Underdetermination in principle holds that there are always plausible empirically equivalent rivals regardless of what evidence comes to light.

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