 Many of these questions, in the end, turn upon the issue of what constitutes “physically reasonable” spacetime structure. General relativity admits spacetimes exhibiting a vast and variegated menagerie of structures and behaviors, even over and above singularities, that most physicists and philosophers would consider, in some sense or other, not reasonable possibilities for physical manifestation in the actual world. But what is to count as “reasonable” here: who is to decide, and on what basis (Curiel 1999)? Manchak (2011) has argued that there cannot be purely empirical grounds for ruling out the seemingly unpalatable structures, for there always exist spacetimes that are, in a precise sense, observationally indistinguishable from our own (Malament 1977; Manchak 2009a) that have essentially any set of properties one may stipulate. Norton (2011) argues that this constitutes a necessary failure of inductive reasoning in cosmology, no matter what one's account of induction. Butterfield (2012) discusses the relation of Manchak's results to standard philosophical arguments about under-determination of theory by data. 