 Related to the issues of essence, Markman (1989) and Markman et al. (2003) have argued that children’s early language learning is guided by several principles, one of which is the mutual exclusivity constraint. This constraint is that in learning words a guiding assumption should be that distinct words refer to distinct things. While this is not always true, Markman and others provide evidence that this constraint is operative, and that in the language learning environment of young children it is a very helpful, if imperfect constraint. If Rosch is correct about the existence of a preferred basic level of objects, it may be that speakers generally use words at that level of objects and thus the exclusivity constraint may be supported by the basic level object hypothesis. However, the experimental data are apparently conflicting on these questions (Mervis et al. 1994). Scholl (2008) provides a useful discussion of whether and how philosophy and psychology can interact beneficially in understanding these issues.