 In an influential discussion, Larry Laudan and Jarrett Leplin (1991) argue that philosophers of science have invested even the bare possibility that our theories might have empirical equivalents with far too much epistemic significance. Notwithstanding the popularity of the presumption that there are empirically equivalent rivals to every theory, they argue, the conjunction of several familiar and relatively uncontroversial epistemological theses is sufficient to defeat it. Because the boundaries of what is observable change as we develop new experimental methods and instruments, because auxiliary assumptions are always needed to derive empirical consequences from a theory (cf. confirmational holism, above), and because these auxiliary assumptions are themselves subject to change over time, Laudan and Leplin conclude that there simply is no guarantee that any two theories judged to be empirically equivalent at a given time will remain so as the state of our knowledge advances. Accordingly, any judgment of empirical equivalence is both defeasible and relativized to a particular state of science. So even if two theories are empirically equivalent at a given time this is no guarantee that they will remain so, and thus there is no foundation for a general pessimism about our ability to distinguish theories that are empirically equivalent to each other on empirical grounds. Although they concede that we could have good reason to think that particular theories have empirically equivalent rivals, this must be established case-by-case rather than by any general argument or presumption.