Historical linguistics is a field that, perhaps more than other branches of linguistics, can be said to exhibit a certain conservatism. To be clear, this term is not meant in any traditional political sense. Rather it is meant to capture the notion that, as a discipline, diachronic studies seem to accept and build on previous theories and empirical findings to a greater extent than do most synchronic subdisciplines. This may be because data are comparatively rare and hard to come by. One result of this scarcity is that, once analyzed, there are fewer opportunities for reanalysis predicated on new data. There are, of course, occasions when more or less radical proposals are brought forward subsequently, which result in debates of the kind which are much more common in synchronic syntax, say, or phonology. The reconstruction of the Indo-European consonant system (Beekes 1995: 132–4 provides a summary), for example, continues to be debated almost two hundred years after it was first proposed.

Generative grammar has its beginnings in the late 1950s with the work of Noam Chomsky and emphasizes innate linguistic knowledge, or Universal Grammar. Children use their innate knowledge and, on the basis of the language they hear spoken, also known as the E(xternalized)-Language, come up with a grammar, also known as the I(nternalized)-Language (see Chomsky 1986: 19–24). Generative grammar focuses on the ability of native speakers to speak and understand grammatical sentences.

Plainly, the effort to apply to historical language study the insights to be derived from synchronic linguistic analysis is fraught with difficulties. The problem is usually conceptualized – as it is by several of the contributors to the present collection of studies – as a difficult marriage of disciplined methods to obstreperous data, a mismatched union somehow to be mediated by the Uniformitarian Principle. To understand the issues properly, then, it would seem a prerequisite to be able to identify what, exactly, the Uniformitarian Principle is. Yet that question itself has no simple answer, in part because the question can be interpreted in at least two ways, both of them bearing directly upon the aims of this collection.