This is a potentially stupid question, but how come Tchaikovsky's Manfred is not a numbered symphony?
Probably for the same reason that some violin concertos have names instead of numbers.
It is a "Tone Poem" or "Symphonic Poem," a genre that tells a story in music that need not follow the classical symphonic-movement format. There are many, many others by many composers.
Same reason Scheherazade is not a numbered symphony.
Wikipedia knows more than anyone here
Wikipedia isn't a numbered symphony either.
Sander, but what is that reason?
+1 to Sander.
It's a good question and has puzzled me too. I attended a Tchaikovsky day a few years ago, in which all of the symphonies were played in three concerts - same conductor, different orchestras. Each concert was one of the earlier symphonies paired with one of the later: 'Manfred' was not programmed! Yet it has a structure that is definitely symphonic by the context of the late 19th century. It qualifies strictly as a symphony better than Berlioz' 'Fantastique', for example.
Feeling a bit like putting my hand up in class, the answer is surely Berlioz who really stretched the definition of Symphony. Not just the Fantastique, there's also the Symphonie Funebre et Triomphale, Harold in Italy (like Manfred, based on a poem by Lord Byron) and Romeo and Juliette, all described as "Symphonie" but not given numbers. Berlioz made two long trips to St Petersburg, during which part or all of these pieces was played. Tchaikovsky may have stated his own reasoning but unfortunately I don't have my biographies to hand (Alexander Poznansky's is VERY revealing).
And while we're about it we have the further examples of Liszt's unnumbered Faust and Dante symphonies, also literarily-inspired
I agree Steve - Berlioz really loosened the controls on the symphony. Right now I can only think of Saint Saens as a major French composer who numbered symphonies. Did French romanticism intrinsically reach out towards the tone poem?