Today, that complexity of human motivation is being stripped bare. The art of building is pursued almost exclusively for single-minded commercial objectives. Increasingly, buildings are commissioned by distant companies owned by shareholders with no allegiance to the local community. New buildings are perceived as little more than commodities, entries in company balance sheets. Our "bottom line" economies ensure that there is no incentive to invest in ecological technologies that only pay off in the long run. There is no incentive to provide a public space such as an arcade nor any reason to landscape a building, or even plant a tree.
Look closely at an average commercial development and you can see just how pared down, how crude it is. After a century of refinement, the steel or concrete building has never been so cheap to build, nor built so cheaply. These barren structures, with their classical, neo-vernacular or modern facades chosen from catalogues have no allegiance to place or people.
While buildings of all types are being packaged and standardised, architects are being selected on the basis of lowest fees rather than the quality of their work. The profession is being restricted to turning round the cheapest, largest building in the fastest time. To complete this grim picture, our buildings are consuming half of the world's energy, dramatically destabilising the ecosystem in the process.
And yet architecture is such a fundamental part of our lives. It is the art form to which we are continuously exposed. It has the potential to nurture us all. Buildings should inspire, and our cities should celebrate society and respect nature.
It is often argued that modern life is about staying in the security of our homes, about shopping, working, and interacting across an electronic network, without any physical contact.
This is a view I wholeheartedly reject. Face-to-face contact, the chance encounter, proximity, these have been the essential components of humanity. Cities respond to this need. Modern technological opportunities make the public life of cities more essential than ever. The issue is how to evolve the concept of public life to embrace these new opportunities.
It is the mass of each building that defines the three-dimensional form of the public realm - a seamless and constantly changing sequence of spaces - the city's signature. We feel this in the compressed public realm of walled cities, when we are led through narrow alleys, then streets, eventually to emerge into the drama of an expansive city square or, in more open cities like Bath, into circuses, crescents and squares which define pure and generous geometric volumes. Even across the grid of New York, there is a checkerboard pattern of interconnected public spaces, from the pocket Paley Park, to the Rockefeller Plaza, and the magnificent Central Park.
Today, the overbearing enforcement of security, the imposition of entry fees to cultural institutions, the lack of public amenities and the dominance of the car are reducing public spaces to narrow pavements. And buildings are being designed as if they were stand-alone objects, rather than elements that enclose and shape the public realm.
Buildings enhance the public sphere in a variety of ways; they model the skyline, landmark the city, lead the eye to explore; they celebrate the corner of a street. They can contribute to a landscaped space, a public arade or a covered way. But even at the most modest level, the way in which the building's details - its paving, handrails, kerbs, sculpture, street furniture, or signage - are related to the human scale or to the touch has an important impact on the streetscape. A beautiful building must address these concerns.
In 1984, we entered, and lost, a competition to design the National Gallery's extension in London. We started by extending our survey well beyond the site, which had lain derelict since the war. To our surprise, we found that this small site held the key to unlocking the isolation of Trafalgar Square - once the heart of the Empire, now a polluted tourist trap encircled by traffic. It is only on the rare occasions when it hosts rallies, demonstrations or celebrations that it regains its civic role. We proposed to reintegrate the square by creating a pedestrian route, rising from Trafalgar Square to Leicester Square, through the site.
The linking of the two squares became the driving concept of the project. We proposed an open flight of public stairs from Leicester Square down to Trafalgar Square. The route passed through our building - taking up almost half of the ground floor and connected Trafalgar Square, through a generous galleria, beneath the busy road. Two independent, but crucial, public places were physically woven together by a relatively small building.
As well as framing public life, buildings are the servants of the people. This essential function raises the practical issue, of how to ensure that architecture keeps pace with their needs. A building that is a financial market today may need to become an office in five years, and a university in 10. A building that is easy to modify has a longer useful life, and uses its resources more efficiently. In social and ecological terms, a building designed for flexibility enlarges the sustainable life of a society.
Designing greater flexibility into our modern buildings inevitably moves architecture away from fixed and perfect forms. But when a society needs buildings that are capable of responding to changing requirements, I believe we must search for new forms that express the power of change.
My partner Renzo Piano and I designed the Pompidou Centre with this in mind. The building was not conceived as a monument but as a people's place where different ages, interests and cultures can come together. The centre contains a broad range of activity, from cinema, music and libraries to restaurants, events and exhibitions. We aimed to create a building where the activities themselves could dictate the building's form.
Our architectural response was to propose a framework of spaces that could be added to or removed, opened up or divided. And by organising all the columns, ducts, lifts and corridors externally, the floors - each at its largest the size of two football pitches - were left free of obstacles.
Access to activities was treated like a system of external public streets - free for anyone to use, and enjoying views of the piazza and the beautiful Parisian skyline. The escalators, streets in the air and viewing platforms, extended the public piazza on to the facade of the building, and created a series of open terraces and glazed galleries from which to see and be seen.
The scale of a building is not defined by its size alone but by the articulation of its parts. To reduce the apparent bulk of this large building, we created a facade that could catch and sculpt the light. A layered facade, a series of transparent screens, metal structures, with terraces and balconies, one behind the other. In order to make a building that could be dramatically altered in unpredictable ways, and still retain a coherence, we crafted a kit of parts that could be assembled in different patterns - a stacked and ever-changing medieval village, rather than a neo-Classical temple.
We must also consider how to adapt the vastly greater number of existing buildings. The issue of how our general architectural heritage is to be preserved raises fundamental questions.
Restoring old buildings to their supposed original condition is, I would argue, a spurious notion. Buildings have always been adapted, reshaped, redecorated, replumbed and relit. But this living process grinds to a halt in the face of over-zealous preservation. As a result, buildings become more inflexible, more expensive to convert and constrict new activity. Worse still is the practice of preserving the facades and hiding an entirely new building within. This expedient solution to preservation reduces an interesting building to a historical shell - heritage camouflaging a modern, and usually banal, commercial building.
By contrast, history shows us that even our very best buildings can be robustly altered and modernised to respond to contemporary needs, and do so by creating a dialogue between old and new. Here I am thinking of such examples as Scarpa's Castelvecchio in Verona, or Norman Foster's Sackler Gallery at the Royal Academy. Looking at the history of a building such as the Louvre, which has lived through almost permanent change for hundreds of years, yet still retains its unity and speaks eloquently of every passing age, I marvel at the continuum of culture that has resulted in its present form - glass pyramid included.
Today we are letting our architectural heritage choke our future. Preservation must be favoured over demolition of a good building and replacement by a poor one, but a building must not be preserved at the cost of stifling innovation. Making museums of our cities ossifies society. The historian Roy Porter sums it up: "When buildings take precedence over people, we get heritage not history."
Half of all commercial energy - that is, energy derived from fossil fuels - is consumed by buildings. Long-term running costs far outweigh the original capital outlay on constructing a building, and yet that is what we tend to focus on. Roughly three-quarters of "everyday energy use" is consumed in equal shares by artificial lighting, heating, and cooling. The challenge for architects is to develop buildings that incorporate sustainable technologies, and so reduce their pollution and running costs.
The typical office block was designed to create a sealed internal environment, that operated despite, rather than in conjunction with, the natural environment. This wasteful, high- energy approach led to the design of deep buildings with highly artificial internal environments. These buildings generate vast amounts of heat, which is caused by large, heavily occupied floors and the intensive operation of computers and machines. Add to all this windows so distant from most people that they require high levels of artificial lighting.
This heat load requires powerful machines to draw out hot, stale air and to pump in chilled, fresh, filtered and humidified air. The result is an environment that isolates people from nature, disconnects them from the life of the city - and achieves this in a grossly polluting manner.
Yet there are alternatives that can reduce energy consumption by as much as three-quarters. A halving of the energy consumed by buildings alone would reduce global energy consumption by a quarter. According to Scientific American, buildings of the industrialised countries consumed a value of energy estimated at $250bn in 1985.
Rather than rely on high energy consumption, architects are now beginning to explore building forms and technologies that can harness natural resources - landscape, wind, sun, earth and water.
In a competition project in Nottingham, the Inland Revenue specified a low-energy building. We set out to investigate all the practical means available in nature to produce a tempered environment. As so often in city developments, two sides of the site were polluted and noisy. However, one side bordered a quiet canal. We pushed the building to the edge of the roads and opened up a small public garden beside the canal. Since opening windows on all facades was not possible, we divided the building in two - basic administration at the back, and social functions and communal facilities nestling around the new garden. Between the two buildings, we created a central landscaped courtyard - a type of small ravine. Around this gently curving landscape, the two lines of building were linked by glazed bridges.
This landscape formed the visual focus of the building but also produced a useful micro-climate. An average tree, for example, absorbs carbon dioxide, gives off oxygen, transpires 380 litres of water a day and purifies the air in its vicinity. In the summer, trees give shade, limit heat-gain from the sun and reduce glare into buildings. Together with water, shrubs and plants, landscape filters pollution, humidifies and cools the air. Breaking up very deep buildings with landscaped courtyards creates local micro-climates that can condition air, while slimmer buildings allow more people to have views out, place windows close at hand, and reduce the need for artificial lighting.
Inside the building, air can be circulated by shaping ceilings and roofs aerodynamically and by connecting floors to a larger space, or atrium, that draws air through. As air in the atrium rises in temperature, the "stack effect" pulls warm air upwards, and this draws air out of connected spaces. The use of an atrium in a building allows for large floors, with visual contact between people, but also allows healthy ventilation.
To increase the natural draw of air out of the building, its roof profile can be smoothed to respond to prevailing winds. Buildings can be shaped to reduce their drag and consequent air turbulence in the vicinity. Architecture is becoming more streamlined as its forms interact with natural forces.
Low-energy techniques commonly reduce the total commercial energy consumption of a building by between a half and three-quarters. And Britain's temperate environment is well suited to this type of system.
Computer technology is one of the breakthroughs in the design of low- energy buildings. Programs now available can generate models that predict air movement, light levels and heat gain while the buildings are still on the drawing board. This significantly increases our ability to refine each aspect of the design of a building, so as to maximise the use of its natural environment. And it is computers that are giving buildings increasingly sensitive electronic nervous systems, able to register internal and external conditions and respond to individual needs.
New materials exist that are capable of changing from high insulation to low, from opaque to transparent, that can react organically to the environment, respond to the daily environmental cycle and transform themselves through the seasons. The future is here, but its impact on architecture is only just beginning.
Architecture is about pursuing the broadest needs of society, and about working our buildings into the cycle of nature. In my view, this is what brings our art back to its very roots.