There’s a ripe, autumnal aura to the work of Jasper Johns, now 91.

For over 70 years he’s been bringing forth sculptures, paintings and drawings, prints and books, costumes, stage sets, rubber stamps, photographs, and more, with annual regularity.

Flags and numbers so associated with his imagery have given way to skeletal figures in recent years. But the content can still be difficult to characterize.

“I think you’ll have to interpret that for yourself,” is a typical Johnsian soft-shoe past the intent and meaning of it all.

But while meaning may be slippery, critical judgment has long marked Johns as one of the preeminent painters of his generation and the one who is perhaps most responsible for ending the reign of the titans of abstract expressionism and ushering in the era of the new American gods of pop and minimalist and conceptual art.

Now, in a virtually unprecedented two-museum exhibition that opens Wednesday at the Philadelphia Museum of Art and the Whitney Museum of American Art in New York City, the fruits of that career will be laid out with an exhaustive, epic completeness.

“Jasper Johns: Mind/Mirror” will run simultaneously in both Philadelphia and New York through Feb. 13; it has been actively in the works for well over five years.

The cocurators of the show, Carlos Basualdo, curator of contemporary art at the PMA, and Scott Rothkopf, the Whitney’s senior deputy director and chief curator, have amassed a trove of works from all over the world in making their case.

Why two exhibitions containing a staggering 500-plus artworks collectively and why New York and Philadelphia together at the same time?

For one thing, both museums have a number of works by Johns in their collections. Johns lived and worked in New York City for years, but he literally made pilgrimages to Philadelphia and the PMA beginning in the late 1950s to soak up the museum’s rich holdings of Marcel Duchamp, a critical part of the museum’s Arensberg Collection, and an artist Johns holds in highest esteem.

Beyond that, Johns became very fond of the art museum’s late director and champion of Duchamp, Anne d’Harnoncourt, who died in 2008. In fact, it could be argued that the origins of this exhibition as presented in Philadelphia at least can be found in the friendship and shared enthusiasms of the artist and late director.

Two parallel exhibits? Both Basualdo and Rothkopf were mulling ideas for Johns exhibitions several years ago when Basualdo suggested that they go all in: Two simultaneous shows, each complete in itself and completing the other.

“The double space would allow us to show the work in depth, across mediums, avoiding a show of highlights that I felt was not sufficient to account for the complexity of the work,” Basualdo said the other day. Besides, he added, “both Scott and I felt the time was ripe to challenge some of the conventional wisdom around the work and bring a new generation of scholars to consider it.”

In a telephone interview, Rothkopf said he told Johns, when presenting him with a copy of the brand-new exhibition catalog, “‘It’s kind of hard to believe, when you look at this, that one artist made all these works.’ And he said, ‘I agree.’ ”

But back in the 1950s, when Johns was in his mid-20s and fresh out of South Carolina, there was no profusion and there were no certainties as he surveyed the landscape and the artistic future. The art world was dominated by de Kooning and Pollock and their abstract expressionist brethren and nobody had the vaguest idea who Jasper Johns might be. Even the great gallerist, Leo Castelli, coming to see the studio of Johns’ lover and guide to the art world, Robert Rauschenberg, didn’t have a clue about Johns.

That didn’t last long, thanks to Rauschenberg, who lived in a loft above Johns on Pearl Street in Lower Manhattan. One day in 1954 Johns told him about a “crazy” dream the previous night. “‘How crazy was it?’ Rauschenberg asked,” according to the writer Calvin Tompkins.

“‘Well,’ Johns replied, ‘in this dream I was painting the American flag.’ The American flag? Rauschenberg didn’t think it was crazy at all. ‘That’s a really great idea,’ he said.”

Johns went out and acquired the makings of what would change the entire direction of art, beginning with an old bed sheet and some wax. Museum of Modern Art director Alfred Barr Jr. snapped up the work — an American flag — the moment he saw it in Johns’ first show at Castelli’s gallery in 1958.

Johns did not stop at flags. He went on create targets, numbers, and maps. All were things that were images of themselves, what the artist called “things the mind already knows” — flags that were flags, numbers that were numbers, maps that were maps.

Johns’ artistic imagination found inspiration in virtually everything that was not abstract and not expressionistic — Ballantine Ale cans, flashlights, and light bulbs became bronze sculptures; tin cups became collaged elements swinging on canvases, newspaper clippings.

As he was incorporating these fragments of the world into his work, encouraged and inspired by his friendships with composer John Cage, choreographer Merce Cunningham, and Rauschenberg, Johns made a pilgrimage to Philadelphia to take in the artworks of Marcel Duchamp that were part of the PMA’s newly installed Arensberg Collection. Duchamp’s works, like The Bride Stripped Bare by Her Bachelors, Even (1915-1923), also known as The Large Glass, made up a large part of the collection.

Asked about these visits, Johns said in a recent email to The Inquirer that the Arensberg Collection “offers many pleasures but it seemed the logical place to become familiar with the work of Marcel Duchamp (something I wanted to do).”

Duchamp’s “work seemed to suggest new doors into art and thought,” said Johns who first visited Philadelphia in late 1959, and followed up with many subsequent visits, particularly after Anne d’Harnoncourt became the museum’s curator of 20th century art in 1971 and then director in 1982.

She put together the museum’s landmark Duchamp show of 1972, and shepherded the artist’s last major work — the mysterious and lubricious Étant Donnés (1946-66) — into the PMA collection. Johns was fascinated by that piece, calling it “the strangest work of art in any museum.”

Duchamp died in 1968, but his widow subsequently introduced Johns to d’Harnoncourt.

“I first met Anne in the 1970s, I think, when Teeny Duchamp brought her to my studio on Houston Street, in New York,” Johns said. “At some point during the evening, red wine was spilled on Anne’s dress and I insisted that she change into a Japanese yukata that I had brought from Tokyo, and, later, that she wear it home.

“She did so, reluctantly, saying that her mother would be horrified. The garment was returned the following afternoon, freshly laundered.”

No dirty laundry here. Just a vast exhibition, fruit, at least partially, of a long-ago accident and a complicated chain of friendship.

“Sometimes you fall in love with an idea because you think it is right,” said Basualdo.