 The argument from underdetermination is contested in a number of ways. One might, for example, distinguish between underdetermination in practice (or at a time) and underdetermination in principle. In the former case, there is underdetermination only because the data that would support one theory or hypothesis at the expense of another is unavailable, pending foreseeable developments in experimental technique or instrumentation. Here, realism is arguably consistent with a “wait and see” attitude, though if the prospect of future discriminating evidence is poor, a commitment to future realism may be questioned thereby. In any case, most proponents of underdetermination insist on the idea of underdetermination in principle: the idea that there are always (plausible) empirically equivalent rivals no matter what evidence may come to light. In response, some argue that the principled worry cannot be established, since what counts as data is apt to change over time with the development of new techniques and instruments, and with changes in scientific background knowledge, which alter the auxiliary assumptions required to derive observable predictions (Laudan &amp; Leplin 1991). Such arguments may rest, however, on a different conception of observation than that assumed by many antirealists (defined above, in terms of human sensory capacities). (For other responses, see Okasha 2002; van Dyck 2007; Busch 2009; and Worrall 2011.)