Strict Interfaces and three kinds of Multiple Grammar

Modern linguistic theory introduces the question of how interfaces are to be mechanically represented. It is argued that there are strict innate interfaces where semantics, pragmatics, and syntax must coincide. The Strong Minimalist Thesis (Chomsky 2005) is a prime example. We argue that so-called 3rd Factors, which are part of these interfaces, must be present in core aspects of grammar. It follows language variation cannot and should not, as suggested by Richards (2008) be abstracted out of core grammar as entirely due to 3rd Factor or Externalization processes. Several instances where variation is compatible because it can be represented are introduced.