[In October 1907] Sibelius met the composer Gustav Mahler, who was visiting Helsinki. The two colleagues noticed that they had experienced the same phenomenon: with each new symphony both of them always lost listeners who had been captivated by the previous symphony.
But they disagreed about the essence of the symphony as a musical form. “I said that I admired its strictness and style and deep logic, which requires that all its motifs must be linked to each other,” Sibelius recollected later. “Nein, die Symphonie muss sein wie die Welt. Sie muss alles umfassen,” answered Mahler. (“No, the symphony must be like the world. It must encompass everything.”)
– www.sibelius.fi
I love both Sibelius’ and Mahler’s music, their symphonies especially. But, I have lately (with the reading of the excellent new Mahler biography by Jens Malte Fischer) decided that Mahler’s own comment about what the symphony should be is actually somewhat ironic. I’ll explain. I believe that Sibelius actually did what Mahler intended in the literal sense. His symphonies are profoundly influenced by nature – nature without man, I’d say. Just the vast expanses of green forests, blue skies, and white snow of his beloved Finnish landscapes. Mahler, on the other hand, I believe, illustrates the interior world of man’s psyche – more specifically, Mahler’s own. And this is where the power of each composer’s works lies. Sibelius depicts the majesty, 
power, and awe-inspiring beauty of nature with such vividness – that the power of his orchestral climaxes are almost unbearable, like trying to look directly at the bright, midday sun. Mahler, on the other hand, finds equally powerful climaxes, but they are triumphs and tragedies of the human spirit, not of the physical world.
What do you think, and what are your favorite moments in Sibelius’ and/or Mahler’s symphonic output?