HERTOGENBOSCH, The Netherlands — A lithe young woman, seemingly nude, slithered out of an enormous human ear, twisting and turning until she fell to the ground, only to be followed by another woman, and another, emerging like escaping parasites as part of a hallucinogenic extravaganza that had, I assure you, nothing at all to do with Dutch weed.

Welcome to the home town of the magnificent and mysterious painter Hieronymous Bosch, which is marking the 500th anniversary of his death, starting this weekend, with three months of festivities inspired by his work and what is certainly the most extraordinary collection of his paintings—20 of 24—ever assembled.

His work in this little Dutch town in the late 15th and early 16th centuries was right on the cusp of the Renaissance, when the stiff, stylized formality of Medieval art exploded with humanity. And it seems to this day wildly original. His paintings were a precursor of Breughel the Elder, on the one hand. (“About suffering they were never wrong, the old Masters,” as W. H. Auden wrote.) More obvious to us today, his dream-like imagery foreshadowed the work of surrealists 400 years later. In the age of psychedelia, in the 1960s, Bosch’s work became iconic on album covers for, among others, Deep Purple and Pearls Before Swine. To this day, any fetishist must be fascinated by his curious treatment of the human backside.

To come to this town now is to be immersed in Bosch’s world, which might seem a nightmare if you know his work only by glimpsing his visions of Hell and his curious notion of “earthly delights” populated by tortured souls and outlandish creatures.

Certainly, the dance production choreographed by Nanine Linnning, “between hell and paradise,” as the program tells us, is a surreal rendition of the seductive terror in a nightmare: human earwigs, fish-headed knights, knocked-up Medieval ladies, tortured men hung on rings or climbing through cages.

But once one enters the heart of the matter, the exhibition of paintings by “The Master,” the experience becomes a matter of intellectual, historic, artistic and indeed emotional exploration.

Who the hell was this man who signed his works Jheronimus Bosch? And what the hell was he thinking?

The answer to the first question, in very general terms, we know. He was born around 1450 into a family of painters whose name was van Aken, meaning “from Aachen,” although they had lived in ‘s Hertogenbosch, the duke’s woods, also called Den Bosch, also called Bois-le-Duc, for generations. His father, grandfather, and uncle were recognized as accomplished painters, and he joined their ranks early on.

As Jeroen van Aken began to acquire a reputation in his own right, he changed his professional name to Jheronimus (or Hieronymus) Bosch, apparently so potential clients and patrons would know where to find him.

It’s not easy, at first, to think of this visionary artist as a commercial painter working on commission, but he surely was. And, more than that, he was accepted by the elite of his little city, a very Catholic, very conservative upper crust epitomized in the Confraternity of Our Illustrious Virgin Mary, of which he became a distinguished member.

In fact, the confraternity is still quite active, not least in the Bosch anniversary celebrations, and what little we know about Bosch’s life we know mainly from its records dating back to the 14th century.

One might surmise, and this is rather surprising, that his work, which sometimes bordered on the heretical, was accepted at the time because he, as a man, as a “brother,” was such a staunch pillar of the community.

But no records—none at all—explain what was going on in Bosch’s head, which has left scholars, scientists, and casual students of his work speculating endlessly.

One of the major revelations of the Bosch exhibition is the collection of sketches revealing his skill as a draftsman. They shed a whole new light on his work for those of us who have never seen them; they are alive, almost animated, and have a clarity quite distinct from the paintings.

One especially interesting drawing shows an owl (a favorite Bosch icon) in a hollow tree. Behind it, in a forest, are two enormous ears. Beneath it, peering out of the ground, are several eyes. He was, scholars tell us, aware of his imitators and, perhaps, his detractors, and this was a commentary. In Latin at the top of the sketch he has inscribed a Latin quotation: “Poor is the mind that always uses the inventions of others and invents nothing itself.”

The sketches became one of the keys to the research begun in 2007 by the organizers of this show to authenticate—or not—paintings in collections around the world that have been considered works by Bosch.

Methodically and meticulously, comparing hands and ears and owls, brushstrokes, colors, and the drawings and paintings revealed beneath the paint by various photographic techniques, the researchers were able to determine with new clarity what was Bosch, what was his workshop, and what was by someone far removed from his tutelage.

One happy result: authentication of a painting of St. Anthony in Kansas City.

One less happy result: the experts for this show called seriously into question three of the six “Bosch” paintings in Spain’s Prado Museum, leading to speculation that this is one reason the Prado, which will host the show after it leaves The Netherlands, did not share the most well-known of Bosch masterpiece, “The Garden of Delights.”

(If there is any possibility you have seen a Bosch, this triptych of Adam and Eve falling from grace, a centerpiece showing a vast collection of naked people experimenting with their appetites, and the definitive, monstrous vision of Hell where they pay for their excesses—that is the one you’ve seen.)

Ron Spronk, a professor of art history in Canada, and a key member of the verification team, acknowledges “de-attribution” of three Prado paintings might have been a thorny issue, but, “We certainly weren’t the first to question them,” he said. Many scholars had doubted the attribution of those three paintings and the Prado was “far too optimistic” about them.

We were talking in front of a painting of St. Jerome, a favorite Bosch subject, that the investigators here confirmed as authentic while some other scholars called it into question.

The fruits of the scientific and scholarly research were sometimes amusing. It was common in the late Middle Ages, for instance, for a painter to depict the patrons of his work as participants or spectators, especially in biblical scenes.

But in several of Bosch’s paintings, infrared photos and restoration work made it obvious that certain figures, probably the patrons and their families, had been painted over, possibly because they refused to pay when they saw the peculiar settings created for them.

“I think they might have been shocked,” surmised Prof. Spronk: “They said, ‘Sorry, Mr. Bosch, this is not quite what we ordered.’”

Given that Spronk has lived so intimately with Bosch’s work for so long, we asked him what he thought was going on inside The Master’s head. Why is it that he painted things no one had painted before—not only the creepy little creatures, and the visions of an infernal world, but beggars and whores, pickpockets and murderers?

Spronk mentioned the theories that Bosch might have suffered from Saint Anthony’s fire, or ergotism, brought on by eating rye grain with a mold that has hallucinogenic qualities (LSD originally was derived from it). But that’s the kind of glib armchair pharmacist explanation that makes for a nice tabloid headline while doing little to illuminate the real context of the work.

“I don’t think he was regarded as so weird at the time,” said Spronk. “There was so much going on in popular culture that we haven’t realized, because it wasn’t preserved. He lifted up popular culture.” Thus some of his paintings hold visual puns, others reflect, at least in the background, the daily life of the Middle Ages in the raw.

One memorable example is The Wayfarer, which shows an itinerant in tattered clothes, with a slipper on one foot, a shoe on the other, a cloth over his head, a hat in his hand, a cat skin pinned to his pack, while in the background one man is taking a piss and another man is kissing a woman next to a birdcage, the classic Dutch symbol for a whorehouse.

Bosch’s technical virtuosity was secondary to his vision. His contemporary, Jan van Eyck, had a more luminous mastery of light and texture. “Van Eyck leaves you in awe because of his technique,” said Spronk. “But Bosch really doesn’t care what you think of his technique—really, he doesn’t give a rat’s ass.”

Which brings us back to all those images of rats and owls and demoniac creatures that are utterly surreal—ghoulies and ghosties and things you really, really hope never go bump in the night.

They were not common references in popular culture or any culture at the time. As the catalogue, Hieronymous Bosch: Visions of Genius says on its opening page, Bosch “must have lived in two different worlds: the real one around him, and the universe of his imagination.”

He could, as Milton’s Satan said, make a Heaven of Hell, a Hell of Heaven. And if Bosch’s standing as a pillar of the community protected him from charges of heresy when he was alive, his supporters later in the 16th century, after his death, were hard pressed to defend him.

At a moment when the Reformation had not yet begun, and the Renaissance was still aborning, Bosch had tapped into something transcendental. In fact, Bosch realized, in a way no painter of his time had realized, that he had the power not only to imitate and reproduce and stylize, but to create unforgettable images—to be his own god in the world of line and color.

“Bosch,” said Spronk with evident emotion, “is the personification of the moment in time when the Medieval craftsman became an artist.”