Astrobiology.net has a lengthy interview with Gerald Joyce describing his contributions to the search for exobiology-- i.e. life outside of Earth. Joyce was on a group selected to define what was being searched for-- he's careful to clarify that they did not try to "define" life in the scientific sense , which would be a whole topic in itself- but instead to extrapolate from properties of Earch life to define what they would search for outside of earth. As the old adage goes, "the universe is not only stranger than we imagine, it's stranger than we can imagine." Their search is going to have constraints, and constraints will exclude some phenomena even from consideration. I think it's an important habit of scientific investigations to be as explicit as possible, right at the outset, about constraints.