Graffiti is the way that a society talks to itself. That's the theory of Axel Albin and Josh Kamler, a couple of designers from San Francisco who started photographing examples in the area around their studio and then became infected by the idea and began to garner examples from all around the world. Mind you, you know what they say about people who talk to themselves.
If graffiti is a kind of conversation it's a pretty solipsistic one, a dialogue of the deaf, peppered with Pinter pauses, for there are none so deaf as those who will not listen. The standard polarity is between those who insist that graffiti is a modern art and those who think it a vandalising nuisance.
There is, of course, something about graffiti that humanises. The oldest ones I've ever seen were in the Ephesus, the Greek city shaken to ruins by an earthquake AD614. The tourist guide pointed out scratchings in the great white stones which he said were an antique advert for a local whorehouse. In Pompeii the scorching ash wiped out human life but preserved the drawings and phrases etched into the walls recording abuse of the powerful, magic spells, declarations of love, smutty aphorisms and political slogans. They are the mere marginalia of a culture but they summon as in dreams the voices of long since buried men and women.
In the old days it was blade on stone, though a friend once stayed in the gatehouse in Windsor Castle where the staff pointed out to him the tiny initials A B in a pane of ancient sagging glass; they were scratched by Anne Boleyn, with her diamond ring, as she awaited transportation to the Tower of London. But the tools of modern choice are the spraycan, the marker pen, the sticker and the stencil. Much of this is mere self-indulgence – calligraphic signatures by which taggers mark out their territory as tomcats do with urine. When I lived in New York in the mid-1980s the city was a battleground between taggers, the most active of whom left his mark on 500 buildings, and the authorities who ran intensive graffiti removal programs. The city's mayor, Ed Koch, convinced that graffiti fuelled a general sense of squalor and a heightened fear of crime, spent $22m on zero-tolerance policing and a chemical wash for trains that dissolved the paint. The B-boy rebellion was crushed. But other rebels have taken their place.
Rebellion is at the heart of graffiti which, by definition, is inflicted on someone else's property. That revolt can be political, as in the words once daubed on the Berlin Wall or the images on today's Israel security barrier where Banksy painted a satiric hole revealing an idyllic beach on the other side. It can be poujadist, as in the curmudgeonly resident of Barcelona who painted: "Why call it the tourist season if we can't shoot them?" It can be cultural: "Your TV wants to own you" in New York. It can be ironic: "I just want to be your housewife". It can be humorous, as in "I still hate Morrissey". (Well, I thought that one was funny.) It can be all those things: "Wanted: dead kids for war" in Manhattan or "Create beautiful children: marry an Arab" in Tel Aviv. It can be brave, like the anarchist's "Against all authority" in Tehran. Or chilling, as was "Remember the flowers I sent..." in the same city.
But its great joy is its evanescence. It is, like the life it celebrates, impermanent. "Please don't take this sticker down," said the sticker in San Francisco. But somebody will. Somebody will.
'Written on the City', by Axel Albin and Josh Kamler, is published by How Books, £16.99. To order a copy at a special price, including free p&p, call 08700 798 897