Site unseen: A city, a process, a cancer? In his new book, the historian roy porter dissects London and its social history. Here is a specimen

Covent Garden has worn numerous guises, some planned, some accidental. Its history has been shaped by the Crown, by grandees, by commerce, by local government and by the will of the people. In Little Dorrit Dickens documented all the different Covent Gardens: Courtly ideas of Covent Garden, as a place with famous coffee-houses, where gentlemen wearing gold-laced coats and swords had quarrelled and fought duels; costly ideas of Covent Garden, as a place where there were flowers in winter at guineas a-piece, pine-apples at guineas a pound, and peas at guineas a pint; picturesque ideas of Covent Garden, as a place where there was a mighty theatre, showing wonderful and beautiful sights to richly-dressed ladies and gentlemen, and which was forever far beyond the reach of poor Fanny or poor uncle; desolate ideas of Covent Garden, as having all those arches in it, where the miserable children in rags among whom she had just now passed, like young rats, slunk and hid, fed on offal, huddled together for warmth, and were hunted about (look to the rats young and old, all ye Barnacles, for before God they are eating away our foundations, and will bring the roofs on our heads]); teeming ideas of Covent Garden, as a place of past and present mystery, romance, abundance, want, beauty, ugliness, fair country gardens, and foul street-gutters; all confused together . . .

It has rarely been just one thing at a time. Despite everyone from Inigo Jones to the GLC, it has never remained what its planners desired - and who can predict what it will be next? And such is the story not just of Covent Garden but of the metropolis at large. Everywhere continuity and change coalesce; forms and functions mutate; past buildings and townscapes enhance but inhibit the present; the future refashions the debris of the past.

People make cities, and cities make citizens. The townscape each generation inherits is a support but sometimes also a strait-jacket. The great city is a wonder of nature, or rather of civilisation. In a castle or camp, living-patterns may be stringently controlled. Lord and priest, soil and crops dictate rules of life to villages, regulating activities, policing peasants. But the giant city knows no great dictator. It has a varied cast - magistrates, landlords and property-owners, markets, migrants, manufacturers -who improvise the urban drama - conflicting, competing, creating new scenes. And all such human players collaborate with great impersonal forces: rivers, climate, soil, trading opportunities, population pressures, market forces. Not least, cities function within force fields determined by distance, communications systems and layout, and are shaped by exogenous factors - regional, national, continental, global. They possess internal topographies, part spontaneous, part planned, zonings for homes and markets, for rich and poor. And all the time, as noted with Covent Garden, the elements fluctuate. Cities are not machines, built once and for all, but remarkable organisms. Like a coral reef, the city is always becoming a product, but that is because, first of all, it is a process.

Cities thus afford special fascination for the historian, being outgrowths of human aims but also the effects of a concatenation of activities beyond central control. The great city assumes a life of its own, and super-cities such as London are always spilling beyond their limits, by eating up not just the countryside but also the imagination - transcending ideas about what a city should, or even could, be. London grew bigger than any imagined city, provoking observers to call it a cancer, fatal to itself, lethal to its citizens, mortal to the kingdom.

London: A Social History is published next week by Hamish Hamilton at pounds 20