humans are known in the stars for our poetry. In its choice of music for Pioneer's cargo, NASA seems to be signalling either that it hadn't heard this saying, or perhaps simply that it hadn't understood it. Philistines at NASA? That would be disappointing. It's more attractive to think that they were being subtle in this case, wanting to suggest by their choice of Glenn Gould that there might be ample poetry in his playing to acquaint other forms of life with the best of both our music and our poetry at the same time. In Gould's case, that would make great sense.

This year is the 20th since Glenn Gould died of a stroke early in October, 1982. It's hard to suggest he might have been the greatest pianist ever, because the record is so limited. But it seems fair to say this: No other pianist in living memory has had such a profound effect on the public's understanding of classical music.

The first recording he published during his life, of Bach's 'Aria with Different Variations' - known popularly as the 'Goldberg Variations' - released in 1956, was extraordinary. Those who knew the piece, heard it again for the first time, as they say. It was a door into the demanding world of classical music through which a multitude passed, who might never have taken an interest without hearing it. It is a fascinating fact that the last recording to be published during his lifetime, issued only a few days before his death in 1982, was also the 'Goldberg Variations' - a second interpretation very different from the first.

To mark the 20th anniversary of his death, the Sony Classical Legacy record label released, a few weeks ago, a two-disk CD of the two recordings, called 'A State of Wonder'. It also includes a 50-minute interview of Gould by Tim Page, who won a Pulitzer prize for his writing about music in the Washington Post. This is the must-have Christmas stocking-stuffer of the year, maybe even of the decade.

A comparison of the two recordings is just riveting. A life's-worth of experience came between them. Gould was 22 when Columbia Records made the first recording. He was nearly 50 when he made the second. I can't resist using lines from a TS Eliot verse quoted in the liner notes of this CD - from 'Little Giddings', the fourth of the 'Four Quartets':

We shall not cease from exploration

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time.

It's a smart quote, not only because it is apt on its face, but also because of the echo. 'Four Quartets' comprises four poems, each written during a different time in the poet's life, each containing different philosophies and views of existence. Like 'A State of Wonder', the feeling it conveys is one of rare intensity and intellectuality. Like 'A State of Wonder', it is a biographical account of one of the great minds of the 20th century.

Glenn Gould was born in Toronto, in Canada, in 1932. He was an eccentric who did not like to leave his apartment on St Clair Avenue. When he did, whether it was winter or summer, he bundled himself up in sweaters, cap, scarf, gloves and overcoat.

He sang Mahler Lieder to giraffes at the Metro Zoo. He drove his boat in circles on the lake near his cottage to scare fish away from fishermen. He loved Holiday Inns, shopping malls, radio, and TV. He disliked, and often refused to perform in public. He liked womb-like recording studios. He called his friends and acquaintances in the middle of the night for talk marathons they thought would never end.

The dislike of public performances, and his preference for recording studios was more than simply a visceral reaction - he believed that modern recording techniques made it possible to deliver a far better performance to an audience than was possible in a concert hall. He wrote a long and complex radio script for the Canadian Broadcasting Corporation, called The Prospects of Recording, that he later turned into an article and published in High Fidelity in 1966. Anyone interested in the effect of technology on music would be fascinated. It is available on the Internet, as is a wealth of other material by and about him at a variety of locations, but I'd mention particularly an excellent National Library of Canada site at http://www.gould.nlc-bnc.ca/.

Glenn Gould was also the most eccentric of performers. He sat low at the piano. He slumped. He sat on a chair with a back support. He made faces. He hummed and sang audibly when he played. When one hand was free, he used it to conduct himself. Perhaps to deal with these against-the-grain habits, he developed and perfected a 'pure finger' technique, which meant that he played with his fingers, with hardly any use of his hands, his arms or his trunk.

Gould was left-handed, and with him it was an acquired rather than an inherited trait. Although he used both his hands, the power and agility of his left was said to be special, to the extent that it is thought probable that the two sides of his brain had both developed creative and motor abilities. He disliked Mozart, at least in part because he felt Mozart the composer neglected and ignored opportunities for a pianist's left hand.

He had another talent that was extraordinary. Not only did he have excellent long-term memory, he also had a supreme haptic memory, or memory of touch. Gould did not read music from a score when he played. He used his mind to memorise it and to experiment with its interpretation and delivery. Before he ever sat down at a piano to rehearse, he had the notes in his head and sometimes rehearsed them, measure by measure, in his head. Finally, when he had it all worked out, he rehearsed it over and over at the piano, for the benefit of the memory of his fingers. His long memory was legendary. He was once asked on a Thursday evening to substitute for another pianist, in a performance the next morning of a Beethoven concerto with the Toronto Symphony Orchestra. It was a piece he had not touched for four years. To everyone's amazement, he played it flawlessly.

These gifts enabled him to produce a wide range of interpretations of a composer's work, varying from one exactly to the letter of the composer's wishes, to others that he modified with respect to rhythm, dynamics, form, etc. The result was always fascinating and sometimes, as with the 'Goldberg Variations', sublime.

Aldous Huxley once wrote that after silence, that which comes nearest to expressing the inexpressible is music. Gould seemed to know just what he was talking about. He was already moving from music towards silence when it enveloped him in death. Bruno Monsaignon, a French film maker who made two series of films for television about Gould and who was the author of three books about him, wrote in 1983 that Gould was about to withdraw from the world entirely when he died.

"As with Bach's 'Art of the Fugue', however, it is above all an aura of withdrawal which pervades the ultimate production of Gould. In his quest for anonymity, which according to him every artist should pursue, he 'was in fact withdrawing from the pragmatic concerns of music-making into an idealised world of uncompromised invention'.

"Once he carried out a few more projects that were particularly close to his heart, Gould - only some of us know about that - was going to stop making records; after a few years of a glittering but enigmatic concert career, after a quarter of a century spent in the recording studio, after having produced innumerable radio programmes and published a huge amount of writings on a wide variety of subjects, Glenn Gould was about to withdraw into a world of silence, of total and ascetic solitude in which, cleansed of the coarse manifestations of incarnated sounds (which had anyway become unnecessary to his inner hearing), only the spirit hears."

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