Cities are extraordinarily resilient places, as we witnessed immediately after September 11th in New York City, and adversity, far from undermining civic confidence, can bring about a renewed and determined spirit of community and common endeavor.

I must confess that it is something of a mystery as to why I have been asked to join you for this most important debate about how, where, or, dare I say, whether - a new generation of tall building should be built in our towns and cities.

I can only assume that, like a bottle of HP sauce, I am to be used as a means of adding a bit of piquance to the menu.... However, I can also imagine that my presence is about as welcome as a police raid on a brothel!

Having said that, I think it is greatly to the credit of Invensys that they have gone ahead with this conference as I have little doubt that the issues being debated here now have rather more significance even than they did before the appalling New York tragedy three months ago to the day.

I think I must explain that I accepted to come here today because I suspect there are many people who remain profoundly uneasy about the current plans for their city and who often feel powerless to affect them in any way.

Like me, I daresay, they may want to ask how the horrific events surrounding the World Trade Centre may affect the way we live and work in our cities, and the implications for the way we build in them? I rather doubt whether everything will go on as though nothing has changed, but neither, I am quite sure, will terrorism drive people away from urban centres.

Cities are extraordinarily resilient places, as we witnessed immediately after September 11th in New York City, and adversity, far from undermining civic confidence, can bring about a renewed and determined spirit of community and common endeavor.

Nor too, is it yet possible, I believe, to do more than speculate on the precise effects these events will have on the way future buildings are designed and constructed. After all, cities have endured, and absorbed, all manner of physical calamities throughout history and, in most cases, they have emerged stronger, with new ideas, techniques and technologies to help overcome their problems.

Here, in London, a rich renaissance heritage of brick, stone and stucco buildings owes much to the outlawing of building with timber after the Great Fire of London in 1666. Necessity is truly the midwife of invention.

Similarly, I suspect that the destruction of the World Trade Centre is unlikely to mark the end of tall buildings in cities, although it may require rather more to be expected of them before new ones are constructed. Doubtless, the challenges of finding ever more sophisticated ways to evacuate people in the event of emergencies, or devising more resilient engineering, will become more dominant in the future architecture of skyscrapers.

However, I do believe, it has now become more important than ever to question how such buildings should be built in future, and most crucially, how they may be designed to create not just safe, but also truly civilised environments?

These questions relate not just to the matter of height, but of scale and context, and London offers an ideal backdrop to the issues at stake.

London's built form stands firmly in the tradition of most European cities, dominated by low to medium-rise buildings. The city's buildings and skyline are overwhelmingly the product of an era when neither technology, culture nor economics enabled the construction of very tall buildings.

This has left a remarkable array of buildings that, whether large, medium or small, have lent a coherence and human scale to the urban environments they compose. In almost all cases, they worked within a code - either explicit or implicit, that expected, encouraged and enabled harmonious streets and neighbourhoods.

And they must be pretty harmonious for have you noticed how many architects actually prefer to live in such neighbourhoods and conservation areas - very often the ones constructed within an 18th Century context?

In my view, very tall buildings can undoubtedly threaten this sensitive balance. Indeed, they may very well wreck it.

Towers, of course, have long been very much a part of many historic city skylines (although at a considerably lower height than those being proposed today). But these Renaissance, Georgian and Victorian contributions to the skyline were usually as much associated with the notion of balance and hierarchy as the lower buildings around them.

This, of course, was because towers were almost entirely reserved for monuments with a special ecclesiastical or civic status. Yet the "skyscraper" in its modern form is something very different. Most obviously, it is a building whose function is utilitarian and commercial, rather than civic or sacred; a so-called "statement building" that is self-referential, and fulfilling no communal purpose whatsoever.

This, it seems to me, requires an exceptional degree of humility on the part of those who proclaim the benefits of such buildings, since, by implication at least, they are less concerned with civic status, community expression or public good, than they are with the benefits and pre-occupations of the businesses that pay for them.

These are giant buildings, with immense public visibility, but serving only a private, indeed, a privatised, purpose.

Now it has, of course, inevitably been suggested that holding this somewhat critical view amounts to little more than a nostalgic, and uniquely English, obsession with the past, and places too great an emphasis on the conservation of the historic city.

Yet if one looks at London's skyline, and compares it, say, to Paris, where building heights are regulated far more precisely, one is immediately struck by how much less is protected here than abroad! The current debates about tall buildings here in London would be unnecessary and superfluous in Paris!

In Berlin, too, where an immense programme of reconstruction and regeneration is going on - larger than in any other European city - the city leaders are insisting upon rigorous limitations to the height of new buildings. These kinds of approach can, I believe, help to achieve a far more coherent sense of harmony and civic self-confidence, than the alternative "free-for-all" that will leave London with a helter-skelter skyline.

To seek to protect historic views and vantage points, and oppose the planning of random new towers, is not, I believe, synonymous with supporting what some have rather disparagingly called a "museum city".

London is perhaps the most culturally and economically diverse city on the planet and the millions who flock here will, I imagine, continue to do so because of the cultural riches and historic architectural wonders that help it retain its unique vibrancy and attraction.

It is precisely this historic urbanity of a city built at a reasonable height, with dignity in its buildings and life in its streets, that continues to attract visitors in their millions. This isn't history as a museum; it is living history, and that's what needs to be protected.

The skyscraper or tower block has had a difficult history in the United Kingdom. One thinks of the gas explosion at the Ronan Point slab block in east London in 1968 - an event that triggered a widely welcomed moratorium on the building of more residential towers, but, sadly, too late for the hundreds of thousands who were already trapped in their new flats. Or the Centre Point block at the end of Oxford Street, built entirely for speculative purposes, and unoccupied for years.

Such examples - and there are countless others - have given credence to the notion that these structures are alien; that in their very scale and their functionalist aesthetics they simply don't "fit" within the city and are doomed to long-term failure.

It is this sense of inappropriateness, rather than any nostalgic reverence for the past, that has caused so many people to feel offended by very tall buildings.

So, the fundamental problem facing those who plan and design tall buildings is only partly, I believe, to do with the simple matter of building height (problematic though this can be), but rather more to do with the difficulties faced in connecting these heights into the fabric of a city.

The essential value and virtue of a city is almost entirely defined in how successfully it is able to help people connect, whether for formal or informal exchange. This crucial function is defined and expressed in the networks of city streets, squares, parks and plazas, all of which require disciplined and well-articulated buildings to form and frame them.

This, of course, has been the basic building block of urbanism for all of recorded history, and in all cultures, until the 20th century, when this template for the traditional (or timeless) city, became challenged by new notions of urbanism, the most potent of which was the Modernist vision of a city of towers in a new parkland landscape - a city on stilts and steroids.

Its greatest polemicist and practitioner was Le Corbusier, who looked forward to the day when the entire city of Paris would be razed and rebuilt; when the - and I quote - "wretched pitched roofs are swept away, along with the casual cafes and places for recreation?.that fungus which eats up the pavements of Paris"!

The consequences of making this vision a reality, as most now recognise, have been disastrous, producing the shattered urban wastelands that have desolated entire communities and disembowelled some of our greatest cities.

The human and economic costs of this catastrophic experimentation with industrialised building, and mechanistic planning, has been huge, and the consequences will remain for decades, even centuries to come.

Tall buildings will, by their very geometry and scale, always struggle to relate to the spaces that a city needs in order to work successfully. They cast long shadows; they darken streets and suck life from them; they tend to violate the sense of public space so vital to a living street, either by requiring a plaza to give them light, or by refusing to align with existing buildings, because of the difficulties of entering and servicing them without a large and cluttered forecourt.

They also create "hot spots" of pedestrian movement that are a major burden on nearby stations and pavements, and because their forms of architectural expression are towering pieces of sculpture, rather than a well-mannered contribution to the public realm, they seldom blend into either the streetscape or the skyline with good aesthetic manners.

"The city is a place where people learn to live with strangers" wrote the urbanist Richard Sennett. A place, in other words, where the values and virtues of sociability and exchange are learnt casually, intuitively, and for necessity as well as pleasure.

It is the very density and connectedness that the traditional city offers that makes such socialisation possible, and yet I fear that the tall, utilitarian building is ultimately alien to that purpose.

In geometric terms, it has more in common with an up-ended suburban cul-de-sac than with the creative networked urbanism of streets and squares.

Now it is true that there are some places where towers and streets have worked successfully together, and one thinks immediately of the example of Manhattan, with its uniform grid.

Yet Manhattan is, and will remain, unique in sheer scale and wealth, and its towers are, of course, far from the whole story of the city of New York.

Jane Jacobs' Greenwich Village with its tenements and brownstones, has been as much, or more, a part of Manhattan's liveability as the Trump Tower and the Rockefeller Centre.

It is interesting to think of the greatest, and most successful skyscrapers of Manhattan - like the Chrysler, Empire State and Woolworth Buildings. These astonishing achievements, built through the 1920s and 1930s, still obeyed many of the basic rules of the city, holding the street line and delighting the eye at street level with their craft embellishments.

Although they framed a new skyline with their pinnacles and spires, these were essentially elongated classical or gothic buildings, which did little damage to life at street level, and this is, I am convinced, why they are still so well loved and used today. Far more so, I hasten to say, than many of the glass perpendicular towers that replaced them, which snub the scale of the street, and stand in self-important isolation from the surrounding city.

The point I want to make is that I am not opposed to tall buildings purely because they are tall buildings. My concern is that they should be considered in their context; in other words, they should be put where they fit properly.

Mind you, I also happen to think there is a very legitimate concern about how tall these buildings should actually be. Trying to make them ever taller than the other person's building is surely taking the commercial macho into the realms of adolescent lunacy.

Just as a residential bungalow would be an absurd building in the City of London, so, perhaps, is a skyscraper, for similar, yet opposite reasons. Both are out of scale in a place that remains essentially a tight mediaeval grid of streets and lanes, enclosed by historic buildings of modest height.

So, if new towers are to be built, then it seems self-evident to me, that they should stand together to establish a new skyline, and not compete with or confuse what is currently there - as has already happened to a depressing and disastrous extent.

If clustered, then the virtue of height becomes something that can, in the hands of creative architects, be truly celebrated. This solution, so clearly the case in Manhattan or La Defence in Paris, requires locations where intrusion into historically protected views, either at height or at street level, can be avoided, and is, therefore, difficult to justify in places such as the City of London, where the pressure to build at height is often greatest.

Some have suggested that unless this pressure is released, by allowing new towers, then London's status as a financial capital will be endangered. I really do wonder.

Frankfurt, often quoted as the likely destination for financial services that may be denied accommodation if London refuses new towers, has failed to capture London's market, despite ruthlessly pursuing a glass and steel modernist vernacular.

I rather suspect that the enduring success of London's financial services economy has more to do with its legal environment, the tradition of honest dealing, the network of business connections, long-standing firms and an ethos of probity - what may be called "living traditions" - than with new glass towers.

This raises a further point too. The City of London is, without doubt, a hugely successful financial centre. But it is a social disaster! Only a handful of people live there; only the City churches and a few schools perpetuate the memory of its social life and culture; and the bleak towers, and many of the surrounding streets, stand deserted at night.

But just a few hundreds yards away, where low-level, smaller-scale buildings remain, people are flocking back. (Perhaps the architects and some of their clients are as well?!) In Clerkenwell, Spitalfields, Smithfield and the East End, the streets are teeming day and night, and the neighbourhoods are mixed and vibrant. The reasons aren't hard to fathom. These are places that have retained a genuinely urban architecture, built to a human scale and easily adapted to change.

Factories, workshops and warehouses become offices, loft apartments and shops. These are buildings that fit, that give something worthwhile to the streets and help them thrive; streets where the sunlight occasionally slants along the pavement and where the good manners, variety and craftsmanship of the facades are timeless.

People need to "fit" into the public realm and this is why we always used to cultivate manners, modesty, and gentleness. The same should be true of buildings, although I fear that so much of the Modernist aesthetic is based on the notion of "standing out" rather than "fitting in".

And what of the viability and sustainability of tall buildings? There can surely be no doubt that structural and security concerns will weigh heavily on those designing tall buildings in the future, but I daresay, we will be assured that ever more ingenious ways to overcome these problems will be found.

Yet, I can't help but suspect that the price of meeting these new demands, whether in direct construction costs, or valuable space forgone for new servicing, will place a considerable strain on the economic rationale of the buildings themselves.

For the high building is, for many reasons, already a very expensive and inherently unsustainable form of building. The huge costs of construction, the loss of useable space through building set-backs, circulation and service space, the maintenance and management obligations, all place strains on the commercial viability of building high.

One wonders how some of the new security and structural concerns that are now being raised, may further call into question the economic case for high-rise?

A new sporting stadium that seats 80,000, requires, by law, every one of the 80,000 spectators to be able to find a place of safety within eight minutes. (Interestingly, almost all such stadia derive the access and egress designs from the model used in antiquity, in particular the Colosseum in Rome). Yet, for tall buildings, there is no similarly strict rule for escape - and, indeed, were there to be so, then one wonders how viable the buildings would remain.

Similar concerns about tensile strength in the event of a serious collapse will now, I suspect, have to be incorporated into future designs. Thinking, and planning for the unthinkable has now become a genuine, and perhaps a permanent concern.

There are further viability challenges too, especially in social and environmental terms. The rush-hour strains caused by large numbers of people arriving and leaving from these new buildings, can only cause more congestion for a public transport system that can barely cope with existing numbers; what hope will there be if new towers are further to increase the commuters that use the trains and tube?

There is no doubt that towers, because of their very structural demands, also rely on huge amounts of electricity to power their lifts, air conditioning and other infrastructure. You cannot usually open windows - so nature's own cooling system is shut down. Heat losses escalate as one builds higher, because neighbours can no longer help to keep you warm. Externally, conditions can deteriorate, because of shadow, wind shear or echo.

I'm afraid I remain somewhat sceptical of the claims that are now being made for overcoming these and other problems, as they are often based on new technologies that remain unproven and in some cases even untested. I'm sure we would never contemplate using such approaches if buildings were cars or aeroplanes!

One of the fundamental principles of sustainable development, and I hope, of sustainable architecture, is the idea of building for change; the construction of buildings that can "learn" and adapt to new and unforeseen circumstances over time.

This is extremely difficult to achieve with very tall buildings, which are often constructed to meet the particular needs of financial institutions or individual global corporate organisations at a single point in time. As utilitarian buildings of great expense and inflexibility, they may be "fit for purpose", but that is almost always a very fixed "purpose" in a very changing world.

What happens when an occupier moves on, or needs a different kind of space? The American architectural critic, Vincent Scully, has written that "Skyscrapers mark the first architecture ever of which the basic principle is impermanence".

His point is clear in this country, where so many of the last generation of tall buildings are now obsolescent, and require either draconian refurbishment, or demolition.

If we are obliged, and there is no alternative, to having new tall buildings, at least here in the United Kingdom, then there could at least, surely, be some way of overcoming some of the difficulties I have mentioned.

Measures could include a clear policy on the location of new buildings, ideally in clusters away from any likely intrusion of important site lines or skylines. Other possibilities may include a requirement for buildings beyond a certain mass or height to be mixed in use, a home for a variety of businesses and residents as well, and capable of meeting differing needs over time.

Perhaps, too, any new towers should pay more attention to the base and the top of their structures.

At the base, new buildings should properly address the streetscape and help define a public realm that is truly public in form and function. Ground floors need to include shops, restaurants and other amenities that bring people into the building and help it integrate with the surrounding townscape. Let's have no more of the left-over spaces that so often masquerade as public amenity - what Thomas Wolfe entertainingly described as "a turd in every plaza"!

And at the top of these new structures, let's see genuine artistry that truly reaches the hearts and souls of those who look on, rather than the overblown phallic sculptures and depressingly predictable antennae that say more about an architectural ego than any kind of craftsmanship.

New buildings with their heads in the clouds should most certainly keep their feet firmly on the ground.... I have to say that I suspect the coming years will test the skyscraper model rather more fundamentally than ever before. The failure of the Modernist city of towers and the crude notions of zoning and segregation, has forced a re-think among planners that is, once again, beginning to encourage more traditionally-inspired city building at a human scale.

At last, people are beginning to see that what I can only call "living traditions" can help create lasting sustainability and enjoyable social exchange. I fear, though, that unless these concerns are shared more openly, honestly and widely, and especially perhaps, beyond these shores, then we risk witnessing a rapidly urbanising world population plunged into solutions that have already failed in much of the developed world.

There are some truly terrifying predictions that billions will be living in high-rise towers within the next few decades, nearly all in the developing world, and unless those responsible choose to learn from recent experience, both good, but also bad, then the consequences for the longer term may haunt many generations to come.

Ladies and gentlemen, there are many, like myself, who are not experts like yourselves but who are inspired and incredulous at the skills and technologies you have come to master.

However, this mastery of technique is different from a responsibility in its stewardship, and it is here, I fear, that errors have been, and continue to be made; never more so than when clever technique and technological innovation masquerades as a philosophy that rejects tradition, the inherited wisdom of our forbears. Must we go on throwing the baby out with the bathwater?

It has taken thousands of years to achieve the sophistication to create living cities, but just two or three generations to destroy them.

Ladies and gentlemen, it is, I believe, only by striving to integrate the best of the past with the best of the new; by tradition once again being defined and practised as something living and not dead, that we can be offered the choices that a truly contemporary architecture demands.