In this bewildering world, in which barely a week passes without some cruel and bizarre atrocity, it is not surprising that people seek a deeper pattern or meaning in events.

Some turn not to newspapers and television but to the fervid speculations of professional rabble-rousers and internet obsessives. In doing so they often become “conspiracy theorists”, a disparaging term for those who make it a point of honour to take nothing at face value. Typically, conspiracy theorists assert that atrocities were committed not by terrorists or lone fanatics but by those who claim to protect us, using them to impose ever more repressive restrictions on us or to advance secret plans for world domination.

A “conspiracy” is a combination of people intent on evil deeds. The word is first recorded in the fourteenth century, in Chaucer’s Canterbury Tales. Its source is the Latin verb conspirare, “to agree”. In 1997, the OED added an entry on “conspiracy theory”, which it defined as “a belief that some covert but influential agency (typically political in motivation and oppressive in intent) is responsible for an unexplained event”.