These days, a relentless impulse to throw everything lying around the studio floor into contemporary art seems to dictate that a giant-size, interactive photo-text installation with video monitors and Dolby sound is inherently better than a plain old painting or sculpture. Early modernism's heroic attempt to distill visual art down to its absolute essentials - to create transcendently beautiful works that held their own amid skyscrapers and automobiles - has been largely, and conveniently, forgotten. Two magnificent retrospectives - one of the Romanian sculptor Constantin Brancusi (1876-1957), and the other of the Dutch painter Piet Mondrian (1872-1944) - not only resuscitate the glories of reductive abstraction, but also aspire to act as esthetic consciences in today's fragmented, junkyard scene. After a couple of hours with Brancusi (at the Philadelphia Museum of Art through Dec. 31) and Mondrian (at New York's Museum of Modern Art until Jan. 23), you can almost hear the two masters telling us it's time once again for a little simplicity.

Brancusi and Mondrian took very different paths to purity. Brancusi's transformation from a neophyte realist to a visionary abstract sculptor was sudden; Mondrian's evolution from a quiet landscapist to what he called "neoplasticism" was gradual, methodical and arduous. Mondrian tried out cubism - albeit halfheartedly - while Brancusi simply avoided it. Although they lived in Paris simultaneously for more than 20 years, the two apparently never met.

Brancusi was born in a peasant village. At the age of 28, he set out (allegedly on foot) for Paris. He worked for Auguste Rodin for a month and then quit. "Nothing grows under big trees," he said. He also rejected Rodin's method of modeling in clay before casting in bronze, in favor of the ancient technique of carving. Suddenly, in 1910, he was there: heavy chunks of stone were honed into graceful curves, folding in on themselves like birth and death rolled into one.

But Brancusi never turned entirely abstract. "Sleeping Muse," from 1910 - a polished bronze cast, taken from molds made from a carving - retains a melodic stylization of a head. Looking almost as if it occurred through some exquisite crystallzation we don't yet understand, the piece poetically commands a space much greater than it occupies. Compared with painting, however, sculpture is inherently limited in attempting to stand as a metaphor for a grander harmony in the universe - sculptures, no matter what the artist does to them, are still so obviously things in themselves. Brancusi's achievement in overcoming that limitation makes him the greatest sculptor of the 20th century.

In the 1920s and '30s, Brancusi's work took on a sleekness echoing the popular art deco and "machine age" styles of the time. Once, much earlier, at an aeronautics exposition, his friend Marcel Duchamp had been looking at a propeller and turned to Brancusi to ask, "Tell me, can you do that?" He certainly could, and his increasingly elegant work attracted a loyal coterie of what curator Ann Temkin calls "fancy American ladies," such as Nancy Cunard.

Brancusi's only known personal link to Mondrian is coincidental - the sculptor had a son by an English concert pianist named Vera Moore, to whom the painter later sold a work. …