Listen. Here that? That is the sound of nothing being said. Sure, the banal everyday 'rain again?', 'see the telly last night?' can be overheard. But where is the conversation that matters?

Around the world, whether in the lands of free speech or in the regimes of despotic society, we are being increasingly censored. How else explain the prolific expansion of graffiti? Since the 1970s graffiti has been the scourge of politicians and the right. But the aesthetic value of graffiti has highlighted its form to artists as well as the open-minded public. In recent times, and here it is obligatory to mention Banksy, it has crossed over in to the mainstream art world, with pieces selling for thousands of pounds. Yet the authors of Written on the City, San Francisco residents, Axel Albin and Josh Kamler began to notice a new form of graffiti. Gone is the stylistic typography, beautiful to some, hideous to others, and in is the simple spray or a marker pen. For the first time it is what it said, rather than what it looked like, that matters. Such was the growth and impact of these sentences, Albin and Kamler set up www.writtenonthecity.com in 2006. It was an immediate success and has since logged millions of views. Now they have collated the most interesting in a compendium called Written on the City: Graffiti Messages Worldwide. They don't clutter the book with faux sociological explanations and only touch upon its history, instead they print more than 200 photos of messages scrawled on toilet walls, pavements and train carriages. The authors identify this phenomenon as a massive illegal conversation. Some of it is funny: 'hero for hire', some poignant: 'If your [sic] reading this you've won half the battle': some heartbreaking: 'you've brought me to my knees'.

Who writes these? What did they mean? The questions you ask yourself while flicking through this well designed book flood in your mind. And despite the lack of aesthetic merit, that is art in my book.