In nine essays and lectures composed in the last years of his life, Werner Heisenberg offers a bold appraisal of the scientific method in the twentieth century - and relates its philosophical impact on contemporary society and science to the particulars of molecular biology, astrophysics, and related disciplines. Are the problems we define and pursue freely chosen according to our conscious interests? Or does the historical process itself determine which phenomena merit examination at any one time? Heisenberg discusses these issues in the most far-ranging philosophical terms, while illustrating them with specific examples.