 Structural realism is another view promoting selectivity, but in this case it is the natures of unobservable entities that are viewed skeptically, with realism reserved for the structure of the unobservable realm, as represented by certain relations described by our best theories. All of the many versions of this position fall into one of two camps: the first emphasizes an epistemic distinction between notions of structure and nature; the second emphasizes an ontological thesis. The epistemic view holds that our best theories likely do not correctly describe the natures of unobservable entities, but do successfully describe certain relations between them. The ontic view suggests that the reason realists should aspire only to knowledge of structure is that the traditional concept of entities that stand in relations is metaphysically problematic—there are, in fact, no such things, or if there are such things, they are in some sense emergent from or dependent on their relations. One challenge facing the epistemic version is that of articulating a concept of structure that makes knowledge of it effectively distinct from that of the natures of entities. The ontological version faces the challenge of clarifying the relevant notions of emergence and/or dependence. (On epistemic structural realism, see Worrall 1989; Psillos 1995, 2006; Votsis 2003; and Morganti 2004; regarding ontic structural realism, see French 1998, 2006, 2014; Ladyman 1998; Psillos 2001, 2006; Ladyman &amp; Ross 2007; and Chakravartty 2007a: ch. 3. See Frigg &amp; Votsis 2011 for an extensive critical survey).