'Chamber music has been part of the mother's milk of my education,' says the 74-year-old Isaac Stern, whose quizzical, mischievous smile frames profound wisdom and humility. Stern is an energetic, Old-Testament Peter Pan, forever jetting here and there, but with an inner repose that protects the youthful core of his artistry. The members of 'his' piano quartet, all of whom he has either known or collaborated with almost since they were children, plainly adore him - and the feelings are obviously mutual.
'We just listen to each other and we match. It's a kind of joyous exuberance, and I think that comes across. And we can also be deeply introspective in our music-making, because none of us has at any time worried about the strength of music being more important than we are.' That's very much a shared philosophy, although taken individually, each of the other quartet members has his own very marked personality.
The Polish-born pianist Emanuel Ax sees himself as keeping everyone's nose hard to the grindstone. 'Of the four of us,' he says, 'I'm the biggest worrier, the one who always sees the glasses half-empty.'
The violist Jaime Laredo confirms the idea of Manny as 'the worrier - even when he's playing beautifully', but the cellist Yo-Yo Ma thinks of him as the quartet's 'intellectual policeman', the group's musical conscience, someone who makes sure 'that things are done properly'.
A prize-winning pianist with a fluent technique and a much-feted sense of musical balance, Ax has remained refreshingly humble. 'We were recording the Mozart Piano Quartets a couple of weeks ago,' he told me recently. 'It was very demanding and I was annoying everybody over various things. At one point I was talking to Yo-Yo and said to him that if, years ago, anybody had suggested to me that I would one day be giving grief to Isaac Stern, I would have said they were nuts]' And yet 'giving grief' is never seen as a problem, certainly not to Ma, whose consummate technical skill Stern describes as 'quicksilver', and who is variously characterised as 'the showman' by Ax and as 'the searcher' by Laredo.
Ma confessed to occasional group disagreements, with the proviso that 'they really do get resolved very quickly: but they are confronted. It's done with whatever method of persuasion someone feels. Manny is the more verbal, whereas Isaac - well, he has these unbelievable instincts. He likes to flirt with danger and is always willing to try new things. With Jaime, however, you see it in his eyes.'
Jaime Laredo is well-known in Britain through his work with the Scottish Chamber Orchestra. Isaac Stern refers to his 'marvellous control of the instrument and wonderful sound', his ability to make the music smile, while Ax thinks of him as a 'unifying force'.
Stern, too, confirms that the quartet always reaches a consensus, albeit 'sometimes noisily. But we reach it because we have to, and quite how we interpret the composer's message and make it our own - what accents we give it with our tongues and what role with our impetus - that very much depends.'
He confesses that things can happen on the spur of the moment. 'We all play along in something very familiar, and then, without saying the word, one of us will lift a shoulder, or cock an eyebrow, or wink - except that we don't, we do it in the music and everybody hears it.'
Mutual trust is of course paramount - 'although I don't think they'd always trust me to remember the music]' said Stern, referring to a specific incident that Ma recalls in detail. 'We were in Palm Springs,' he told me, 'when Isaac forgot the music for a Brahms Piano Quartet - and he only realised towards the end of the interval. He then decided to make the 20-minute drive back to the hotel, turned to Manny and me, and said, 'You boys can take care of things.' So we played a Beethoven cello sonata, off-the-cuff and without any music. Isaac only returned by the time we had finished - but it was great fun all the same.'
Shared humour is an essential ingredient for the quartet's mental well-being, just as a shared love of good food makes for their nourishment. 'One of the great things about travelling with Isaac Stern,' says Ax, 'is that you move in very exalted gastronomic circles.'
All four players have taken an active part in promoting new music, Ma and Laredo particularly. As a group, they've recently been considering Aaron Copland's Piano Quartet for inclusion in their repertoire, and Stern has developed a passion for Charles Ives' crafted but crazy Piano Trio. 'I love it,' he said, with obvious relish, 'but it was sometimes a bit of problem for Manny, because he wasn't brought up in America. Still, for us the bumptiousness of the music, its raucous, country-style guffawing, its allusions to childhood themes and folk-songs - they're quite hilarious. We'd giggle, but Manny would say, 'What's so funny?' You see, they weren't part of his childhood - the jokes weren't Polish. But then he came along, and we all had a ball playing it.'
There are also telling differences in their attitudes to the pluses and minuses of recording versus performance. 'It's very much like an actor who does something on stage and then has to do it for a movie,' says Ax. 'When the camera is up there, you can't make the same gesture, you can't speak in the same way - you have to diminish your range of speech and gesture.'
Laredo agrees that 'you can't help but play differently, because in a recording studio you try to make every take as perfect as possible.' Ma, on the other hand, has a very creative attitude to recording. 'The whole idea, whether in performance or in the studio, is to try and match concept with reality - and with recording you have the benefit of hearing a play-back and the choice of changing things.' Stern refers to the microphone's X-ray ears, which serve to inform you as to whether 'the fourth little hair in the left ear is curled to the left or to the right - which is not something you'd ordinarily think about in terms of an audience.'
He's quite adamant, however, about the quartet's spontaneity in the studio. 'Some people think that these are simply recording 'sessions' rather than live performances, and that's not true,' he insists. 'We're disciplined enough to do whatever we have to do, to listen to each other, to give and take, and because of that, it is a live performance - one that can't repeat itself.'
Yet, having taken such pains to transfer the living moment to tape, isn't there a chance that all will become lost if the listener fails to reciprocate with a parallel degree of concentration? We've heard a great deal about the art of performance, but isn't there also an art to listening? 'Of course there is - for us, too,' replied Stern, pausing for thought, 'because the art of playing can only develop once you've learnt to listen.'
Tomorrow 7.30 Symphony Hall, Birmingham (021-212 3333); Monday 7.30 Barbican Hall, London (071-638 8891)