This dissertation asks how we should understand the distinction between semantic and pragmatic aspects of linguistic understanding within the framework of mentalism, on which the study of language is a branch of psychology. In particular, I assess a proposal on which the distinction between semantics and pragmatics is ultimately grounded in the modularity or encapsulation of semantic processes. While pragmatic processes involved in understanding the communicative intentions of a speaker are non-modular and highly inferential, semantic processes involved in understanding the meaning of an expression are modular and encapsulated from top-down influences of general cognition. The encapsulation hypothesis for semantics is attractive, since it would allow the semantics-pragmatics distinction to cut a natural joint in the communicating mind. However, as I argue, the case in favor of the modularity hypothesis for semantics is not particularly strong. Many of the arguments offered in its support are unsuccessful. I therefore carefully assess the relevant experimental record, in rapport with parallel debates about modular processing in other domains, such as vision. I point to several observations that raise a challenge for the encapsulation hypothesis for semantics; and I recommend consideration of alternative notions of modularity. However, I also demonstrate some principled strategies that proponents of the encapsulation hypothesis might deploy in order to meet the empirical challenge that I raise. I conclude that the available data neither falsify nor support the modularity hypothesis for semantics, and accordingly I develop several strategies that might be pursued in future work. It has also been argued that the encapsulation of semantic processing would entail (or otherwise strongly recommend) a particular approach to word meaning. However, in rapport with the literature on polysemy—a phenomenon whereby a single word can be used to express several related concepts, but not due to generality—I show that such arguments are largely unsuccessful. Again, I develop strategies that might be used, going forward, to adjudicate among the options regarding word meaning within a mentalistic linguistics.