Donald Rumsfeld could have been a contender. In 1980 he was at the Republican National Convention in Detroit, pacing round his hotel room, waiting for Ronald Reagan to call and tell him that he wanted him to be his running mate. George H. W. Bush (the father) was in another room, awaiting the same call. Bush was chosen. His camera fixed on Rumsfeld's face, Errol Morris recalls the moment when it all slipped away:

He meets the camera's gaze, inscrutable. Nothing else is vouchsafed. A known unknown has been registered, not to be discussed. There will be more of those to come.

Famously the youngest and the oldest Secretary of Defense in the history of the United States, Rumsfeld is still in the ring. His complacent but disastrous tenure of that office from 2001 to 2006 under President George W. Bush (the son) is the focus of The Unknown Known (2014), a documentary based on thirty-three hours of interviews with its subject. Its director is in every sense an auteur. As filmmaker, interviewer, private investigator and blogger, Errol Morris has form. He has taken on a Secretary of Defense before, in The Fog of War (2003), an Academy Award-winning study of Robert McNamara; and he has tackled the grisly subject of Abu Ghraib, in Standard Operating Procedure (2008), at once an inquiry into the nature of the evidence and a compilation of witness testimony, out of the mouths of the perpetrators themselves.

Morris has been called an erudite gumshoe. It is an apt description, as his ‘Opinionator’ blogs in the New York Times amply confirm. ‘The certainty of Donald Rumsfeld’, a four-part series drawing on everything from Pascal's Pensées to Lewis Carroll's Alice in Wonderland, displays his characteristic blend of intellectual voracity and professional idiosyncrasy.