 Before considering the nuances of what scientific realism entails, it is useful to distinguish between two different kinds of definition in this context. Most commonly, the position is described in terms of the epistemic achievements constituted by scientific theories (and models—this qualification will be taken as given henceforth). On this approach, scientific realism is a position concerning the actual epistemic status of theories (or some components thereof), and this is described in a number of ways. For example, most people define scientific realism in terms of the truth or approximate truth of scientific theories or certain aspects of theories. Some define it in terms of the successful reference of theoretical terms to things in the world, both observable and unobservable. (A note about the literature: “theoretical term”, prior to the 1980s, was standardly used to denote terms for unobservables, but will be used here to refer to any scientific term, which is now the more common usage.) Others define scientific realism not in terms of truth or reference, but in terms of belief in the ontology of scientific theories. What all of these approaches have in common is a commitment to the idea that our best theories have a certain epistemic status: they yield knowledge of aspects of the world, including unobservable aspects. (For definitions along these lines, see Smart 1963; Boyd 1983; Devitt 1991; Kukla 1998; Niiniluoto 1999; Psillos 1999; and Chakravartty 2007a.)