Marcel Duchamp served for many years as both a prince and court jester to modern art in the 20th century. While creating some well-known works, he also punctured pretensions with jokes, pranks, aphorisms and a perpetual hunt for new byways of art. Then he announced he was abandoning art, giving it all up to play chess. But he was not telling the truth.

He worked in secret for 20 years, assembling a huge, fanciful and puzzling diorama. When he died in 1968, only a few people knew about his secret.

A year after his death, the Philadelphia Museum of Art installed the secret work and displayed it to the public. While some patrons were shocked by its sexuality, it soon became a magnet for young artists looking for new paths to take their own work. Duchamp’s masterpiece, known as “Etant donnes,” a shortened form of its French title, is now regarded as one of the most powerful and dynamic influences on contemporary art.

Museums rarely devote an entire exhibition to a single work. But the Philadelphia museum holds Etant donnes in such high esteem that, on the 40th anniversary of its installation, it has mounted a show -- simply titled “Marcel Duchamp: Etant donnes” -- that describes in great detail how Duchamp conceived and constructed the work. The show, which closes Nov. 29 and goes nowhere else (though “Etant donnes” remains as before on permanent display), comprises more than 100 objects, including body casts, drawings, photos and other pieces that shed light on the creation. There is even a loose-leaf notebook in which Duchamp set down detailed instructions, in handwritten French, on how to take the work apart and put it back together again.

On top of this, the 447-page catalog includes 35 letters that Duchamp wrote to his lover, the Brazilian sculptor Maria Martins, in the late 1940s and early 1950s. Her body served as the model for the nude that is a central piece of “Etant donnes.” The letters, obtained by the museum a year ago, have not been published before.

‘Strangest work of art’

The displays enhance the experience of seeing “Etant donnes” itself. The installation, according to the painter Jasper Johns, is “the strangest work of art any museum has ever had in it.”

A visitor walks into a darkened room and stands before an antique Spanish door arched by thin red bricks that come from Cadaques, Duchamp’s summer vacation site in Spain near the home of Salvador Dali. There are two almost imperceptible peepholes in the door. Through the door, the visitor sees a brick wall smashed open enough to allow a view of almost all of what lies behind.

A voluptuous nude woman dominates the scene. There are wisps of blond hair, but her face cannot be seen. She is lying, in a provocative pose, on a bed of heavy twigs. The exhibition has shown that she is fashioned from painted parchment shaped mainly from a mold of the body of Maria. Her left hand is holding a French Bec Auer gas lamp. Beyond her is a forested mountain landscape that includes a waterfall with little dots of light that create the illusion of cascading water.

The visitor is a voyeur of a startling three-dimensional scene within an installation 10 feet long. It is an experience that cannot be captured in a photographic reproduction.

Duchamp left no explanation of what it all means. His full title is not much help. In English, the title is Given: 1. The Waterfall. 2. The Illuminating Gas. This echoes old-school problems in mathematics and logic. “Given that we have (1) and (2),” a problem would say, “the conclusion must therefore be _______.” But Duchamp has not set down any conclusion in his title.

Since Duchamp always insisted that the spectator was a vital participant in a work of art, he probably would be satisfied with any conclusion reached by someone peering into the peepholes of “Etant donnes.” But some speculation does go wild. Some writers, for example, believe that Duchamp was trying to reflect the grisly Black Dahlia murder in Los Angeles in 1947. But this theory is debunked by Michael Taylor, curator of the exhibition, who points out that several of Duchamp’s preliminary studies were completed long before aspiring Hollywood starlet Elizabeth Short was murdered.

“Etant donnes” is so complex and replete with so many allusions, Taylor insists, “that it actively resists any single critical interpretation and ultimately remains unfathomable.”

Taylor, the curator of modern art at the museum, is a distinguished Duchamp scholar who has no doubt about the status of Duchamp today. “If you had said in 1968 when he died that Duchamp was as important as Picasso,” Taylor says, “no one would have believed you. Today it is self-evident. Duchamp was the most influential artist of the 20th century. He changed the world of art.”

Young artists, according to Taylor, do not believe they can learn anything from Picasso. “But you can’t go to a gallery in Chelsea without seeing Duchamp,” he says, referring to the art gallery district in Manhattan.

Even before the public installation of “Etant donnes,” Duchamp had champions such as Jasper Johns, Robert Rauschenberg and Andy Warhol. In his catalog, Taylor lists more than 40 younger artists, including Jeff Koons, Cindy Sherman and film director David Lynch, who were influenced much later by “Etant donnes.”

Taylor says echoes of the work have been found in “life-size installations and peep shows, photographs and light-box constructions, film, video, and performance art as well as painting, sculpture and works on paper.”

An independent artist

Duchamp, who was born near Rouen in France in 1887, first attracted attention at the 1913 Armory Show in New York with his Cubist painting “Nude Descending a Staircase.” The painting stirred up so much fuss at the show that one reporter described it as an “explosion in a shingle factory.” To former President Theodore Roosevelt, it looked like a poor imitation of a Navajo rug, but Duchamp became an icon of modern art.

Duchamp was allied somewhat with the Dadaists (and later the Surrealists) and, in 1917, he performed what looked like a pure nihilist act of Dadaism. He bought a porcelain urinal, placed it upside down, signed it R. Mutt, titled it Fountain, and submitted it anonymously with the $6 entrance fee to the New York Society of Independent Artists for its annual show. Although the society had pledged to display any work submitted with the entrance fee, the urinal was rejected.

This became less of a joke years later when Duchamp began offering what he called “ready-mades” as works of art. These were manufactured goods that an artist, under Duchamp’s theory, could turn into a work of art through the act of selecting and displaying them. The ready-made, he said, was “a form of denying the possibility of defining art.”

In 1923, Duchamp completed an enigmatic, sexual painting on 9-foot-high glass called “The Bride Stripped Bare by Her Bachelors, Even.” He then announced he was leaving the world of art for the world of chess, and the glass painting, now part of the Philadelphia museum’s permanent collection, was regarded as his masterpiece for the next 45 years.

During World War II, he moved to New York, making it his home from then on. (He received American citizenship in 1955.) He started his secret work on “Etant donnes” in a minuscule room in his small apartment on 14th Street. Maria served as the model and muse. He implored her to marry him, but she refused to give up her life as the wife of the Brazilian ambassador to Washington and later Paris.

In 1954, Duchamp married Alexina Sattler, the divorced wife of art dealer Pierre Matisse. Alexina, an American, supported the secret project and, in fact, became a model for part of it. When the original left arm of the nude was damaged, Duchamp substituted a cast of his wife’s arm. Moreover, he decided to substitute blond hair like that of his wife on the nude for the original brown hair like that of Maria.

A few years before his death, Duchamp told Calvin Tomkins of the New Yorker that he was fed up with the commercialization of art and the tendency of artists to use only their eyes and not their minds when they painted.

“No, the only solution for the great man of tomorrow in art is to go underground,” he said. “He may be recognized after his death, if he’s lucky.”