 The Basic Underdetermination Argument. Feminist empiricists reply to this challenge by extending Quine’s argument that theory is underdetermined by evidence (Longino 1990, Nelson 1993). Any body of observations counts as evidence for particular hypotheses only in conjunction with certain background assumptions. Vary the background assumptions, and the same observations will support different hypotheses. For example, the failure to observe stellar parallax in the 16th century was taken as evidence that the Earth stands still by geocentrists, and as evidence that the stars are very far away by heliocentrists. No logical principle stops scientists from choosing different background assumptions. In practice, scientists face constraints in selecting background assumptions, based on cognitive values such as simplicity and conservatism (resistance to revising assumptions on which many other beliefs depend). But with respect to open questions, such cognitive values rarely limit the scope for choice down to one option, and their interpretation and weights are contestable in any event (geocentrism was overturned by overriding conservatism). Feminist empiricists conclude that, given the scope for choice in background assumptions, no methodological principle forbids scientists from selecting their background assumptions on account of their fit with social and political values. Hence, feminist scientists may select their background assumptions on account of their fit with feminist values.