The architect Renzo Piano is unpredictable. He has designed museums of extraordinary beauty and refinement, from the Menil Collection in Houston to a recent addition at the Art Institute of Chicago. And he has produced work that is downright bombastic, especially the Broad Contemporary Art Museum in Los Angeles, done around the same time as his work for Chicago. What attracts so many different clients to Piano is the sophisticated yet playful feeling for intervals, proportions, and materials that he brings to the cool geometric forms of mid-twentieth-century modernism. He gives the old modern religion a flash of youthful swagger. The only way to explain his bewilderingly erratic performances has to be by considering his clients, whose particular expectations and aspirations he apparently responds to with uncanny intuition. When Renzo Piano works for a pretentious art mogul like Eli Broad, the results are glib and showoff-ish. When he works for a great connoisseur, like the European art dealer Ernst Beyeler, what you get is a wonder of visual discretion.

Renzo Piano’s latest success, his freestanding addition to the Isabella Stewart Gardner Museum in Boston, is a perfect jewel-box of a building. Although Mrs. Gardner died nearly 90 years ago and had made it abundantly clear that she did not want her museum to be changed in any way, Piano’s magnificent new work can only be explained as a heartfelt response to a brilliant client—or at least the ghost of a brilliant client. Piano has responded in a bold and original way to the extraordinary institution Mrs. Gardner opened to the public in 1903. She was determined to bring some of the splendors of a Venetian palace to New England, and what she created on the Fenway in Boston is nothing less than a revelation, the quintessence of sophisticated and luxurious fin-de-siècle taste. The paintings—especially Titian’s Rape of Europa and the Giotto and the Fra Angelico—are among the richest and subtlest expressions of the Renaissance imagination, with intellect and emotion seamlessly united. And the settings—from the central garden with its ferns and hothouse flowers to the galleries chockablock with Venetian rococo furniture and layers of deeply colored brocades—are as subtly sensuous as the Venetian interiors in The Wings of the Dove, the novel by Mrs. Gardner’s friend Henry James that came out a year before she opened her museum.

To all of this century-old poetic beauty, Piano has responded with his own kind of beauty, so that the addition to the Gardner plays the role of a sleek, gallant courtier—never upstaging the main event. Of course, I was nervous about seeing the Gardner again. I knew there had been controversy around the demolition of a carriage house behind the museum. Like many in Boston, I feared that a great institution was going to be diluted. Would museumgoers still plunge into the glorious overload of experience that Mrs. Gardner had devised? As I discovered when I visited last week, the experience is now even stronger, even more concentrated. By removing the mechanics of museumgoing—the ticket-taking, the coat-checking, the restaurant, the gift shop—from Mrs. Gardner’s Venetian fever dream of a museum, the current administration has managed to intensify a visitor’s experience. Renzo Piano’s new building handles all the mundane matters involved with a visit. Then we step through a modest glass passageway into the world that Mrs. Gardner made. If anything, the galleries are more beautiful than I remember them, with a general sense of surfaces cleansed and polished and freshly illuminated. Titian’s Rape of Europa has never looked more beautiful, its fiery mythological mysteries rendered all the more mysterious by a setting where everything old really is new again.

Renzo Piano’s new building is a triumph of dialectical thinking. Piano has responded to the gilded and brocaded inwardness of Isabella Stewart Gardner’s Venetian palace with a structure that is all gleaming, transparent modern geometry. As an homage to Mrs. Gardner’s great central garden, Piano offers a magnificent, hi-tech greenhouse that you can look into as you enter the new building. And the Gardner’s legendary concert series will continue, now in Piano’s sensuously severe Calderwood Performance Hall, a tall, square, airy yet intimate room, with three balconies overlooking a central performance space. In its own way, Piano’s building is every bit as luxurious as Mrs. Gardner’s Venetian palace. This is, come to think of it, exactly the sort of museum that a Milanese billionaire would want to hold his art collection today; he might well commission Renzo Piano, a fellow Italian, to do the job. Boston has once again, through the generosity of the Gardner, been the beneficiary of the best that Italy has to offer. Where else in the museum world has a building program designed to bring an old institution into the twenty-first century demonstrated this kind of loving respect for the past? Under the guidance of Anne Hawley, the museum’s director, the Isabella Stewart Gardner Museum has brought off what amounts to a miracle.

Jed Perl is the art critic for The New Republic.