Philippe Auguin. (Dario Acosta)

‘Tristan und Isolde” is about longing for something you never reach. The consummation of a relationship. The resolution of a chord. The ideal performance. The next intermission.

Is it, in fact, Wagner’s hardest opera? It depends on whom you ask.

“I never feel this as a hard piece,” says Philippe Auguin, the music director of the Washington National Opera, which will be presenting the work — in a production from Australia that’s new to the company — as its season-opening tribute to Wagner’s bicentennial year, starting on Sunday afternoon. “There is so much substance before the piece and in the piece . . . Behind every single bar there is something,” some source, some rationale.

As examples, he cites the philosophy of Schopenhauer, Novalis’s cycle “Hymnen an die Nacht,” published in 1800 — “the first time night was put in a positive contrast to the world of day” — and a particular kind of 12th-century troubadour song, the chanson d’aube, a form in which two lovers rail against the approach of day while a watchman warns them of impending perils.

“If you have the appetite for this intellectual speculation, then it’s wonderful,” Auguin says of the opera, in a voice that even over the phone line is audibly wreathed in smiles.

Irene Theorin. (Miklos Szabo/Miklos Szabo/Courtesy of WNO)

This is exactly why opinions are divided on Wagner’s operas in general, and “Tristan” in particular. To some, they’re an exciting field of intellectual and artistic exploration, stimulating on many levels at once. Others may find Auguin’s description to sum up the reasons the opera is less to be enjoyed than endured.

Wagner was never a great one for gauging the dimensions of a project in advance. “Tristan,” in contrast to the massive “Ring” cycle that he was in the middle of writing at the time, was supposed to be relatively small and easy to stage. The resulting opera, it’s true, has only a few lead characters and three sets, but it’s also one of the biggest workouts for singers and orchestras in all of opera. In 1863 in Vienna, attempts at a first production were abandoned after 77 rehearsals, and the work was deemed unperformable, though it finally did make the stage in 1865. For his next opera, Wagner opted to change pace with a lighthearted comedy — and produced the five-hour-long “Die Meistersinger von Nürnberg.”

Nonetheless, “Tristan” does indeed stand apart in the Wagner oeuvre for a certain compactness. Most of Wagner’s operas were agonized over for years; “Tristan” was written relatively quickly, within a span of two years. And where some of his operas sprawl, “Tristan,” for all of its length, is musically and dramatically unified, elegantly and symmetrically constructed. Wagner, who had long struggled to work out a theory of music drama and realize it in his operas, had finally internalized some of his techniques, enabling him to compose fluidly. He himself referred to the process of writing it as a “fixed improvisation,” folding the flexibility and innovation of improvisation into the more permanent framework of a written score.

Detractors observe that not much “happens” in the opera plot-wise: Tristan and Isolde drink love potion, embark on a passionate love affair, and finally die. Most of the activity is in its ideas and in its music. It’s based on a musical hypothesis: What would happen if you wrote a work in which the chords continually shift toward resolution without ever quite getting there? This musical embodiment of the idea of insatiable yearning starts with the opening chord of the prologue, known as the “Tristan Chord” and itself responsible for reams of musicological analysis. The tension ebbs and shifts for the entire opera, the oceanic orchestra expressing the scale of the love that is unfolding above it.

The story was compiled from medieval lays and legends, Gottfried von Strassburg’s epic 12th-century poem “Tristan” chief among them. “Nine hundred years ago, it was a really popular story, and went all over the place,” said the composer David Lang last year when interviewed about his own vocal work “Love Fail,” which is based on the same material. Wagner was becoming adept at synthesizing elements from a wide range of sources to produce his own mythology. In the “Ring,” he created his own pantheon of gods based on Norse epic; in “Tristan,” he leaves Western religion altogether. “There is no mention of God in Tristan,” Auguin points out — in contrast to many of Wagner’s other works, such as “Tannhäuser” or “Parsifal.”

Instead, “Tristan” orients itself toward the philosophy of Schopenhauer and, more obliquely, Wagner’s interest in Buddhism. The opera in fact aspires to be Schopenhauer made manifest, starting with the idea that music can express concepts beyond the ability of words. It gives narrative form to the philosopher’s idea of physical love as a manifestation of an insoluble yearning that can only find release and absolute consummation in death. As for its Buddhist overtones: one of its main tropes is the contrast between the daytime world of illusion — what most people think of as “reality” — and the nighttime world of the higher spiritual plane on which the lovers dwell.

The music illustrates this: “The chromatic world becomes reality,” Auguin says, “and the diatonic world” — that is, what 19th-century ears would hear as regular tonality — “is the exception, illusion.” Each act begins with a simple tune from the normal, daytime world — a sailor’s song, hunting horns, a shepherd’s pipe — that is quickly pushed aside by the lovers’ heightened music. The text is borne along on a particularly dense orchestra; in “Tristan,” Wagner used his technique of leitmotifs in association not with characters or objects, as he did in the “Ring” (where you hear themes denoting the giants, the sword, the Rhein river, and so forth), but with emotions, in a score that is only loosely associated with the words, carrying the singers like foam on a wave — and sometimes drowning them, unless the conductor can intervene.

“You have to adjust constantly,” Auguin says. “You want to follow the ideal goals Wagner set for you; at the same time, you have to help the people on the stage.” It’s almost impossible, he says, to do the opera exactly as written. “If we do these tempi in the second act, the singers will not be able to sing the third act.”

For singers and audiences alike, getting through Wagner’s operas requires stamina. But the rewards are many, and tend to grow on acquaintance — to a point where the music is found, for some, addictive. The singers who love them, certainly, keep coming back for more.

“She’s one of the most interesting characters in the repertoire,” said Deborah Voigt, who was originally scheduled to sing the role in Washington but has decided to retire it from her repertoire.

“It’s the most fulfilling of any role I’ve done so far,” says Alwyn Mellor, who along with Irene Theorin has been called in to replace Voigt; she will sing Isolde at WNO’s final performance on Sept. 27. “I feel when I sing this it’s a kind of cleansing experience, even though it’s about pain as well as love and joy. It kind of opens you up — and as a member of the audience it does as well.”