You say to brick, “What do you want, brick?” And brick says to you, ‘I like an arch. “And you say to brick, “Look, I want one, too, but arches are expensive and I can use a concrete lintel … What do you think of that, brick?” Brick says, “I like an arch. "

What Kahn meant with this little fable was that great architecture is never arbitrary. Even the lowliest material should be used for what it does best.

But only a fool would accuse Kahn of sounding New Agey. Though he’s been dead for 17 years, Kahn has a powerful appeal in our depressed times. His buildings’ big, hefty forms and down-to-earth materials are a strong antidote to the flimsy construction and slick design all around us, and his ideas for a transcendent architecture speak to a certain spiritual emptiness. This huge show (which will travel to Paris, New York, Japan, Los Angeles, Ft. Worth, Texas, and Columbus, Ohio) tries hard to get inside Kahn’s head as well as to illustrate his complex career. It traces his life from his Philadelphia boyhood (he was 5 when he came from Estonia), through the Depression and World War II, when there was little architectural work except public housing and planning project 50 when he got his first major commission, the Yale University Art Gallery.

Kahn was a modern architect who asked some very unmodern questions. “What does the building want to be?” he would inquire of a proposal for a school, a museum, a church. He didn’t look for literal ideas the way a postmodern architect might quote a classical pediment; he searched for deep, subconscious associations and tried to make abstract forms reflect them. He thought the Salk Institute, overlooking the Pacific in La Jolla, Calif. (1959-65), shouldn’t be just laboratories but include places where scientists could contemplate. He seized the idea of a monastery and added small studies. In his National Assembly Building in Dhaka, Bangladesh (1962-1983), he wrapped the structure in concrete screen walls with gaping cutouts, a primeval fortress against the heat.

In Italy in 1951, he’d absorbed Roman architecture. “Our stuff looks tinny compared to it " he said. What marked his work more and more were massive walls and the honest beauty of materials like poured concrete. He wanted his work not to look like ancient architecture, but to have the same timeless essence.

And the massiveness was countered by the natural light that animated his interiors. Nowhere did Kahn join monumentality and light more eloquently than in the Kimbell Art Museum in Ft. Worth (1966-72). With its rows of concrete barrel vaults, each slit lengthwise to let light filter into the serene galleries, it is one of the great American modern buildings.

Architect Arata Isozaki has designed the exhibition, with fragments of walls and arches, based on an unbuilt plan of Kahn’s for a synagogue-so the show, too, suggests ruins. It is packed with Kahn’s architectural sketches and wonderful pastels from his travels, as well as models and photographs. But at times the arrangement is confusing.

Kahn was one of our great architects, even if some of his monolithic buildings are hard to love. He arouses strong passions (right now there’s a movement to save the magnificent Salk Institute from an intrusive addition). The passion isn’t just for what Kahn built, but for the poetic thinking he brought to the form and function of modern architecture.