Pub Date : 2020-05-25DOI: 10.1093/oso/9780190915650.003.0008
S. McMahon
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.
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Pub Date : 2020-05-25DOI: 10.1093/oso/9780190915650.003.0005
Emily C. Parke
This chapter argues that the concept of “life” is used in several rather distinct ways: sometimes as an all-or-nothing phenomenon and other times as a matter of degree; sometimes referring to individual organisms and other times to communities; sometimes based on specific chemistries and other times on functions. In contrast to biologists in general, astrobiologists cannot take the status of their subject matter as living or nonliving for granted. There are at least two reasons to think astrobiologists need an understanding of what counts as life. The first is to set search criteria for finding “life as we don’t know it” in the universe. The second is to set success conditions conducive to agreement about when life has been found and when it has not. In addition to particular cases like the recent Mars finding by NASA, the meaning of “life” figures into a broader agenda in astrobiology: looking for biosignatures.
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