Finally available in English, this book is a major contribution to the theory of knowledge and to twentieth century philosphy in general. The two fundamental problems of knowledge that lie at the centre of the book are the problem of induction, and the problem of demarcation. Containing the seed of many of the celebrated arguments that were later to find definitive expression in Popper's most famous work, The Logic of Scientific Discovery, this book is essentional reading for anyone interested in Karl Popper, in the history and philosophy of science, and in the methods and theories of science itself.