Anyone searching for hard evidence of improvements in human thought will be buoyed quite a bit by a visit to the corner of Pennsylvania Avenue and 10th Street downtown, where a most instructive architectural face-off between the 1960s and the 1980s has taken shape.

On one side of 10th Street we have the hulk of the east, the J. Edgar Hoover FBI headquarters, a building many people love to hate (with much justification). On the other side we have the bulk of the west, 1001 Pennsylvania Ave., a building many will like at first sight.

The FBI building, though not completed until 1975, is the 1960s entry, a behemoth that pushed reinforced concrete and the then-reigning brutalist style to absurd dimensions. It strains to impress -- and how! By contrast, the half-finished office building at 1001, not nearly so big but still a heavyweight (its perimeter measures 1,200 feet), takes pains to be civil and look old -- and is, and does.

This contrast between architectural styles and ways of thinking about cities bespeaks a healthy change in both. There are, to be sure, flaws. Each of these projects is a superblock -- not the best model for in-city development. When we build at this scale we guarantee the loss of much history, texture and charm. And the proliferation of upscale retail shops downtown is a mixed blessing: The new stores presumably can pay the rising rents, but as the old ones die the city loses something of its character.

Despite these reservations, and a few others to follow, the new office building is no small triumph. It was designed by the Washington architectural firm of Hartman-Cox (with assistance from Smith, Segreti & Tepper); researched for historic preservation by Mary Oehrlein; developed by Cadillac Fairview Urban Development Inc., the Canadian giant; and guided along by the Pennsylvania Avenue Development Corp., whose nervy counsel at difficult moments was important.

The first of its distinctions is that it pleases the eye. If one stands, say, on the southeast corner of the intersection of Pennsylvania and 10th, and looks for a while at the new building, one sees how quietly, comfortably, it occupies its sizable space. Like a big site-specific sculpture, it looks as if it belongs.

Partly this is because of the limestone slabs that sheathe the building's outer layer, up to 10 stories high -- a really beautiful material that catches the city's soft light. More important than the material itself is the skillful way it was handled by the architects. Taking cues from every building in the immediate vicinity except the FBI, they developed a crisp, subtle, classical vocabulary that suits the material and practically guarantees a contextual fit.

One advantage the classical mode has over the conventional modernist one is that it establishes rules by which part can be related to part in pleasing ways.

The long, horizontal Pennsylvania Avenue fac,ade of the FBI building is indeed an impressive thing, powerful in the way its recessed windows use sun and shadow to sculpt space. But it is a repetitive, inflexible system that emphasizes the building's bulk and extent. The subtleties of the newer building's fac,ades, on the other hand, are manifold. Variations in fenestration alone repay attention, from two-bay storefronts at the street through a succession of double-hung sash windows of differing depths and sizes that emphasize a clear progression from bottom to middle to top.

Details, such as the changing depths and forms of window lintels and sills or the curved corner cornices of the avenue fac,ade, are beautifully, if simply, modulated throughout. So, too, are the limestone surfaces: Note, for instance, how the framing effect of the middle-level piers is reinforced by diagonal scorings in the stone. That this treatment is more visible from a sharply angled point of view than from a head-on perspective enhances one's experience as one walks around the building. (This is definitely a structure to be seen in the round; each of the four fac,ades differs subtly from the others.)

It all comes so handsomely together that it looks effortless, although of course it wasn't. Designing 1001 was almost akin to reinventing the wheel, according to Graham Davidson, project architect for Hartman-Cox (George Hartman was the partner in charge). "A knowledge of architectural history was useful," he said, "but different from an understanding of how to manipulate and arrange columns, entablatures, cornices and so on to make a building. We set aside our experience of 10 or 20 years and the architectural history of the last 50 or 60 to use a different manner of designing, of making a building, of thinking."

I have emphasized the surfaces and the details because they are what first attract the eye, and hold it. The outsized skeleton they flesh out was conceived by Hartman-Cox five years ago in answer to two vexing questions: How does one disguise the bulk of such an enormous building, and how does one accommodate historic (or at any rate quite nice) older and smaller structures within such a potential monster?

The very idea of asking these questions is a sign of change. The FBI building, designed in the mid-'60s, again is a convenient foil, for it reflects all too well three qualities of the Pennsylvania Avenue plan at that time: a fixation with self-evident size, a predilection for abstract modern architecture and a pronounced hostility to historic preservation.

(It also reflects the desire of its principal client, J. Edgar Hoover, for one-of-a-kind monumentality. The irony is that, had the original plan been carried out, Hoover's building would have been replicated, more or less, on the north side of the avenue from the White House to the Capitol, though I suppose none of the duplicates would have had that towering battleship bridge in the back -- a structure so unbelievable it likely will be declared a historic landmark some distant day.)

Fortunately, in response to a number of forces, among which was a less than kind reaction to the FBI building itself, the avenue plan as reconstituted in 1974 and refined in the late '70s set the stage for responsible architects to ask those questions about size and preservation, and to answer them as brilliantly as Hartman-Cox did in its initial massing studies for 1001. The idea was strikingly simple: to give the building three distinct layers -- one right on the street, incorporating several old fac,ades; another higher and set back 20 feet; and the third still higher and set back yet another 20 feet.

This general scheme, suggested by PADC requirements to step back a certain amount from the avenue before attaining the permissible 160-foot height and to save a few older pieces on the site, has become the familiar Washington resolution to conflicts between old and new. But in few places has it been carried out with such certitude and finesse as here, and never in a project of such size. The setbacks are masterfully proportioned; the old buildings are by and large well integrated with the new construction; and each layer is given an appropriate change in character, shifting from the fully inflected street fac,ades to a sort-of midlevel depth in the middle to a "dumb box" in the rear.

In addition to the felicities of the exterior design, the public spaces on the ground floor are superbly planned and beautifully detailed. More lobby than atrium, the domed octagonal space at the center of the plan is accessible from four directions -- or will be this fall, when the northern portion of the building is complete. One can readily imagine taking a shortcut (or even a long cut) through the building, just to experience this space and the arched passageways leading to it.

However, the project is not altogether charmed. The reddish bricks of the handsome 1909 U.S. Storage Building on 10th Street, for instance, stand out a bit too much against the limestone and neutral blond bricks of the newer pieces. More important, the third-level box, with its rows of barely recessed double-hung windows, is unappealing where it becomes too visible. Thus the least fortunate view of the building is from the northwest, where the new pieces step down most dramatically to accommodate the low 19th- and early 20th-century commercial structures at the corner of 11th and E streets NW. Even more important is the realization that the rush to develop must stop somewhere: If we apply the "Washington solution" throughout the downtown historic district, it no longer will deserve the name.

But there is much to cheer about. To remind oneself how much, one simply has to look next door.