At a press conference in October 1981, Ronald Reagan quoted Ibn Khaldun (1332–1406) in support of what is known as supply-side economics. Although the 14th- century politician and thinker wrote extensively about economics and was almost unique among medieval Arab writers in so doing, it is quite ‘marvellous’ writes Robert Irwin, the author of a new intellectual biography of this famous North African, that he ‘should have anticipated American Republican party fiscal policy’.