Ich weiß für mich, daß ich, solang ich mein Erlebnis in Worten zusammenfassen kann, gewiß keine Musik hierüber machen würde.
For myself I know that, as long as I can summarize my experience in words, I would certainly not make any music about it.
Variant: If a composer could say what he had to say in words he would not bother trying to say it in music.
Letter to Max Marschalk (26 March 1896). Original German text cited from: Blaukopf, Herta (ed.). Gustav Mahler. Briefe. 2nd edition. Zsolnay, Vienna 1996, ISBN 3-552-04810-3, p. 171.
For a month now I have been an out-and-out vegetarian. The moral effect of this way of life, with its voluntary castigation of the body, causing one's material needs to dwindle away, is enormous. You can judge for yourself how utterly I [am] convinced of it, when I tell you that I expect of it no less than the regeneration of humanity. All I can tell you is: let yourself be converted to a natural way of living, but one in which you eat suitable food (compost-grown, stone-ground, wholemeal bread) and soon you will see the fruits of your endeavours.
Letter to Emil Freund (1 November 1880), in Gustav Mahler: New Insights into his Life, Times and Work by Alfred Mathis-Rosenzweig, translated by Jeremy Barham, Ashgate Publishing, 2007, ISBN 978-0-7546-5353-0, pp. 165-166.