This month marks the 20th anniversary of 9/11, the day two planes, hijacked by members of Al Qaeda, flew into the world trade centre in New York City, killing thousands. A third plane hijacked plane crashed into the Pentagon that day, the headquarters of the US military, while a fourth crashed in Pennsylvania, after its passengers managed to divert it from its original target.  A 20-year war in Afghanistan was supposed to have eradicated Al Qaeda and Islamic terrorism, but last month, as the United States army was evacuating its personnel and allies from Kabul airport, ISIS K, a different Islamist terrorist  organisation, attacked the airport with suicide bombers, killing at least 60 civilians and 13 US troops.   Is it the willingness to use violence what defines an extremist? Or is it perhaps their extreme ideas, occupying the far ends of the ideological spectrums of politics and religion? Can the status quo ever be considered extremist? And what do people mean when they say that extremes meet - that extremists of all political orientations and religions have something deep in common? Quassim Cassam is professor of philosophy at the University of Warwick, and author of the just published book Extremism: A Philosophical Analysis. Leave us a review on Apple Podcasts. This podcast is created in partnership with The Philosopher, the UK’s longest running public philosophy journal. Check out the autumn season of online philosophy webinars: https://www.thephilosopher1923.org Artwork by Nick Halliday Music by Rowan Mcilvride