[...] However, it occurred to me that it would be injudicious to publish Tristan as yet. Between a poem designed for music and a drama which is to be expressed solely in poetry, the differences in plan and in execution are so radical that the former, regarded with the same eye as the latter, must necessarily fail to reveal its meaning—until it finds its completion in the music. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 405