[2] The opera, which explores existential themes such as that of mankind's insatiable striving and the transcendental nature of a love beyond death, incorporates spirituality from Christian mysticism as well as Vedantic and Buddhist metaphysics, subjects that also interested Schopenhauer.

[4] Tristan und Isolde is widely acknowledged as one of the greatest achievements of Western art music, intriguing audiences with philosophical depths not usually associated with opera, and the "terrible and sweet infinity" of its musical-poetic language.

[5] Its advanced harmony, immediately announced by the famous opening Tristan chord of its prelude, marks a defining moment in the evolution of modern music, characterized by unprecedented use of chromaticism, tonal ambiguity, orchestral colour, and prolonged harmonic suspension.

[6] While these innovations divided audiences initially, the opera grew in popularity and became enormously influential among Western classical composers, providing direct inspiration to Anton Bruckner, Gustav Mahler, Richard Strauss, Alban Berg, Arnold Schoenberg, and Benjamin Britten.

[9] According to his autobiography, Mein Leben, Wagner decided to dramatise the Tristan legend after his friend, Karl Ritter, attempted to do so, writing that: He had, in fact, made a point of giving prominence to the lighter phases of the romance, whereas it was its all-pervading tragedy that impressed me so deeply that I felt convinced it should stand out in bold relief, regardless of minor details.

The cadences first introduced in the prelude are not resolved until the finale of Act III, and, on a number of occasions throughout the opera, Wagner primes the audience for a musical climax with a series of chords building in tension – only to deliberately defer the anticipated resolution.

Resolution of the music does not occur until the very end of the opera, after Isolde sings the closing excerpt commonly referred to as the "Liebestod" ("Love-Death"), after which she sinks down, "as if transfigured", dead onto Tristan's body.

Not all composers, however, reacted favourably: Claude Debussy's piano piece "Golliwog's Cakewalk" mockingly quotes the opening of the opera in a distorted form, instructing the passage to be played 'avec une grande emotion'.

The opera opens with the voice of a young sailor singing of a "wild Irish maid" ("Westwärts schweift der Blick"), which Isolde construes to be a mocking reference to herself.

Tantris was found mortally wounded in a barge ("von einem Kahn, der klein und arm") and Isolde used her healing powers to restore him to health.

Tristan relapses and recalls that the shepherd's mournful tune is the same as was played when he was told of the deaths of his father and mother ("Muss ich dich so versteh'n, du alte, ernst Weise").

Isolde appears to wake at this and in a final aria describing her vision of Tristan risen again (the "Liebestod", "love death"), dies ("Mild und leise wie er lächelt").

[27] The music itself embodies Schopenhauer's concept of the Will, a force that is inherently restless and never fully satisfied that drives all human urges and desires, leading to a cycle of longing and suffering.

The passion of the music is often referred to as being "sensual" and "erotic",[28] this not only reflects the desires of the illicit lovers but is consistent with Schopenhauer's position that the sexual urge is the most powerful manifestation of the Will.

[30] The Day represents the external world of social obligations, duties, and constraints—embodied by King Marke's court, where Tristan and Isolde must suppress their love and live according to the norms and expectations of society.

When Tristan and Isolde willingly drink the potion at the end of Act I but do not die, their eyes are opened to the illusions of material Day and to the higher spiritual insight of Night.

The 5 July 1865 edition of the Allgemeine musikalische Zeitung reported: Not to mince words, it is the glorification of sensual pleasure, tricked out with every titillating device, it is unremitting materialism, according to which human beings have no higher destiny than, after living the life of turtle doves, 'to vanish in sweet odours, like a breath'.

"[40][41] The first performance in London's Drury Lane Theatre drew the following response from The Era in 1882: We cannot refrain from making a protest against the worship of animal passion which is so striking a feature in the late works of Wagner.

[45] In The Perfect Wagnerite, the writer and satirist George Bernard Shaw writes that Tristan was "an astonishingly intense and faithful translation into music of the emotions which accompany the union of a pair of lovers" and described it as "a poem of destruction and death".

Richard Strauss, initially dismissive of Tristan, claimed that Wagner's music "would kill a cat and would turn rocks into scrambled eggs from fear of [its] hideous discords."

[47]Arnold Schoenberg referred to Wagner's technique of shifting chords in Tristan as "phenomena of incredible adaptability and nonindependence roaming, homeless, among the spheres of keys; spies reconnoitering weaknesses; to exploit them in order to create confusion, deserters for whom surrender of their own personality is an end in itself".

[citation needed] Friedrich Nietzsche, who in his younger years was one of Wagner's staunchest allies, wrote that, for him, "Tristan and Isolde is the real opus metaphysicum of all art ... insatiable and sweet craving for the secrets of night and death ... it is overpowering in its simple grandeur".

In a letter to his friend Erwin Rohde in October 1868, Nietzsche described his reaction to Tristan's prelude: "I simply cannot bring myself to remain critically aloof from this music; every nerve in me is atwitch, and it has been a long time since I had such a lasting sense of ecstasy as with this overture".

[51] In the years before World War II, Kirsten Flagstad and Lauritz Melchior were considered to be the prime interpreters of the lead roles, and mono recordings exist of this pair in a number of live performances led by conductors such as Thomas Beecham, Fritz Reiner, Artur Bodanzky and Erich Leinsdorf.

There is also a technically flawed, but historically important video recording with Birgit Nilsson and Jon Vickers from a 1973 live performance at the Théâtre antique d'Orange, conducted by Karl Böhm.

The performance stars Robert Gambill as Tristan, Nina Stemme as Isolde, Katarina Karnéus as Brangäne, Bo Skovhus as Kurwenal, René Pape as King Marke, and Stephen Gadd as Melot, with Jiří Bělohlávek as the conductor, and was recorded on 1 and 6 August 2007.

[62] Leopold Stokowski made a series of purely orchestral "Symphonic Syntheses" of Wagner's operas during his time as conductor of the Philadelphia Orchestra, bringing to concert audiences of the 1920s and '30s music they might not otherwise have heard.

[65] Other works based on the opera include: Aubrey Beardsley's pen and ink drawing The Wagnerites shows highly coiffured men and women attending a performance of Tristan und Isolde.

[69] The following year Beardsley produced a print depicting a stylised image of a woman, standing in front of a half length yellow curtain, wearing an ornate flowered hat and holding a large drinking vessel to her mouth.

[71][72] In Alfred Hitchcock's 1963 film The Birds, a recording of Tristan is prominently displayed in the scene in which Annie (Suzanne Pleshette) resignedly reveals to Melanie (Tippi Hedren) her unrequited love for Mitch.