In this thesis I trace ideas about naturalistic inquiry into commonsense understanding through Chomsky's work. I argue that the resulting picture significantly illuminates both the nature of `common sense' and existing interdisciplinary debates surrounding it. Specifically, I claim that progress in investigating the nature of humans' commonsense understanding of psychology (folk psychology) has been hampered by the same kind of methodological dualism which for so long haunted scientific accounts of language. Following Chomsky, I discuss in general how a rationalist inquiry into cognitive domains other than language could proceed by positing `learning theories' for organisms in given domains, LT (0, D), and attempting to characterize their interaction with experience and the resulting knowledge structures. I further consider how the Language Faculty and a Science Forming Faculty might contribute to our introspective awareness of the understanding of the world delivered to us by core commonsense principles. In particular, taking into account methodological insights gained from the study of language, I put forward a view of a Psychology Faculty and I adapt some ideas from Grice (1975) as a first step towards a positive proposal. Evidence from current developmental and ethological research is presented in support of my position. I thus draw out substantial similarities both between the study of language and the study of commonsense psychology, and between the objects of study in each case. By defusing possible philosophical objections to the comparison, I articulate a detailed defence of my claims.