Jasper Johns : A MOMA retrospective chronicles the many lives of an artist who, ultimately, is a true American master.

It’s said there are no second acts in American life, but apparently no one bothered to tell Jasper Johns. For, against all odds, the artist managed a dazzling reinvention of his distinguished career in 1972.

Johns’ challenging artistic motives didn’t change. A consuming need to maintain a freshness of visual experience has always driven his artistic program, ever since his distinctive work of the late 1950s that was pivotal to a wholesale transformation in contemporary art. Instead, what changed in 1972 was the avenue for achieving Johns’ driving ambition.

The sturdiness of that unbroken thread is made plain by the 40-year retrospective of his paintings, sculptures, drawings and prints that opened Sunday at the Museum of Modern Art. So is the nimbleness of mind essential to maintaining it.

Having spent nearly two decades making those now-famous pictures of flat, two-dimensional objects--the American flag, targets, numbers, the alphabet, maps--Johns suddenly did an about-face. He returned to abstract art, in visually dazzling pictures made from densely worked patterns of linear crosshatching, often painted on top of a layer of newsprint collage. The gorgeous, endlessly fascinating crosshatch paintings showed that stylistic questions of representation or abstraction were finally beside the point.

So Johns surprised everyone by performing an extraordinary Act 2. And if second acts are exceedingly hard to come by--well, the sense of dissoluteness and retreat in Johns’ art of the 1980s and 1990s might therefore be more than expected. How could we possibly hope for a riveting Act 3?

The MOMA exhibition sags badly in its final galleries, but only after some of the most thrillingly beautiful rooms I’ve seen in an artist’s retrospective. Indeed, the show begins full-tilt.

Johns destroyed most of what he made before 1955, when, at 25, he completed the amazing rendition of a 48-star American flag that launched his career. The stars and stripes greet you cheerfully at the show’s entrance, offering shining evidence of a fully formed artistic intelligence that deepens at an astonishing clip in succeeding rooms.

“Flag” is less a traditional painting than a laboriously hand-built construction. Canvas stretched over three wood panels creates a sturdy 5-foot-wide surface for the tactile application of pieces of newsprint, slathered with sensuous strokes of red, white and blue pigments suspended in translucent wax. Each brush stroke is a separate, distinct bit of visual mortar that holds the image together.

The painting--part two-dimensional image and part three-dimensional object--is thus both a representation of an American flag and, inescapably, an actual American flag. A delightfully paradoxical conundrum, “Flag” shows how images and symbols have their own lively, independent existence in the world, just the way any tangible object does.

The painting also shows how an image doesn’t just represent an object; more important, it represents an audience, too.

Think about it: A national flag is an object that represents a constituency of like-minded individuals. Whatever meaning it possesses is not intrinsic--not an essence contained within its decorative pattern. Instead, a flag’s meaning depends on the dynamic social use to which it is put.

Johns made the radical proposition that art is no different. When you look at a flag--or at “Flag”--you are being asked to pay attention, size up a situation, come to some decisions about where you stand in relation to it and weigh the consequences of your commitment. You are asked, in short, to give meaning to your own experience.

Forget the aristocratic turn of mind, in which the authority of artistic truth is handed down from above. Johns dares to mix it up instead on the contentious, horizontal plane of democracy. That’s finally what makes his art as American as apple pie, despite the acknowledged debts it owes to Marcel Duchamp and other aristocratic precedents in European Modern art.

It’s also what makes Johns a central cultural figure of our time. His subsequent work certainly has peaks and valleys; but, given his established stature, the brilliant transformation he then accomplished in the 1970s is all the more remarkable.

The dramatic nature of the change is emphasized in the show’s savvy installation: Work from 1954 through 1971 is laid out in a loose chronology on the museum’s first floor, while the downstairs galleries open with the startling introduction of the crosshatch motif in a big, untitled, multi-panel painting from 1972.

Looking at the subtle but complex linear patterns of such crosshatch paintings as “Scent,” “The Dutch Wives,” “Dancers on a Plane” and the “Corpse and Mirror” series is like watching pure thought unfold--but it’s thought experienced as disarmingly sensuous.

The crosshatch patterns are sophisticated in their design. In “Dancers on a Plane,” linear patterns of red, yellow and blue repeat or mirror one another within each of six rectangles, rather the way couples do in a square dance or like the movement of a corps de ballet. (For many years Johns designed sets and costumes for the choreographer Merce Cunningham.) As you scan the surface in an effort to make sense of the complex patterns, checking rational vision against gut intuitions, your visual memory keeps slipping, like overloaded circuitry.

Your mind shifts gears and you begin to feel the patterns in your bones. Using art to enliven bodily knowledge has been important since the Renaissance and Johns is a master at it.

As MOMA curator Kirk Varnedoe writes in an insightful catalog introduction, artistically there’s a mindless quality to executing carefully planned crosshatch motifs. But, for the viewer, the visible “mindlessness” has the surprising effect of unlocking bodily responses. Oddly, these abstractions thus recall Johns’ representational flags, targets and numbers from the 1950s and 1960s--pictures of “things the mind already knows,” as the artist once famously described their motifs.

Using things the mind already knows has the salutary effect of opening up a powerful territory. The vigorousness of lived experience struggles with the constraints of memory.

The struggle remains central to Johns’ paintings from the 1980s and 1990s, but with an important difference. Now he incorporates highly personal images--a remembered floor-plan of a childhood home, tests from old perceptual psychology textbooks, disguised fragments of favorite paintings--which together give the pictures a distinctly autobiographical edge. For the viewer, though, such highly personal images are just so much esoterica.

These disappointing paintings suggest puzzles or rebuses. They intimate that a “correct” experience is hidden deep inside, waiting to be revealed--an intimation that shuts down all possibility for lively interaction.

The retrospective thus ends in an academic slump. But in the 1950s and again in the 1970s Johns made some of the most remarkable works in all postwar art. The achievement is immense.

* Museum of Modern Art, 11 W. 53rd St., New York, (212) 708-9400, through Jan. 21. Closed Wednesdays. The show will travel to Cologne, Germany, and Tokyo.