Arguing from Ignorance

It might seem that it can never be rational to argue from a state of ignorance. But this is not the case. Ignorance arguments abound in our everyday thinking such as when we conclude that a train does not stop in York because York is not listed in the timetable. These arguments also tend to be the first line of defence when the safety of food or prescribed medications is called into question. The statement ‘There is no evidence that beef is unsafe’, which is a premise in an ignorance argument, was used so extensively by health officials and government ministers during the BSE epidemic in the UK that it came to be called the mantra of the BSE affair by the public inquiry into the epidemic. A similar ignorance premise is often used to frame the conclusion of systematic reviews into the effectiveness of a particular medical or health intervention. This chapter examines the various purposes for which ignorance arguments are advanced in medicine and health and considers the epistemic conditions under which these arguments are more or less rationally warranted.