The National Idea as a Research Problem SOW, Warszawa 2001 Zbigniew Greri Institute of Slavonic Studies, Polish Academy of Sciences, Warsaw MECHANISMS OF LINGUISTIC AND ETHNIC CONVERSION AT THE BORDERLAND OF CLOSELY RELATED LANGUAGES AND NATIONS The aim of this paper is to define the mechanisms and causes of linguistic and ethnic conversion at the border of closely related languages and ethnoses. As materials for analysis I have used the results of my research on contemporary ethnic and linguistic consciousness of the Polish-Czech borderland in the Cieszyn Silesia done in the years 1996— 2000 using the method of a comprehensive questionnaire survey, and my study of historical consciousness of the region based on researching the archives and confronting the findings with the current state of historical knowledge on the subject. The starting point was an analysis of linguistic conversion, and ethnic consciousness was included as a factor influencing linguistic conversion. By investigating - on the basis of concrete material — the correlations between conversion, ethnic multi-identification and national indifferentism, I will try to determine the wide—ranging causes and roots of the phenomenon of conversion in the Polish-Czech borderland, with the View of applying the conclusions in research on similar borderland territories. Naturally, some statements that are true of the territory under ‘ examination could be true or even self—evident for other territories, but some other statements may prove false.