The language faculty, the ability to understand and acquire human language, is a feature of our neurobiological make-up which has been passed down through the generations in every human being as part of our genetic endowment.1 It is the language faculty which allows us to acquire any language as long as we are exposed to it in our early childhood. Although it cannot be directly observed, the language faculty imposes structural conditions which must be met by all the languages of the world, that is, it provides limits to what can occur in a human language by containing a framework within which language variation can arise.