 Less formally and perhaps more typically, realists have attempted to explicate approximate truth in qualitative terms. One common suggestion is that a theory may be considered more approximately true than one that preceded it if the earlier theory can be described as a “limiting case” of the later one. The idea of limiting cases and inter-theory relations more generally is elaborated by Post (1971; see also French &amp; Kamminga 1993), who argues that certain heuristic principles in science yield theories that “conserve” the successful parts of their predecessors. His “General Correspondence Principle” states that later theories commonly account for the successes of their predecessors by “degenerating” into earlier theories in domains in which the earlier ones are well confirmed. Hence, for example, the often cited claim that certain equations in relativistic physics degenerate into the corresponding equations in classical physics in the limit, as velocity tends to zero. The realist may then contend that later theories offer more approximately true descriptions of the relevant subject matter, and that the ways in which they do this can be illuminated in part by studying the ways in which they build on the limiting cases represented by their predecessors. (For further takes on approximate truth, see Leplin 1981; Boyd 1990; Weston 1992; Smith 1998; Chakravartty 2010, and Northcott 2013.)