 A fairly standard reply to this line of argument is to suggest that what Laudan and Leplin really show is that the notion of empirical equivalence must be applied to larger collections of beliefs than those traditionally identified as scientific theories—at least large enough to encompass the auxiliary assumptions needed to derive empirical predictions from them. At the extreme, perhaps this means that the notion of empirical equivalents (or at least timeless empirical equivalents) cannot be applied to anything less than “systems of the world” (i.e. total Quinean webs of belief), but even that is not fatal: what the champion of contrastive underdetermination asserts is that there are empirically equivalent systems of the world that incorporate different theories of the nature of light, or spacetime, or whatever. On the other hand, it might seem that quick examples like van Fraassen’s variants of Newtonian cosmology do not serve to make this thesis as plausible as the more limited claim of empirical equivalence for individual theories. It seems equally natural, however, to respond to Laudan and Leplin simply by conceding the variability in empirical equivalence but insisting that this is not enough to undermine the problem. Empirical equivalents create a serious obstacle to belief in a theory so long as there is some empirical equivalent to that theory at any given time, but it need not be the same one at each time. On this line of thinking, cases like van Fraassen’s Newtonian example illustrate how easy it is for theories to admit of empirical equivalents at any given time, and thus constitute a reason for thinking that there probably are or will be empirical equivalents to any given theory at any particular time we consider it, assuring that whenever the question of belief in a given theory arises, the challenge posed to it by contrastive underdetermination arises as well.