“Visible” and “invisible” changes

The term “linguistic change” is ambiguous because it may refer to two fundamentally different aspects of the historical evolution of language. Our aim could be to describe changes that can be observed in the texts, and latterly also the sound recordings, which have come down to us – that is, in the documentary record or (to borrow the convenient term coined by Noam Chomsky) in historical performance data. On the other hand, we might want to go beyond these data and use them to make inferences about the changes that must have occurred in the underlying rule systems; that is, in native speakers' linguistic competence. It is clear that hypotheses about the second type of language change will be more difficult to arrive at and more controversial, because they are relatively more theory-dependent. The decision about which of the two perspectives on change to adopt will also crucially influence the chronology one is able to establish. For example, important changes in individual speakers' competence might not show up in the documentary record for centuries, because a traditional construction and its newer, reanalyzed variant may look identical in surface structure in the vast majority of cases. This inevitable time-lag may explain why advocates of the second perspective have generally focused on the broad outlines of major changes in the remote past of the language, rather than on developments in the recent past and the present.