 Consider first the issue of how best to identify those theories that realists should be realists about. A general disclaimer is in order here: realists are generally fallibilists, holding that realism is appropriate in connection with our best theories even though they likely cannot be proven with absolute certainty; some of our best theories could conceivably turn out to be significantly mistaken, but realists maintain that, granting this possibility, there are grounds for realism nonetheless. These grounds are bolstered by restricting the domain of theories suitable for realist commitment to those that are sufficiently mature and non-ad hoc (Worrall 1989: 153–154; Psillos 1999: 105–108). Maturity may be thought of in terms of the well established nature of the field in which a theory is developed, or the duration of time a theory has survived, or its survival in the face of significant testing; and the condition of being non-ad hoc is intended to guard against theories that are “cooked up” (that is, posited merely) in order to account for some known observations in the absence of rigorous testing. On these construals, however, both the notion of maturity and the notion of being non-ad hoc are admittedly vague. One strategy for adding precision here is to attribute these qualities to theories that make successful, novel predictions. The ability of a theory to do this, it is commonly argued, marks it as genuinely empirically successful, and the sort of theory to which realists should be more inclined to commit (Musgrave 1988; Lipton 1990; Leplin 1997; White 2003; Hitchcock &amp; Sober 2004; Barnes 2008; for a dissenting view, see Harker 2008; cf. Alai 2014).