to be transposed to a new key, using completely different notes, but still retain its identity. Wertheimer took the more radical line that "what is given me by the melody does not arise ... as a secondary process from the sum of the pieces as such. Instead, what takes place in each single part already depends upon what the whole is", (1925/1938). In other words, one hears the melody first and only then may perceptually divide it up into notes. Similarly in vision, one sees the form of the circle first – it is given "im-mediately" (i.e. its apprehension is not mediated by a process of part-summation). Only after this primary apprehension might one notice that it is made up of lines or dots or stars. Gestalt-Theorie (Gestalt psychology) was officially initiated in 1912 in an article by Wertheimer on the phi-phenomenon; a perceptual illusion in which two stationary but alternately flashing lights appear to be a single light moving from one location to another. Contrary to popular opinion, his primary target was not behaviorism, as it was not yet a force in psychology. The aim of his criticism was, rather, the atomistic psychologies of Hermann von Helmholtz (1821–1894), Wilhelm Wundt (1832–1920), and other European psychologists of the time. The two men who served as Wertheimer's subjects in the phi experiment were Köhler and Koffka. Köhler was an expert in physical acoustics, having studied under physicist Max Planck (1858–1947), but had taken his degree in psychology under Carl Stumpf (1848–1936). Koffka was also a student of Stumpf's, having studied movement phenomena and psychological aspects of rhythm. In 1917 Köhler (1917/1925) published the results of four years of research on learning in chimpanzees. Köhler showed, contrary to the claims of most other learning theorists, that animals can learn by "sudden insight" into the "structure" of a problem, over and above the associative and incremental manner of learning that Ivan Pavlov (1849–1936) and Edward Lee Thorndike (1874–1949) had demonstrated with dogs and cats, respectively. The terms "structure" and "organization" were focal for the Gestalt psychologists. Stimuli were said to have a certain structure, to be organized in a certain way, and that it is to this structural organization, rather than to individual sensory elements, that the organism responds. When an animal is conditioned, it does not simply respond to the absolute properties of a stimulus, but to its properties relative to its