Fittingly, The Unanswered Question opened the first concert in the Barbican's Charles Ives festival; it also provided the title for the whole grand event. It was a good choice on both counts. You may feel you have a better grasp of the scale and range of Ives's output after this weekend extravaganza, but the questions linger. When Ives threw together hymn-tunes, spirituals, military marches in vibrant, garish non-tonal collages, how clearly did he know what the end result would be - and does that matter? How could the composer of the mawkish, derivative cantata The Celestial Country go on to something as astonishingly original as From the Steeples and the Mountains - with its angular, dissonant brass counterpoint and bells in three different keys - just two years later? Ives is certainly a visionary, but what is the essence of his vision: is there a spiritual core that holds everything together, or is he simply enthralled by the wild diversity of everyday phenomena?
The first two days of the Ives weekend may not have provided answers, but they sharpened one's appreciation of the questions, and of the questioner. Ives may be baffling at times, some of his attempts at innovation may seem naively self-conscious, but there is something so palpably alive in work after work - so vital and courageous - that the best attitude may be simply to take him as he is, rather than judge him according to standards which have little or nothing to do with his unique brand of musical thought.
Part of the fun was simply in hearing so many of his works live instead of in recordings. The Barbican Hall muddied some of Ives's riotously dense textures, and the St Giles Church acoustic drained the Duke Quartet's performance of the First Quartet of a lot of its sonority and power. But there was some splendid playing - from the Dukes, from pianist Philip Mead, grappling with the horrendously complicated writing of the First and Second (Concord) Sonatas, from the BBC Symphony Orchestra and London Sinfonietta. Under Andrew Davis the BBCSO gave a stirring performance of the Holidays Symphony, especially in the "Thanksgiving" finale. Whatever the title may suggest now, it plainly meant something very different to Ives: stern, granite-like polyphony, and a fervent choral climax on an old Puritan hymn, quoted without a hint of Mahlerian "ironical distance".
The Sinfonietta (under Oliver Knussen) took longer to get beneath Ives's teeming surfaces, but the wondrously bizarre The Gong on the Hook and Ladder (a portrait in sound of the local Volunteer Fire Company's annual parade) earnt its encore, and Three Places in New England was full of strange, haunting loveliness in its outer movements. So, too, was the BBCSO's Third Symphony, though the non-appearance of the synthesised bells at the end was a minor catastrophe - someone, somewhere deserves a carpeting for that.
n The last of the Charles Ives Festival concerts are on Radio 3 tonight, at 7.30pm and 9.10pm