Existentialism is a philosophical attitude that we find most clearly in people like Søren Kierkegaard and Friedrich Nietzsche, though the term itself was not used until the 20th century by people like Jean-Paul Sartre. Wikipedia summarizes the basic idea as:
In existentialism, the individual's starting point is characterized by what has been called "the existential attitude", or a sense of disorientation and confusion in the face of an apparently meaningless or absurd world.I will leave you to follow the links for an idea of existentialism. I will try to sketch what I think I hear in the symphony. Existentialism is a big idea about the nature of human existence and the feelings the individual may have confronted by a confusing and possibly meaningless world. The symphony, as one of the very big genres, well into the 20th century at least, is a logical place to find this sort of attitude in music. Indeed, music, with its powerful means of expression, might be an ideal place to explore the existentialist mood or attitude in art. There is an interesting quote by Sartre that captures this, I think:
"... man first of all exists, encounters himself, surges up in the world—and defines himself afterwards."This surging up in the world is exactly the kind of sensation that music, notoriously bad at specific definitions, is very good at.
I think the very first existentialist moments in music, in the symphony at least, are found in the Symphony No. 9 of Beethoven and the "Unfinished" Symphony of Schubert. In the Beethoven it is in the coda to the first movement where a grinding chromatic passage in the basses underpins dotted angst in the winds. In this clip, the passage begins right after the 16 minute mark:
The whole of the first movement of the Schubert "Unfinished" is another example:
There are lots of other examples in Schubert songs, but I am going to just stick to symphonies in this post. What I am calling "existentialism" in this music is a kind of dark, expressive angst, perhaps an aloneness, a confusion, the expression of the suffering of an individual. This may not be entirely new in music, but I think it is new in the symphony which has, up until now, been a largely celebratory medium, one in which we find communal exaltation. But now we are hearing something much more eerie and dark and the orchestra has marvelous means to express this.
I won't try and pick out every example, but just hit a few high points to illustrate what I mean. There is a bit of this in Berlioz, but I really don't hear it in either Schumann or Brahms. I'm not sure it is in Bruckner either, but we find it everywhere in Mahler:
A very powerful example and the one that sparked my thoughts about existentialist symphonies is the Symphony No. 6 "Pathetique" of Tchaikovsky. This is why it has this nickname:
The mood is present in various places in Sibelius, especially in the Symphonies Nos. 4 and 7:
But it is only occasional in Shostakovich where it is diverted into either irony and sarcasm or into what I sense is more of a communal, not individual angst. But the third movement of the Symphony No. 10 might be a good example:
The absolutely best examples of the existentialist symphony is every one of those written by Allan Pettersson. Each one is a journey from despair into serenity:
This little exercise reminds me of something Jorge Luis Borges once said, that every artist creates his own predecessors. I suspect that we only hear the existentialist mood in those early symphonies because we have been sensitized to it by the later music of people like Tchaikovsky and Pettersson.