  Gordon Purves (forthcoming) argues that there are examples of fictions in computational models (his example is so-called “imaginary cracks”), and elsewhere, that do not meet the strict requirements discussed above. Unlike Toon, however, he also wants to delineate fictional modeling elements from the non-fictional ones. His principal criticism is of the criterion of fictionhood in terms of social norms of use—and Purves argues that we ought to be able to settle whether or not some piece of modeling is a fiction in the absence of such norms. Thus, he wants to find an intrinsic characterization of a scientific fiction. His proposal takes as constitutive of model fictions that they fail to have the characteristic that Laymon (1985) called “piecewise improvability” (PI). PI is a characteristic of many models that are idealizations; it says that as you de-idealize, your model becomes more and more accurate. But as you de-idealize a silogen atom, you do not get a more and more accurate simulation of a silicon crack. But Purves takes this failure of PI to be constitutive of a fiction, rather than merely symptomatic of them.