Radical constructivism is a theory of knowing that provides a pragmatic approach to questions about reality, truth, language and human understanding. It introduces a change to many basic ideas, and consequently has a profound influence on the general attitude towards the world we experience. By generating awareness of the thinker's active role in building up concepts, the new orientation liberates the individual from spurious tethers and shows that it is ultimately ourselves who are responsible for what we think and do. To educators, the book suggests that the most important task is not to convey ready-made knowledge but to teach the art of constructing it. In this volume, Glaserfeld offers a theoretical account of radical constructivism. It is an elegantly and thoroughly argued account of this epistemological position, providing a profound analysis of its concepts. The book traces two genealogies of the theory. The first is the constructivist strand in the history of philosophy from the pre-Socratics via Jean Piaget to the present. The second is his own intellectual biography, illustrating how a number of lines of thought became synthesised into radical constructivism. Given its diverse roots, the first full articulation of the theory is likely to have an influence that extends beyond mathematics education.