Chomsky (1965: 4) remarks that "linguistic theory is mentalistic, since it is concerned with discovering a mental reality underlying behavior" and in note 1 (p. 193) appended there states that "[m]entalistic linguistics is simply theoretical linguistics that uses performance as data (along with other data, for example, the data provided by introspection) for determination of competence, the latter being taken as the primary object of its investigation." The research reported in this book is mentalistic linguistics in the above sense, and the book's aim is to put forth and defend a means to determine, with some confidence, when to take a certain set of informant judgments as a reflection of properties of the Computational System.