This book is about Karl Popper's early writings before he began his career as a philosopher. The purpose of the book is to demonstrate that Popper's philosophy of science, with its emphasis on the method of trial and error, is largely based on the psychology of Otto Selz, whose theory of problem solving and scientific discovery laid the foundation for much of contemporary cognitive psychology. By arguing that Popper's famous defence of the method of falsification as well as his elaboration of an evolutionary theory of knowledge are equally indebted to German psychology, Michel ter Hark challenges the received view of the development of Popper's philosophy. The book concludes with a reinterpretation of Popper's theory of the mind-body problem, emphasizing its contemporary relevance.