The genetic classification of languages is a central problem of historical linguistics. It is important for historical linguistics proper because it it gives us natural units ("families") at various levels which provide the sets of languages to which the comparative method is applied. A second aspect of its importance is its bearing on human history. As Darwin himself said, a complete world taxonomy of languages would provide the basis for the history of the origin and spread of the human species. Thirdly its results can be compared with other independent evidence, especially archaeology and genetics to help draw a more complete picture of the main outlines of human history. In such an endeavor linguistics cannot remain isolated from the other human sciences.

The methodology of classification requires that we concentrate on the distinguishing of families at various levels and assign a very subordinate status to the mere notion of relationship which has been so prominent in linguistic thinking on this subject. In my own work I have distinguished three methodological principles. The first is the sole relevance of simultaneous sound-meaning resemblances to the exclusion of typological criteria which were formerly applied widely in Africa and elsewhere. The second is the sole relevance of linguistic data to the exclusion of such properties as mere size of population, racial type etc. The third is all-inclusiveness. All languages for which there are data should be used. This is all that is meant by the term multilateral comparison which so far from being in opposition to the comparative method is really the first and indispensable step in the method itself. This can be seen in the table of a few common words of all the languages of Europe. The division into three groups and the subdivisions of Indo-European are immediately visible. All of the resemblances noted in this table are contained in the comparative dictionaries of these languages and provide the starting point for the application of the comparative method in the narrower sense of the word.