This book presents an analysis of the 'essentialist ideology', which is inherent to class-based societies. The author argues that essentialist ideology is efficient through its unconscious component and is imposed on everyone. It guides school selection and imposes on each class a language specific in its reference to concrete domination relations. It even unbalances the scientific objectivity of researchers in the social sciences, not only among those who abide by the theory of natural aptitudes, but also among its sharpest critics, such as Basil Bernstein, Pierre Bourdieu and J C Passeron, whose work is considered in this book.