'I want to begin by declaring that I regard scientific knowledge as the most important kind of knowledge we have,' writes Sir Karl Popper in the opening essay of this book. In the course of a life of ninety years Sir Karl can look back on positive changes in the world - the vast reductions in mass poverty, the liberalization of penal systems, the defeat of dictatorships. The search for a better world is never complete, but in spite of two world wars and a long and dangerous cold war, it was not in vain. The essays and lectures collected in this book chart many familiar as well as some less known aspects of Sir Karl's thinking, from his interest in the birth of scientific speculation in classical Greece to the destructive effects on the intellect of totalitarianism in twentieth century states. His discusssions range over problems of politics, the history of philosophy, and the great figures of the Enlightenment such as Voltaire and Kant, and the relation of science and art (in an address given at the 1979 Salzburg festival).