Painter Jasper Johns, a South Carolina country boy, an unknown in his 20s, became an art-world giant in 1958.

That was the year of his first show at Castelli's gallery, and he put it all together. He was on the covers of the magazines. He sold three of his new pictures - those bland but exciting flags and targets - to the Museum of Modern Art. "Jasper Johns," announced Time magazine, "is the brand-new darling of the art worlds bright, brittle avant-garde."

Johns is 47 now, and this week his face peers intently from the cover of Newsweek under a headline that yells "Super Artist." The art world is a fickle one in which young painters shine a while, then see their reputations crack. But Johns' has only grown.

His long-awaited retrospective, which opens in New York today at the Whitney Museum of American Art, and goes from there to Germany, Paris, Tokyo and San Francisco (no small-town stops for Johns) is imposing as a monument.

Johns is not a great-souled master. His best painting is restrained. He is not a tragic figure as were Pollock and van Gogh, nor does he have the flash of a Warhol or a Dali.

But he does fit. Teachers love to teach from his works because they make so many points. His austere alphabets and maps made of tiny action paintings, are read as a transition between anguished '50s painting and cooler '60s art. Johns is an ally of Rausehenberg's an heir of Duchamp's, a printmaker of awesome gift, and a conceptualist as well.

His pictures look expensive. His brushstrokes, drips and scribbles are never less then elegant, and his surfaces are luxurious as fine Persian carpets.

There is another reason for his towering reputation. Johns is - and makes his viewers feel - exceptionally smart. In almost every work he asks us to consider what is really real and what merely represented. But his pictures are not baffling, at least not for long, because they provide answers to the questions that they pose.

Is that American flag a symbol? Or a symbol, or just a painting of a flag. Unlike the abstract expressionists, whose handwriting he mastered, Johns does not bare his soul. Instead, he shows us shapes previously unquestioned yet utterly familiar - targets, the letters of the alphabet, numbers, the map of the U.S., flashlights, wire coat hangers - "things the mind already knowns."

Paintings that look "real" but of course are not, have pleased the minds and eyes of men since we lived in caves. Bisons scratched on the wall were not real beasts, but they were the next best thing. The trompe I'oeil game pictures of the 19th century fooled us by agreement - we knew the dead ducks hanging there were not drippling on the sofa. Johns' early works - his ale cans of hand-painted bronze, his alphabets and numbers - ring changes on the same delight. His represented numbers are the real thing.

Later he would mix reality and representation in other ways as well. The word "red" appears, beside a bright red brushstroke, but its letter are bright blue. Forks and spoons, cups and rulers, appear in his pictures sometimes painted, sometimes real.

We recognize an actor in a TV a dinstantly as a phony real person, and we deal with the same discrernment and discovery beore the works of Johns.

The painter entertains us, too, with many mild jokes and non-obscure allusions. The face on the real souvenir plate in his "Souvenir" of 1964 is not that of the President. It is Jasper Johns'. His "High School Days," a metal shoe with a mirror inset at the toe, reflects the viewer's eye, and reminds him of the rekes who placed mirrors on their shoes to peer up the girl's dresses. Often in his paintings Johns will represent some early work by Johns. Those who know his works are almost always in the know.

Critics love to promote his paintings - there is so much to talk about - but Johns doesn't really need them. The essay for his catalog was written not by a learned scholar, but by an amateur, Michael Crichton, author of "The Andromeda Strain."

Johns is at his best in his clearest paintings. Those that look like works by Robert Rauschenberg, his one-time New York painting friend, seem confused and overloaded. John's touch is not light.

It is cerebral, sure, considered. In his newest field paintings, the so-called "cross-hatched" pictures,the brushstrokes seem to oscillate between foreground and background, and the patterning obeys mathematical systems the patient viewer can d* ecipher. Johns draws beautifully. His paintings are intelligent and dense. They look like quality goods, which, indeed they are (prices for his early works have reached $270,000).

Viewers baffled by complete abstraction, or annoyed by self-indulgent painting of the '50s, may find within his pictures intellectual delight as they penetrate his visual puns and play his mental games. Johns teaches us small lessons about the ways we think, remember, represent, and see, and he does not mislead. But here we see him whole (perhaps three-quarters of his entire production is hanging in this show) and we don't see all that much. His intelligence is undeniable, his surfaces are beautiful, his quality is almost always very high, but the universe that he's explored so carefully in the end seems somehow limited by his self-imposed restraint.