particular segment of S that we might examine, we cannot be sure of precisely what regions of the world will behave in ways that are consistent with the patterns identified by molecular biologists. This is not to say, though, that the molecular biologists have failed to give us any interesting information—as we saw, universality (or even a rigidly defined domain of applicability) is no condition on predictive utility. To put the point one more way: though the special sciences are differentiated from one another in part by their domains of inquiry, giving an exhaustive account of exactly what parts of the world do and don't fall into the domain of a particular science is likely an impossible task. Even if it were not, it isn't clear what it would add to our understand of either a particular science or of science as a whole: the patterns identified by molecular biology are no less important for our not knowing if they do or don't apply to things other than some of the systems on Earth in the last few billion years; if molecular biology is forced to confront the problem of how to characterize extraterrestrial living systems, it is certainly plausible to suppose that its list of patterns will be revised, or even that an entirely new science will emerge from the realization that molecular biology as thus far conceived is parochial in the extreme. Speculating about what those changes would look like—or what this new special science would take as its domain—though, is of little real importance (except insofar as such speculation illuminates the current state of molecular biology). Like the rest of the sciences, molecular biology takes its problems as they come, and does what it can with the resources it has.
If we can't say for any given special science what exactly its domain is, then, perhaps we can say a bit more about what the choice of a domain consists in—that is, what practical activities of working scientists constitute a choice of domain? How do we know when a formerly singular