 One problem that has been raised against the theory theory is that it has difficulty in allowing for different people to possess the same concepts (or even for the same person to have the same concept over time). The reason is that the theory theory is holistic. A concept’s content is determined by its role in a theory, not by its being composed of just a handful of constituents. Since beliefs that enter people’s mental theories are likely to be different from one another (and are likely to change), there may be no principled basis for comparison (Fodor &amp; Lepore 1992). Another problem with the theory theory concerns the analogy to theory change in science. The analogy suggests that children undergo radical conceptual reorganization in the course of cognitive development, but many of the central case studies have proved to be controversial on empirical grounds, with evidence that the relevant concepts are implicated in core knowledge systems that are enriched in development but not fundamentally altered (see Spelke &amp; Kinzler 2007 on core knowledge). However, there are certain specific examples where radical conceptual reorganization is plausible, for instance, when children eventually develop a theory of matter that allows them to differentiate weight from density, and air from nothing (Carey 2009).