Charles Rosen, Queen Elizabeth Hall

Asked last week to describe Charles Rosen, the Southbank Centre’s head of contemporary culture replied: ‘A god.’ And it was in that spirit that people packed the Purcell Room for this grizzled New Yorker’s pre-concert lecture.

For his pianism is only the start of it: as an intellectual provocateur, and author of seminal books on Classicism and Romanticism, he’s a more effective populariser of musicology than anyone else alive.

The figure who shuffled on stage had trouble walking, and looked all of his 84 years. Then he hung his walking-stick on the piano, warned us that his lecture would be ‘extremely technical’, and for the next ninety minutes – playing and talking non-stop – he was as good as his word: this was coruscating PhD stuff, and people loved it. They’d got his Cds, and read his books, and they wanted to hear his take on Romanticism. In a nutshell – and his divagations into Mozart and Beethoven were fascinating – his thesis was that the new-minted language of the Romantics was a revolution in itself, in its attitude to tonality and form.

When he shuffled on stage at the Queen Elizabeth Hall the next day, it seemed incredible that anyone so infirm would dare attempt the bravura flights of Chopin’s third sonata and fourth ballade, but he began with two manageable nocturnes. His sound was big, firm, and darkly pedalled, and it felt like a further illustration of his lecture; the Barcarolle, which followed, unfolded organically. Then came two late Mazurkas, and the C sharp minor Waltz. There were no evocations of vanished ballrooms here, no suggestions of chocolate-box fairy tales: this Chopin was at once diffident and rigorous, and about structure, not surface.

But in the ballade and the gigantic third sonata - which can test the most brilliant virtuosi - we saw the results of the inevitable ravages of age. Rosen made it to the end of the ballade, but was struck by a memory-lapse in the third movement of the sonata: he recovered, but it was painful to watch his hands briefly stray like lost souls over the keyboard. After two encores – a Lisztean joke, and an unassuming piece of springtime Chopin – we were left with memories, and a vertiginous historical thought: that this man’s teacher was taught by Liszt.