Scepticism aside, there lurks behind almost all musical aesthetics since the Enlightenment period Wordsworth's idea of "recollected emotion" as if a poet first experiences, then contemplates, then seeks a means of expressing his joy or pain.

Wagner seems to have believed something similar, and clearly, if ever a composer was ripe for relating to his music, it is he, as Joachim Kohler shows.

But everybody has experiences, and the question is whether or how a composer's can inform his music, especially if it is great music.

I don't think that this enormously tricky question is answered by tracing whatever it is that music prompts in the listener.

It will do this by means of the conventions and technical associations of its period: certain harmonies, melodic structure, timbres, tempo and so on. But music is sound, and sound is a thing in itself. Where exactly is the link between sound and non-sound, musical skill and personal life, that Kohler seems to assume? If I hear Parsifal as being about the sadness of ageing and dying, that only means that the world it transports me to is the world of that particular imagining. To claim more would be to try binding the unbindable.

I find more useful than Wordsworth's recollected emotions Michael Oakeshott's idea that the poet does a single thing: "he imagines poetically". The world of imagination to which we are transported in Wagner's music depends on his skill in taking us there, on his grasp of how music works (Oakeshott's "poetic mode"). We have ample evidence that the terrible moment in Gotterdammerung when the poor, misled Gutrune thinks she hears Siegfried's horn, is the work of a man who knows both love and despair. We can tell, smiling at the evil dwarf cooking soup while Siegfried is forging his sword, that its creator knew about annoying domestic distraction. But even in operas, why assume that experience precedes or propels the music? Do the incidents and gestures of Wagner's music express the incidents and gestures of his life, or vice versa? Who can say which way round it is, if either?

Of course, even the self-indulgent, vain, deceitful and dictatorial Wagner was capable of imagining Gutrune's feelings, and could no doubt see himself, too, as an innocent victim used by the gods and deserving sympathy; but such feelings are commonplace while the music is not. So there is more to explain, and great music remains a conundrum. Something in common between Wagner's personal life, his prose and his music is their excess -that feverish, extreme element unmistakable in all three. Is this link, admittedly intangible, one that could in the end prove more revealing?