In art as in philosophy, Zen revels in contradiction. The picture of an ant running endlessly round a grindstone is a comment on futility. A priest, on the brink of spiritual discovery, is not in elegant robes or mystic postures but wearing a battered straw raincoat, resting on a walking stick.

These days, the great devotional paintings of Christianity mostly raise a yawn. Their symbolism and scriptural references are obscure. So, quite apart from being extremely attractive splashes of black ink on white paper, the wit of Zen art may be a revelation. It shows how religious art can bypass the rational part of brain, and cut straight to our intuition. Even more surprising, it can even make us laugh.

"It's powerful, direct and honest," said the American collector Dr. Kurt Gitter, when asked about the appeal of Zen art. Ninety-eight works from his collection form the core of the exhibition at Shibuya's Shoto Museum.

Gitter first came to Japan in the 1960s as a U.S. Air Force flight surgeon stationed in Kyushu. There he started a long love affair with Japanese art, and on many subsequent trips went from dealer to dealer, particularly searching for the paintings and calligraphy of Zen monks.

"I was interested in Abstract Expressionism and when I saw this powerful calligraphy in sumi ink I realized that those great modern artists had seen it and been influenced by it too," he said. "I also like the idea that [Zen art] is by nonprofessionals, by monks, and was not produced for sale."

As a respected eye surgeon, he has been able to build up a considerable collection over the last 30 years, which is housed at the Gitter-Yelen Foundation in New Orleans. Other works here are from the New Orleans Museum of Art and various collections in Japan.

The Zen philosophy, which stresses intuitive insight and living in the "here and now," was refreshingly appealing in post-war America. The erudite D.T. Suzuki had been writing about Zen in English as early as 1897, but the "Zen boom" abroad didn't occur until the 1950s, aided by the interest of the Beat writers and Abstract Expressionist painters.

Yet for all this, the paintings of Zen monks are little valued in Japan. In a revealing catalog essay, Professor Yuji Yamashita, one of the organizers, writes of the general disdain of the Japanese art establishment. While landscape ink paintings by Sesshu, for example, are way up in the academic hierarchy, the unique work of Hakuin, who was an important Zen priest, has been sadly neglected.

Whether this is due to tunnel vision, snobbery, or a quirk of fate, one hopes that the fine work in this exhibition comes as a pleasant surprise to the public, and a timely jolt to the establishment.

The average Western visitor, in blissful ignorance, will no doubt enjoy the paintings at face value.

The central figure is the great Zen master Hakuin Ekaku, born in 1685, who studied calligraphy as a youth, but burnt up all his careful studies when he saw the "unskilled" calligraphy of Daigu Shochiku. In a flash, he had realized that great calligraphy was an expression of human character, rather than a display of technical skill.

The exhibition focuses on his paintings and calligraphy, along with the work of his followers. In addition, there are works by priests from the non-Zen sects, such as Fugai, who lived in a cave, and Nantenbo, who rattled people's cages with his huge staff of nanten (heavenly bamboo). The works date from the mid-18th to early 20th centuries, and bring these wonderfully eccentric characters to life.

Hakuin chose not to live in the wilderness, nor to court influence amid the military elite, but spent his long life among ordinary people, as head priest of Shoin-ji Temple. Some of his finest work was produced in his 60s, such as the massive painting of Daruma in the final gallery, and a year before his death at 83 he was still traveling, preaching and producing powerful work.

What kept him going? His art was not an elegant hobby, but part of his vocation. Perhaps 2,000 scrolls survive, and most were freely given to disciples, or parishioners, who often requested a souvenir from this popular priest. This explains the recurrence of certain images. But no matter how often he dashed off a picture of Daruma, his work seems fresh and sincere.

Daruma, as we know from the round red dolls sold at the New Year, is the founder of Zen Buddhism who was affectionately believed to have sat so long in meditation that his legs fell off. Hakuin's pictures of Daruma often show an elderly, grumpy fellow, who seems just about to speak. But somehow, we know he will let us figure things out for ourselves.

His visual parables are touching and witty. There is one of two blind men feeling their way across a bridge. With just a few strokes of the brush, he captures their postures, fraught with hesitation. Again and again, in such paintings as the young gibbon hanging from a branch, or "Hotei as a begging monk," where a jolly, feckless, near-naked monk races across the paper clutching a sprig of bamboo, one marvels at his talent. As Oscar Wilde said, brevity is the soul of wit. Here it is before our eyes.

Some speak of amateurish brushwork or manga cartoons, in order to cast doubt. But I think many newcomers will recognize his true talent, and sense the modest, playful, humane character of the man behind the brush. Hakuin threw away the rulebook, yet it took great talent to go forward and create something so very fresh and new.

Hakuin's calligraphy is also a treat. Here is energy, power, nobility and mystery. What more could we ask? His calligraphy about anger is an explosion of thorns. His character for "death" hangs in the air with a subtle certainty.

Of his followers, the work of Suio seems particularly pure. His picture of the priests Kanzan and Jittoku makes us smile, the way the tousle-headed duo is caught pointing "off-screen" at the moon. I love the way we can see the artist running out of ink in the inscription, and carrying on with a re-charged brush.

This is one of the delights throughout. The smudges, wobbles and pauses of the brush bring life to these old strips of paper. Nantenbo, particularly, seems to enjoy a great splash of ink, and one wonders if it was occasionally a pose. But his twin scrolls, which show a caterpillarlike procession of monks going to and from the temple to receive alms, is a masterpiece. And like the essence of Zen itself, it eludes definition.

Go see it for yourself, and smile.