Do you believe in Fate? Many composers did, however they interpreted it. Tchaikovsky certainly did. It's what his autobiographical Fourth Symphony is all about, from its blazing opening fanfares to its breathtaking, race-to-the-finish close. And Beethoven certainly believed in it, because we know he confronted Fate, eyeball to eyeball, in his Heiligenstadt Testament, a document often described as something between a will and a suicide note.

Beethoven, deciding that the music inside him was more important than the relentless deafness closing in all around him, basically told Fate to sod off: "I shall seize Fate by the throat." And looked what happened: the music poured out of him and his revolution began to roll with serious purpose.

Tchaikovsky, however, was hemmed in by Fate. He poured his heart and soul into the Fourth Symphony. But when he got to the great finale, he took a different course to Beethoven: he compromised. Basically, he advocated that if there is nothing in life for you - that is, joy - then try and enjoy it vicariously, through the happiness and experience of others.

I don't go with that myself, though most conductors do. For me, that explosive finale is a seething pot of pent-up frustration which nearly broke free in Peter Oundjian's recent account of it with the RSNO, and is completely characterised in my 1957 recorded version of the symphony by Yvgeny Mravinsky and the Leningrad Philharmonic, where the rage boils over in the finale and the music is vicious in its agonised fury. It's incredible.

But it's not Fate itself that I'm on about today. It's the condition a composer finds himself in. It's the moment when a composer confronts himself, or is confronted by, a massive tension, frustration or crisis. It might be the time, it might the place they're at (literally, in the case of Beethoven). It might be the circumstances they find themselves in; it might be the arrival of a third party. But something happens as a result. It's as though a door opens in the composer's s mind and the music streams out out of him, white-hot in its immediacy and dazzling in its freshly minted inspiration. It's a release: a catharsis of great creativity.

It's happened to so many composers. I don't call it Fate; I call it synchronicity. It's absolutely unmistakeable when it occurs, and its impact on listeners can be enormous. I've experienced its effects hundreds of times over the decades, and once, to my acute embarrassment, live on Radio Scotland, when I was overwhelmed by James MacMillan's Tuireadh, his Piper Alpha lament, at its first performance in Orkney in the presence of the relatives of those who lost their lives in that horrendous conflagration. There was a potent mix of communal pain and grief and, in the sonic presence in MacMillan's great piece, of a quietly persistent note of consolation. I was so overwhelmed on air that the presenter, the late Neville Garden, virtually had to pull the plug on me.

Back in 1840, it happened to Robert Schumann who, with his great love, the celebrated concert pianist Clara Wieck, endured some six years of pressure and brutal obstruction by Clara's father, Friedrich. He put every imaginable obstacle in their path towards marriage, including sending Clara abroad on concert tours to keep them apart, and attempting to use legal processes to have Robert declared a drunkard, unable to support his daughter (groomed since infancy by Friedrich for international stardom).

All the pressure caused stresses on Robert that undermined what stability the composer might have had. Eventually, Clara and Robert, flat out of options and desperate, had to drag her father to court. The court ruled in their favour and, in the New Year of 1840, gave them permission to marry.

Instantly, with the cumulative effect of six years of non-stop pressure suddenly lifted, something magical happened: a door unlocked in Schumann's psyche, and, beginning in February 1840, his Year of Song, the music began to pour from him, including all the great song cycles, Dichterliebe, Liederkreis (two sets) Woman's Life and Love.

It was a torrential and unstoppable flow that went on all year and included some of the finest love songs of the century. It was a revolution in song, as well as in the function of the piano in the art of German Romantic song. That 1840 outpouring has been described as the "greatest outbreak of song in the history of music". It was synchronicity: and it resulted in an astounding catharsis of creativity.

Six decades later, the phenomenon loomed again, in a different era and with a different composer, the Moravian Leos Janacek. That's a fantastical, near-incredible yarn. I will tell you about it soon.