In 1963 David Ogilvy, the father of Madison Avenue and author of a classic business book,“Confessions of an Advertising Man,” wrote: “An advertisement is like a radar sweep, constantly hunting new prospects as they come into the market. Get good radar, and keep it sweeping.” Half a century later advertisers are at last taking him at his word. Behavioural profiling has gone viral across the internet, enabling firms to reach users with specific messages based on their location, interests, browsing history and demographic group. Ads can now follow users from site to site: a customer who looks online for flights to Frankfurt will be inundated with German holiday offers. Conversant, a digital-marketing firm uses an algorithm to deliver around 800,000 variations of an ad to its big clients’ prospective customers to make it as irresistible as possible. Kraft, a food company, monitors online opinions on its brands in an office which it calls “the looking glass.”