 For the realist, theoretical entities exist independently of our theories about the world. Also, natural kinds that classify these entities exist independently of our theories (cf. Psillos 1999; Lewis 1984). The instrumentalist picture is commonly reported to account for theoretical entities in terms of mere fictions. The formalist variant of instrumentalism denies that theoretical terms have referents in the first place. Between these two extreme cases there is a number of intermediate  positions.[1]