Few people have influenced the waywe think about art more than John Berger. His television series Ways of Seeing was an attempt to see art and aesthetics as part of the world in which we live. On winning the Booker Prize for his novel G in 1972, he famously attacked the sponsors and announced that he would be sharing the prize money with the Black Panthers. Now 75, he talks to Patrick Wright about writing, painting, politics and over 40 years of tireless intellectual enquiry.