Freedom of speech is one of the most divisive issues in the world today. In many cultures, it’s regarded as dangerous: in China, India, across Asia and the Middle East, and under authoritarian regimes everywhere, men and women are regularly punished for expressing heterodox views about politics, sex, or religion. In the west, we try to uphold the opposite ideal – that freedom of expression is vital to a free society. It’s one of our core values: as Americans, Britons, Germans, Danes, Australians, or however we choose to define ourselves. Yet at the same time, we can never agree on its limits: we fight about free speech all the time. Our national attitudes are so different that the same words can be criminal in France or Italy but legal in Britain or the US. And, of course, even within our national cultures we are hopelessly divided. The meaning of free speech has never been more contested than it is right now.

What is Free Speech? The History of a Dangerous Idea (Penguin and Harvard University Press, 2025) seeks to address two big questions. The first is – Why is the western world so distinctive in its attitude to free speech? Why do we celebrate it, whilst most of the world runs scared of it? But the second question is – Why, in the west, do we disagree so much on what freedom of speech really means? Where does this discord come from, and what’s its significance? Should we even speak of free speech as a ‘western’ value?

These are pressing political problems, which my work is intended to illuminate. But they are also huge historical questions, which have never been asked before. As my book will show, our particular, modern ideals of free speech date back to the 18th century, but our global and national disputes over it have even deeper historical roots.

My three-part BBC radio series The Invention of Free Speech grew out of my early work on this subject.