He's hot on the unreliability of manuscripts, and on compositional whims. "A lot of Mozart's changes were for the worse. Chopin published simultaneously - often differently - in three cities, so there's no way of knowing what his final intentions were. In fact, the idea that a composer has a final intention is a bibliographers' myth. If I think a composer's last version is less good than an earlier one, I'll stick with the earlier."
One widely used Schumann edition he'd consign to the flames. "I won't buy it. The man in charge was responsible for impounding musical instruments from Jews in Paris in the Second World War, including Wanda Landowska's harpsichord. I'm glad to say it's a terrible edition." Coming from a man who looks and sounds like a New York truck driver, this carries some force, but it's a mere passing detail in a flow of words and music so unstoppable that it has to be guillotined by a reluctant chairman.
It later emerges that Rosen is in pain from an elbow that sometimes feels dislocated. But this just goes with the job. "Many of the finest pianists around are very obviously driving themselves to bear pain. They often find in later life that the last two fingers of the right hand suddenly stop working." He blames Chopin and Liszt, with their showy displays of staccato octaves. "But Chopin's sadism is more subtle and interesting, because the physical pain is usually associated with emotional violence."
This apercu is expanded in a substantial tome he has just published - The Romantic Generation, based on lectures he gave at Harvard, and undoubtedly destined to join his earlier book about Haydn, Mozart and Beethoven (The Classical Style) on the short shelf of key musicological works.
It kicks off in a manner that might land it in Pseuds' Corner, studded as it is with tail-chasing aphorisms. "A quotation is a memory made public... A memory becomes a fragment when it is felt as both alien and intimate, when we are aware that it is as much a sign of the present as of the past." But as Rosen develops his idea of a "fragment" - a prelude by Chopin, a bagatelle by Beethoven, or a tone-poem by Schumann - and links this to the architectural concept of the purpose-built ruin, you realise he's on to something.
When he goes on to show the parallels between Schubert's Winterreise and Romantic landscape painting - overlaying memory and sense-perception in exactly the same way - you stop and think some more. When he demonstrates that the ideology of the 20th-century avant-garde was fully developed before the end of the 18th, you see the point of all this history.
The core of the book is an analysis of what made Schumann and Chopin - the one with his pedalling and phrasing, the other with his sonorities - so revolutionary. The unexpected thing is the rock on which Rosen shows these breakthroughs to be founded: the music of JS Bach.
He's surprised that this should come as a surprise. "Berlioz was the only 19th- century composer who didn't like Bach, and he was the only one who didn't play the piano. Everyone else, from Beethoven onwards, learnt music by playing The Well-Tempered Clavier. As did Debussy, as did all of us until about 20 years ago. Bach has been central to everything."
One of Rosen's pleasures is puncturing myths, like that of Schumann the mad composer. "He was terrified of going mad from the age of 17 onwards, but his music exploited that fear. When he did go mad in reality, his music wasn't mad any more." Are Chopin's mazurkas full of folk tunes? "Folk rhythms, maybe. But the tunes owe more to Italian opera." Was Beethoven's mid-life fallow period caused by his fight with his sister-in-law over the custody of his nephew? "No. I think he invented that whole legal battle to take his mind off the much more terrifying fact that his creative juices had suddenly dried up."
Rosen has little time for "authenticity". "If you try to play things as they were expected to be played - and you can never really know what those expectations were - you get into terrible paradoxes. Every so often someone announces they're going to do the Eroica with the same small orchestra as was used at its first performance. But it was performed at the Kinski palace, as a compliment to the patron, in a room so small the audience had to sit outside. That sort of authenticity is nonsense."
How would Beethoven have viewed the modern concert grand? "It's irrelevant to speculate. The French have a saying - 'If my aunt had them, she'd be my uncle.' No, Beethoven would probably have been delighted by the modern piano, but he would have written different music. His music creates big problems of balance on a modern instrument."
Another myth Rosen joyfully skewers is that of the "beautiful sound". "The whole idea is an illusion, despite what piano teachers say. All you can do is play louder or softer, shorter or longer. There's nothing else you can do. You get a beautiful sound through the melodic curve, and by balancing your chords. It's always been so.
"And it's not to do with how you hold your hands. Horowitz played with his fingers absolutely flat, Rubinstein's were completely curved, Iturbi's wrists were three inches below the keyboard - and they all played beautifully. But how you sit does affect things like octaves. Glenn Gould played practically crouching on the floor, which made rapid octaves impossible."
If he were to write a similar book looking back on the 20th century, who would figure? "You mean, which undiscovered geniuses? Well, it's very simple. Nobody is going to survive who is not already famous. People are recognised in their lifetime. If you are any good, it's almost impossible not to be." I'm about to mention Schubert, but he gets there first. "The people who are supposed not to have been recognised in their lifetime actually were. There was never a Bach revival, because he never went away. When Schubert died, Schumann - hundreds of miles away in Leipzig - wept all night. He was recognised, at least by the people who mattered. Beethoven's reputation was settled world-wide by the time he was 35." Rosen regretfully acknowledges that no women composers figure in his history - the "fabulously talented" Clara Wieck being a casualty of social pressures. "But it would do the feminist cause no good to pretend that 19th-century women composers were better than they were."
Then he dispatches another myth. "Mozart may have hit a difficult season or two towards the end, but he was famous throughout Europe. He was exceptionally disliked, but always on the programme. Musicians didn't care what the critics said, they just wanted to play him. It's the same with Schoenberg. I don't think the general public is ever going to love him, but he will always draw musicians to his passionate defence. He'll continue to be played, whether the public likes it or not."
Which brings him to the bottom line. "We all want the public to like what we play, but we play because we love it ourselves. That's why we do it; it has to be this way. Otherwise, it's a really silly life."
n 'The Romantic Generation', accompanied by a CD on which Rosen illustrates his arguments, is published this week by HarperCollins at pounds 30. Rosen's recordings of Bach's 'Goldberg Variations' and Schumann's 'Carnaval' and 'Davidsbundler' are available on SonyReuse content