Douglas’s theoretical apparatus allowed her to think in original ways about almost any topic. In a lecture earlier this year at the Young Foundation, she discussed "enclaves," the small groups which at their most extreme become terrorist cells. Where others emphasise their strengths, she emphasised their weaknesses: how prone they are to splits and sectarianism, and how hard it is for their founders to enforce rules. To survive, enclaves create around themselves what Douglas called a "wall of virtue"—the sense that they alone uphold justice, while all around them are suspect—yet the very thing that bonds them together encourages individuals within them to compete to demonstrate their own virtue and the failings of their peers. The only thing that can override this fragility is fear of the outside world—and so sects, whether political or religious, peaceful or violent, feed off the hostility of outsiders, using it to reinforce their own solidarity. The implication is clear for western governments: in the long term, defeating terrorism depends on ratcheting fear down, not up, dismantling the "walls of virtue" rather than attacking them head on with declarations of war.