 How then does this give rise to underdetermination, a presumptive concern for realism? The argument from underdetermination proceeds as follows: let us call the relevant, overall sets of scientific beliefs “theories”; different, conflicting theories are consistent with the data; the data exhaust the evidence for belief; therefore, there is no evidential reason to believe one of these theories as opposed to another. Given that the theories differ precisely in what they say about the unobservable (their observable consequences—the data—are all shared), a challenge to realism emerges: the choice of which theory to believe is underdetermined by the data. In contemporary discussions, the challenge is usually presented using slightly different terminology. Every theory, it is said, has empirically equivalent rivals—that is, rivals that agree with respect to the observable, but differ with respect to the unobservable. This then serves as the basis of a skeptical argument regarding the truth of any particular theory the realist may wish to endorse. Various forms of antirealism then suggest that hypotheses and theories involving unobservables are endorsed, not merely on the basis of evidence that may be relevant to their truth, but also on the basis of other factors that are not indicative of truth as such (see  sections 3.2,  and  4.2–4.4).  (For recent explications, see van Fraassen 1980: ch. 3; Earman 1993; Kukla 1998: chs. 5–6; and Stanford 2001.)