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.