 There is a striking contrast with discussions of underdetermination among scientists, who often emphasize instead the enormous difficulty in constructing compelling rival  theories.[16]  This contrast reflects a disagreement regarding how to characterize the empirical content of theories. Suppose that the empirical content of theory consists of a set of observational claims implied by the theory. Philosophers then take the existence of rival theories to be straightforward. Van Fraassen (1980), for example, defines a theory as “empirically adequate” if what it says about observable phenomena is true, and argues that for any successful theory there are rival theories that disagree about theoretical claims. If we demand more of theories than empirical adequacy in this sense, it is possible to draw distinctions among theories that philosophers would regard as underdetermined. Furthermore, even when scientists do face a choice among competing theories, they are almost never rivals in the philosopher’s sense. Instead, they differ in various ways: intended domain of applicability, explanatory scope, importance attributed to particular problems, and so on.