One of the most important journalists of his generation, Alan Rusbridger edited the Guardian for two decades, helping to break international mega stories that included Snowden, Wikileaks and phone hacking. Now editor of Prospect magazine, here he reflects on the dangers of misinformation and disinformation on social media platforms, not least on Elon Musk's X, the need to counteract alternative facts, such as those promoted by Donald Trump, the importance of impartiality at the BBC, and what he believes is at the core of good journalism.