Imagine that when you walked in here this evening, you discovered that everybody in the room looked almost exactly the same: ageless, raceless, generically good-looking. That person sitting right next to you might have the most idiosyncratic inner life, but you don't have a clue because we're all wearing the same blank expression all the time. That is the kind of creepy transformation that is taking over cities, only it applies to buildings, not people. Cities are full of roughness and shadow, texture and color. You can still find architectural surfaces of great individuality and character in apartment buildings in Riga and Yemen, social housing in Vienna, Hopi villages in Arizona, brownstones in New York, wooden houses in San Francisco. These aren't palaces or cathedrals. These are just ordinary residences expressing the ordinary splendor of cities. And the reason they're like that is that the need for shelter is so bound up with the human desire for beauty. Their rough surfaces give us a touchable city. Right? Streets that you can read by running your fingers over brick and stone. But that's getting harder to do, because cities are becoming smooth. New downtowns sprout towers that are almost always made of concrete and steel and covered in glass. You can look at skylines all over the world -- Houston, Guangzhou, Frankfurt -- and you see the same army of high-gloss robots marching over the horizon. Now, just think of everything we lose when architects stop using the full range of available materials. When we reject granite and limestone and sandstone and wood and copper and terra-cotta and brick and wattle and plaster, we simplify architecture and we impoverish cities. It's as if you reduced all of the world's cuisines down to airline food. (Laughter) Chicken or pasta? But worse still, assemblies of glass towers like this one in Moscow suggest a disdain for the civic and communal aspects of urban living. Right? Buildings like these are intended to enrich their owners and tenants, but not necessarily the lives of the rest of us, those of us who navigate the spaces between the buildings. And we expect to do so for free. Shiny towers are an invasive species and they are choking our cities and killing off public space. We tend to think of a facade as being like makeup, a decorative layer applied at the end to a building that's effectively complete. But just because a facade is superficial doesn't mean it's not also deep. Let me give you an example of how a city's surfaces affect the way we live in it. When I visited Salamanca in Spain, I gravitated to the Plaza Mayor at all hours of the day. Early in the morning, sunlight rakes the facades, sharpening shadows, and at night, lamplight segments the buildings into hundreds of distinct areas, balconies and windows and arcades, each one a separate pocket of visual activity. That detail and depth, that glamour gives the plaza a theatrical quality. It becomes a stage where the generations can meet. You have teenagers sprawling on the pavers, seniors monopolizing the benches, and real life starts to look like an opera set. The curtain goes up on Salamanca. So just because I'm talking about the exteriors of buildings, not form, not function, not structure, even so those surfaces give texture to our lives, because buildings create the spaces around them, and those spaces can draw people in or push them away. And the difference often has to do with the quality of those exteriors. So one contemporary equivalent of the Plaza Mayor in Salamanca is the Place de la Défense in Paris, a windswept, glass-walled open space that office workers hurry through on the way from the metro to their cubicles but otherwise spend as little time in as possible. In the early 1980s, the architect Philip Johnson tried to recreate a gracious European plaza in Pittsburgh. This is PPG Place, a half acre of open space encircled by commercial buildings made of mirrored glass. And he ornamented those buildings with metal trim and bays and Gothic turrets which really pop on the skyline. But at ground level, the plaza feels like a black glass cage. I mean, sure, in summertime kids are running back and forth through the fountain and there's ice-skating in the winter, but it lacks the informality of a leisurely hangout. It's just not the sort of place you really want to just hang out and chat. Public spaces thrive or fail for many different reasons. Architecture is only one, but it's an important one. Some recent plazas like Federation Square in Melbourne or Superkilen in Copenhagen succeed because they combine old and new, rough and smooth, neutral and bright colors, and because they don't rely excessively on glass. Now, I'm not against glass. It's an ancient and versatile material. It's easy to manufacture and transport and install and replace and clean. It comes in everything from enormous, ultraclear sheets to translucent bricks. New coatings make it change mood in the shifting light. In expensive cities like New York, it has the magical power of being able to multiply real estate values by allowing views, which is really the only commodity that developers have to offer to justify those surreal prices. In the middle of the 19th century, with the construction of the Crystal Palace in London, glass leapt to the top of the list of quintessentially modern substances. By the mid-20th century, it had come to dominate the downtowns of some American cities, largely through some really spectacular office buildings like Lever House in midtown Manhattan, designed by Skidmore, Owings and Merrill. Eventually, the technology advanced to the point where architects could design structures so transparent they practically disappear. And along the way, glass became the default material of the high-rise city, and there's a very powerful reason for that. Because as the world's populations converge on cities, the least fortunate pack into jerry-built shantytowns. But hundreds of millions of people need apartments and places to work in ever-larger buildings, so it makes economic sense to put up towers and wrap them in cheap and practical curtain walls. But glass has a limited ability to be expressive. This is a section of wall framing a plaza in the pre-Hispanic city of Mitla, in southern Mexico. Those 2,000-year-old carvings make it clear that this was a place of high ritual significance. Today we look at those and we can see a historical and textural continuity between those carvings, the mountains all around and that church which is built on top of the ruins using stone plundered from the site. In nearby Oaxaca, even ordinary plaster buildings become canvasses for bright colors, political murals and sophisticated graphic arts. It's an intricate, communicative language that an epidemic of glass would simply wipe out. The good news is that architects and developers have begun to rediscover the joys of texture without backing away from modernity. Some find innovative uses for old materials like brick and terra-cotta. Others invent new products like the molded panels that Snøhetta used to give the San Francisco Museum of Modern Art that crinkly, sculptural quality. The architect Stefano Boeri even created living facades. This is his Vertical Forest, a pair of apartment towers in Milan, whose most visible feature is greenery. And Boeri is designing a version of this for Nanjing in China. And imagine if green facades were as ubiquitous as glass ones how much cleaner the air in Chinese cities would become. But the truth is that these are mostly one-offs, boutique projects, not easily reproduced at a global scale. And that is the point. When you use materials that have a local significance, you prevent cities from all looking the same. Copper has a long history in New York -- the Statue of Liberty, the crown of the Woolworth Building -- but it fell out of fashion for a long time until SHoP Architects used it to cover the American Copper Building, a pair of twisting towers on the East River. It's not even finished and you can see the way sunset lights up that metallic facade, which will weather to green as it ages. Buildings can be like people. Their faces broadcast their experience. And that's an important point, because when glass ages, you just replace it, and the building looks pretty much the same way it did before until eventually it's demolished. Almost all other materials have the ability to absorb infusions of history and memory, and project it into the present. The firm Ennead clad the Utah Natural History Museum in Salt Lake City in copper and zinc, ores that have been mined in the area for 150 years and that also camouflage the building against the ochre hills so that you have a natural history museum that reflects the region's natural history. And when the Chinese Pritzker Prize winner Wang Shu was building a history museum in Ningbo, he didn't just create a wrapper for the past, he built memory right into the walls by using brick and stones and shingles salvaged from villages that had been demolished. Now, architects can use glass in equally lyrical and inventive ways. Here in New York, two buildings, one by Jean Nouvel and this one by Frank Gehry face off across West 19th Street, and the play of reflections that they toss back and forth is like a symphony in light. But when a city defaults to glass as it grows, it becomes a hall of mirrors, disquieting and cold. After all, cities are places of concentrated variety where the world's cultures and languages and lifestyles come together and mingle. So rather than encase all that variety and diversity in buildings of crushing sameness, we should have an architecture that honors the full range of the urban experience. Thank you. (Applause)