A fierce debate and petty spat erupted some months ago between Gilles Kepel and Olivier Roy, two of the handful of heavyweight stars of the strange world of French Arab and Muslim studies. Both are in their sixties and are internationally respected experts on “political Islam”, and they hate each other. Both have a large audience, a lot of students, fans and clients, and both compete for hegemony in academia, the media and political circles. Both have serious and inveterate enemies. Both have widely shared strong ideological biases, and find each other's opinions unbearable. Both seek recognition as the top academic explaining “political Islam” to French and Western audiences. After the 13 November attacks in Paris, both intervened early to “explain” what happened. Each attacked the other's position. Roy focused on “demand”, not on “supply”. His motto is: We are witnessing the “Islamisation of radicality”, not the radicalisation of Islam. In other words: Western societies are “producing” a generation of young people, not necessarily Muslim, who do not like what they see: liberal, permissive and unfair societies that no longer provide norms, morality, guidelines, reliable families and good prospects for a decent future. They feel estranged, they become radicals and they conduct a search for an ideology suggesting answers, proposing a new society, completely different, and they find “jihadism”, be it Salafist or otherwise. The new jihadists know nothing about religion, about Islam; they are more Westernised than we would think. They opt for the jihadist path without really studying religion. If “jihadism” did not exist, they would adopt extreme left ideologies or nihilism, or something else. This theory had been applauded and is popular. First of all, it explains why so many “white Europeans” with a middle-class background become enamoured with jihadism (later on, a debate would occur on the respective importance of converts and people coming from Muslim families within the jihadi movement). Second, it proposes a rigorous explanation of the radicalisation process, which looks sound and credible. Third, it is a robust answer to those saying, explicitly or not, that the problem is with Islam or with Muslim culture. The jihadists who attacked Paris or Brussels knew nothing or very little about this culture. They are Western products of Western societies; they embody the ills and the revolt of a Western generation. Before turning to jihad, their behaviour was typically Western: alcohol, dance, even sex, etc. Fourth, from Cairo we can easily overlook that Roy's scheme is also a clever attack against “tiers-mondisme”, a leftist ideology popular on campuses, saying that we cannot understand Muslims and contemporary Islam if we forget the “colonial” past and the fact that the Arab and Muslim world spent the greatest part of the 20th century in national liberation struggles, with great efforts to protect cultural authenticity. Further to this, we must take into account post-colonial Western society, its alleged racism, and systematic discrimination against non-white people, who are stuck in very poor neighbourhoods with no access to good jobs. In other words, for “tiers-mondisme”, modern jihadism is a reaction to systematic and institutionalised racism, which is the main cause of violent Arab behaviour. Roy says this does not explain why so many jihadists (at least 25 per cent) are converts, with European white, middle-class backgrounds. By proposing another explanation, Roy broadened his “support base”: a lot of people within the Western elites and the middle classes do accept the idea that “terrorism and jihadism have nothing to do with Muslim tradition”, that radicalism is a local Western product, but are fed up by the other “tiers-mondiste” theme: basically, “White people and Western societies get what they deserve.” But the scientific, ideological and political implications of Roy's thesis are huge. First of all, and most importantly, if Roy is right, destroying the Islamic State group is not the solution. It may be necessary, or may not (Roy seems to say it is not). But violent radicalisation will outlive it. The Islamic State tries to capitalise on it, but it does not create or sustain it: as long as Western societies are such a huge disappointment for their young people, you will have radicalisation and terrorist violence. This means that the fight against radicalisation should be an “internal security issue”, and that public policies should seriously address the damage inflicted by “the system”. A scientific (and in some ways political) implication is equally controversial. If Roy is right you do not need to speak Arabic to be a jihadist, and you are not compelled to become an expert on Muslim culture and theology, or study “the jihadist movement in Western society”. But you do have to know these societies, their economic and social systems, their ills, and to understand what public policies can or cannot do. Linked to this, understanding the jihadists' strategic and tactical choices does not require an understanding of ideology, of Ibn Taymiyya, of Maqdissi and Abu Qatada. The jihadists do not know these ideologies; they behave as Western radicals. Kepel saw in this a mixture of scientific stupidity and a direct attack on his work. And his reply was violent. The writer is a professor of international relations of the College de France and visiting professor at Cairo University.