By the time Wagner was exposed to Arthur Schopenhauer's The World as Will and Representation in 1854 he had completed four major dramatic works (Der fliegende Holländer, Tannhäuser, Lohengrin and das Rheingold) and was in the middle of the second Ring opera, die Walküre. Schopenhauer's ideas were to have profound influence on Wagner; he read and reread Schopenhauer's book many times over the next four decades. One thing that struck him was that, though his pre-1854 operas differed considerably from Schopenhauerian ideas in terms of plot, surface events, characterization and Feuerbachian anthropocentrism, his intuitions, especially with regard to the Ring, were quite in line with what he was reading in Schopenhauer. By 1857 Wagner had become bogged down on the third of the Ring operas and it was easy for him to abandon Siegfried and turn to the eminently Schopenhauerian subject of Tristan and Isolde.

The ideas addressed here are discussed with great insight in Eric Chafe's recent book The Tragic and the Ecstatic: The Musical Revolution of Wagner's Tristan and Isolde. For more details on some of these ideas and how they relate to the opera follow the link below:

Noumenal realm vs. phenomenal realm