Do Extraordinary Claims Require Extraordinary Evidence?

This chapter looks at Carl Sagan's famous dictum: extraordinary claims require extraordinary evidence. The gist seems to be that one does not have sufficient reason to credit an extraordinary claim unless one also has commensurately extraordinary evidence to support it. This looks reasonable enough at first glance but its vagueness leaves it open to several interpretations, some of which are incompatible with the norms of rational inquiry. In particular, while Sagan's dictum is a justified skeptical response to claims that are known to be highly improbable or contrary to well-substantiated science, it is irrational and contrary to scientific objectivity to demand extraordinary evidence for those that are merely amazing or bizarre, and thus Sagan's dictum must be handled with caution in astrobiology. The chapter then sets out the conditions under which an appeal to Sagan's dictum is justified and those under which it is not, with special reference to existing and anticipated astrobiological debates.