Does free speech include a right to cause offence? Many thinkers have insisted that it must - but debate has raged for millennia over where the limits to insult can be set. While some maintain Enlightenment values must include permission to shock, offend and even injure, there is a growing sense that rights must be balanced by responsibilities to one's community, in speech as well as action. And as technology has given each of us an worldwide platform to express any idea, anywhere, the potential for instant, global offence has only grown. How are we to define how much is too much - and what really distinguishes insult from injury? Edward Stourton speaks to historians, theologians and philosophers to explore the outer limits of free expression.