 Many philosophers hold that evidence is not sufficient to determine which scientific theory we should choose. Scientific theories make claims about the natural world that extend far beyond what can be directly established through observations or experiments. Rival theories may fare equally well with regard to some body of data, yet give quite different accounts of the world. Philosophers often treat the existence of such rivals as inevitable: for a given theory, it is always possible to construct rival theories that have “equally good fit” with available data. Duhem (1914 [1954]) gave an influential characterization of the difficulty in establishing physical theories conclusively, followed a half century later by Quine’s arguments for a strikingly general version of underdetermination (e.g., Quine 1970). The nature of this proposed underdetermination of theory by evidence, and appropriate responses to it, have been central topics in philosophy of science (Stanford 2009 [2016]). Although philosophers have identified a variety of distinct senses of underdetermination, they have generally agreed that underdetermination poses a challenge to justifying scientific theories.