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, which collects his meditations on the real improvements science has wrought in society, in politics and in the arts in the course of the twentieth century. His subjects range from the beginnings of scientific speculation in classical Greece to the destructive effects of twentieth century totalitarianism, from major figures of the Enlightenment such as Kant and Voltaire to the role of science and self-criticism in the arts. The essays offer striking new insights into the mind of one of the greatest twentieth century philosophers.