In May 2000 the British police were searching the home of a suspected member of al-Qāʿida in Manchester, and chanced on a terrorist manual written in Arabic. After the events of 11 September 2001, extracts from this manual were made available in an English translation. These extracts included instructions to be followed by undercover members of the organisation in order not to blow their cover; such a member should avoid manifesting his religiosity through his appearance or conduct. One point underlined in this connection was, in the wording of the translation, that he should ‘not get involved in advocating good and denouncing evil in order not to attract attention to himself’. In the same way, a brother travelling on a special mission ‘should not get involved in religious issues (advocating good and denouncing evil)’.

The duty which the terrorist manual thereby set aside is a central, and in some ways distinctive, feature of Islamic ethics. As the celebrated Sunnī scholar Ghazzālī (d. 1111) put it, every Muslim has the duty of first setting himself to rights, and then, successively, his household, his neighbours, his quarter, his town, the surrounding countryside, the wilderness with its Beduin, Kurds, or whatever, and so on to the uttermost ends of the earth. Of these demanding activities, all bar the first fall under the rubric of ‘commanding right and forbidding wrong’ (al-amr biʾl-maʿrūf waʾl-nahy ʿan al-munkar) – roughly speaking, the duty of one Muslim to intervene when another is acting wrongly.