  Exploring the Universe is one of the great unifying themes of humanity. Part
of this endeavour is the search for extraterrestrial life. But how likely is it
that we will find life, or that if we do it will be similar to ourselves? And
therefore how do we know where and how to look? We give examples of the sort of
reasoning that has been used to narrow and focus this search and we argue that
obvious extensions to that logical framework will result in greater success.
