The Mentality of Apes by Wolfgang Köhler is a 1925 book. It is a landmark work in ethology, cognitive psychology and the study of the hominid apes. In it the author, a leading gestalt psychologist, showed that chimpanzees could solve problems by insight (intelligence). The importance of this work was to show there is no "absolute" dividing line between the human species and their nearest living relative, at least in this respect. It was also a marker in the struggle between behaviourism and cognitive psychology which continued for the following half century. Köhler's observations and experiments were done on chimpanzees in captivity. Not until Jane Goodall's work later in the century was the behaviour of chimpanzees in the wild studied in depth.