Publisher Summary From the time Niels Bohr formulated his complementarity viewpoint in 1927, it constituted the basis for a wide variety of his thoughts and actions. The viewpoint originated from Bohr's work in physics in the discipline in which he continued to refine it. However, Bohr sought with increasing vigor to explain and promote complementarity to an ever wider audience. In the process, he applied his viewpoint in fields outside physics, such as psychology, biology, and anthropology. The book, Niels Bohr Collected Works, volume 10, presents Bohr's main published statements in this area that are at the same time his main contributions to the philosophical questions.