Starting next week, I will be telling the story of how orchestral music's most famous form has shaped musical history by curating a non-chronological, entirely personal (and therefore doubtlessly controversial!) canon of the 50 symphonies that I think are responsible for telling us most about how the form has changed the musical world, and the world outside the concert hall too.

Over the year, I hope what will come over is the sense that the development of a supposedly abstract musical structure isn't simply about compositional invention or experimentation, but about how we hear ourselves and our place in the world: from the courtly entertainment of the early Rococo symphony to the world-changing idealism of the late 19th and early 20th centuries in Brahms, Bruckner, and Mahler; from a social order bounded by conventions and their transgressions in the 18th century, as in the music of Haydn and Mozart, to a more recent age of creative freedoms and limitless possibilities, the symphonies by Berio (yes, that's what his Sinfonia actually is!), Peter Maxwell Davies, Oliver Knussen, or Per Nørgård.

But before all that comes the most basic question of defining my terms: what is a symphony? It's usually how we refer to the multi-movement form that evolved in the early 18th century in central Europe (from the Baroque suite and the operatic overture) as a self-contained work of purely instrumental music, and which went on to become the single most prestigious expression of musical architecture in the late 18th, 19th and early 20th centuries, the highpoint of many composers' ambition and achievement.

But it's much, much more than that. A symphony isn't just a structure, a musical formula, or a set of containers - three or four movements of contrasting speeds and characters - that composers merely had to fill in to qualify as "symphonic" writers. The symphony is really a way of thinking about what music actually is, what it's really for.

Because if you accept the idea that instrumental music is capable of "saying" anything at all, then it's in the symphony that that power is released most grandly, most extravagantly, and most directly. The symphony is the ultimate embodiment of the idealist notion of music being the "highest of the arts", a place beyond words or representative images in which transcendent feelings were given pure, unadulterated expression. As we'll find out, the crucible of those ideas is the way Beethoven's symphonies have been thought about and performed: such music as the Third (the "Eroica"), the Fifth, or the Ninth - even if that piece was the first symphony to use a choir, and a text.

But the problem with thinking of the symphony as idealistic transcendence is that you lose sight of how it communicates and who it communicates to. A symphony is always public: in terms of who it's written for, in the ever-changing and ever-expanding orchestral forces that composers have been able to call on, and who hears it, from private aristocratic gatherings at the end of the 18th century, bourgeois entertainments in the 19th, to today's huge auditoriums. The story of the symphony from Haydn's genre-defining pieces that were composed for his handful of musicians at the court in Esterhazy to Mahler's symphonies, with their forces of hundreds is a drama that's as much social as it is musical. It's about who paid the composer and the musicians, about what the symphony was heard to represent, and about what role composers were supposed to fulfil in society.

It's often said that the story of the symphony is bounded by historical time, and that we're now living in a post-symphonic age. That's because a symphonic frame of mind, with its associations of order and coherence doesn't fit with our more fractured and fractious sensibilities. What I hope you might hear in exploring the 50 symphonies over the next 12 months is rather the opposite: the extremities, disturbances, and strangenesses at the heart of the symphonies of the 18th and 19th centuries, and the urge to create some kind of order from chaos in the works of the later 20th and 21st.

Other threads I hope we'll pick up along the way: the paradox of pieces that aren't called symphonies but which really are "symphonic" in the musical language they speak - Mahler's Das Lied von der Erde is only the most obvious example. And then there's astonishing range of ways of playing the canonic symphonies of Mozart, Brahms, Bruckner, Mahler - and everyone else! - that we can all now instantly hear at our fingermousetips. That's a phenomenon that amounts to much more than Wilhelm Furtwängler taking longer over a Beethoven slow movement in the 1940s than John Eliot Gardiner in the 1990s. The difference is actually a revelation of two completely different views of what the "same" piece of music means. That's a process of renewal that continues any time these great pieces are properly, intelligently, passionately played - which means that Beethoven's 5th Symphony, for example, isn't a fixed work so much as a palimpsest of musical histories that only gets richer and richer each time it's played, heard, and thought about.

The symphony, then, ain't over yet. And talking of Beethoven's 5th, that's a very good place to start next week… see you then.