from Alma Mahler's Gustav Mahler: Memories and Letters, pages 256-57, "Letters 1904-1905"
Arnold Schoenberg to Gustav Mahler, 12 December 1904 (this date may be incorrect, and the letter likely refers to Mahler's Third Symphony)
My dear Director,
I must not speak as a musician to a musician if I am to give any idea of the incredible impression your symphony made on me: I can speak only as one human being to another. For I saw your very soul, naked, stark naked. It was revealed to me as a stretch of wild and secret country, with eerie chasms and abysses neighboured by sunlit, smiling meadows, haunts of idyllic repose. I felt it as an event of nature, which after scourging us with its terrors puts a rainbow in the sky. What does it matter that what I was told afterwards of your "programme" did not seem to correspond altogether with what I had felt? Whether I am a good or a bad indicator of feelings an experiences arouses in me is not the point. Must I have a correct understanding of what I have lived and felt? And I believe I felt your symphony. I shared in the battling for illusion; I suffered the pangs of disillusionment; I saw the forces of evil and good wrestling with each other; I saw a man in torment struggling towards inward harmony; I divined a personality, a drama, and truthfulness, the most uncompromising truthfulness.
I had to let myself go. Forgive me. I cannot feel by halves. With me it is one thing or the other!
In all devotion,
(Having read this far, now have a treat: Happy Belated Purim! https://www.youtube.com/watch?v=51OnscdUHgw)