The papers in this 1963 collection brought the full range and depth of Popper's work to the attention of the educated public. During the 1950s his range of influence was almost entirely limited to the undergraduate students who attended his annual series of introductory lectures and the people who attended his regular "Tuesday seminar" at the London School of Economics. The 21 papers in this collection include the first paper that he wrote in English and his first fragment of autobiography. Some of the best deserve the status of classics. The Introduction, "On the sources of knowledge and of ignorance", challenged the authoritarian structure of western thought. Chapter 1 contains some autobiographical notes and it reveals how Popper's psychological studies on habit-formation paid off by adding value to his evolving ideas on epistemology. An appendix contains a remarkable list of his research projects at the time. Chapter 2 demonstrated how extra-philosophical problems vitalize and refresh philosophical investigations and warned that philosophy could become introverted and marginalized if those external "roots" are cut. Appendices provide a thumbnail sketch of Popper’s life and work, summarise some of his novel contributions and list the top ten errors made by philosophers in misreading and misunderstanding his ideas. This is the fourth volume in the Popular Popper series.