For centuries after the glory that was Rome had degenerated, good architecture was still understood to be Classic architecture.

"The challenge and response," which according to historian Arnold Toynbee, brings all great periods of art and architecture to their end, did not come until the 12th century. When they did come, when pointed arches with their lofty structures rose all over western Europe, the critics called them ignorant, perverse and barbaric-in a word, "Gothic."

Today, with all the degeneracy of the Modern movement and all the cracks about "form follows fiasco," good architecture is still understood to be bare and square "Functionalism."

For that reason, the only serious challenge and response to date-the most recent work of Philip Johnson-is derided by the New York critics as silly, mannerist and, in a word, "kitsch."

Johnson was in town last week to pick up his $100,000 Pritzker Architecture Prize, an international honor, and the first of its kind. The prize, donated by the Hyatt hotel people, serves, in the words of its jury chairman, Sir Kenneth Clark, "to focus public attention on a branch of human endeavor by which our civilization will be judged in the future."

Johnson, I think, added a little glitter to the branch. The New York oracles, to be sure, do not like his most glittering latest work, the design for the 40-story Pittsburgh Plate Glass Industries headquarters in Pittsburgh. But I think he deserves the Pritzker prize just for what he has done for skyscrapers lately.

A year or so ago, the critics and, for that matter, the entire architectural profession, were outraged by Johnson's design of "the Chippendale skyscraper." It is his proposal for the new AT&T headquarters building on Manhattan's Madison Avenue and features an enormous hallway entrance, a pronounced granite facade and a roof structure in the shape of a broken pediment, an ornamental form often found on 18th-century English furniture.

AT&T was, as I noted at the time, as radical a departure in Johnson's architectural style, as Le Corbusier's irrational Ronchamps chapel of 1956 was a departure from his white, Cubist "machines for living."

Now, in Pittsburgh, Johnson compounded the heresy by creating what at first glance looks like a Disneyland rendition of an English Gothic tower, done in reflective glass, no less.

At first glance, I was as shocked by the sketch as my New York colleagues. Johnson is not easy to cope with. He thinks no more of consistency than Emerson. But worse, his hobgoblins are those of a brilliant mind. And for all his almost mystic in Architecture, he is refreshingly lucid.

That's tough.

It is easy, you see, to stand in awe and write in wonderment about such mystics as Louis Kahn. Nobody pretends to understand. It is a matter of sharing (or not sharing) an experience.

Philip Johnson does not only design architecture, he designs architectural philosophy. He is immersed in its history, he thinks and he talks about it incessantly but clearly. You have to take him at his word. And with Johnson, the Word, the gospel, keeps changing-not once, as with Le Corbusier, but along with a continuing search like that of Picasso. We are not used to architects doing this.

Johnson turned from art historian to practicing architect in his late 30s (young people call this age "mid-life") and started as an ardent disciple of Mies van der Rohe. He was also, to use his own words, the U.S. "propaganda bureau" for the bare and square Modern style. His trouble is that his critics still believe his early propaganda.

Johnson himself, however, just could not forget history. For several decades he kept flirting with - his words again - "sporadic, superficial historicism, mixed lightly over an International Style base." His Lincoln Center Opera House is one example. Much of that historicism smacked of "styling," probably because of that International Style base.

Johnson knows it. He now proudly calls himself "eclectic," which used to be a very dirty word. And he feels "free to roam history at will."

He feels, he says, "new sympathy for 'the-style-to-suit-the-job' attitude." He has become an admirer of the eloquent British architect Sir Edwin Landseer Lutyens, who died almost 50 years ago and sprinkled the British empire with his creations, including Her Majesty's embassy here on Massachusetts Avenue.

Lutyens was the last good architect who got away with designing one building in Norman and the next in Egyptian style, simply because these buildings were audacious, they made people feel good, they seemed appropriate to their purpose and they worked.

Johnson and his partner John Burgee have taken a similar approach to AT&T and PPG.

Johnson's new concerns now are variety, historic continuity (he calls it "recall") and the sense of place (he calls it genius loci. And he has suddenly discovered that skyscrapers, the foremost landmarks of the Modern movement, are plunked into the fabric of our cities like stabs from outer space. With rare exceptions (one is Rockefeller Center), they have no roots in the cityscape. They just shoot up from the pavement or their plastic little plazas, deadening, rather than enlivening city life. Park Avenue or Sixth Avenue, are deserts, despite the hundreds of thousands of people who work there.

The AT&T is tied down to Madison Avenue by hugh inviting passageways and arcades lined with shops. It harmoniously joins the Manhattan sckline, because like most others, it is not an insipid slab but a tower that is properly shaped and dressed.

The PPG headquarters complex is also carefully woven in Pittsburgh's downtown texture. Its lower buildings frame one square, along Market Street, and newly create another at the tower entrance. These squares as well as the rest of the buildings are lined with arcades for shops and cafes. They will make it pleasant to walk through an ever-changing and surprising sequence of different spaces, an experience that makes strolling in Verona or Bologna unforgettable.

Vaguely reminiscent of the High Victorian Gothic towers of Parliament in London and echoing the Pittsburgh University's Tower of Learning, the PPG tower, too, blends into the Pittsburgh skylines. It is a skyline of individual, self-assertive shapes. Only the ugly, massive U.S. Steel Building is nothing but an oversized slab. Johnson's pinnacles or turrets enhance the drama.

The PPG facade is even more strongly articulated than that of AT&T. Its vertical emphasis makes the tower look at once soaring and earth-bound.

Reflective or mirror glass is, to be sure, the worst abomination modern architecture has inflicted on us. But used as Johnson, proposes to use it, on facets and angles reflecting against each other, it will, or so says Johnson, dramatize the play of changing light and shadows. That play, Le Corbusier once said-correct et magnifique "-is the essence of architecture.

The PPG people assure us that the building will also be highly energy efficient and incorporate important innovations still being worked on.

"John Burgee and I had fun the last few years with shapes and funnels, plazas, and all the rest," Johnson said the other day. "What a grand period for us to live in today!"

"Contrariwise," he added, "what will all this sound like in 10 years? CAPTION: Sketch, Philip Johnson's design of the new PPG headquarters