In 1935 the psychologist Karl Duncker published Zur Psychologie des produktiven Denkens, on the psychology of productive thinking. On the first page he defines a problem. Ein ‚Problem‘ entsteht z.B. dann, wenn ein Lebewesen ein Ziel hat und nicht ‚weiß‘, wie es dieses Ziel erreichen soll. A problem arises, for example, when a living being has a goal and does not know how to reach it.
Thinking, Duncker wrote, is called onto the scene wherever the given state cannot be turned into the desired state by simple action. Its task is to design the action that leads from one to the other. In 2003 a seminar at Heidelberg University opened with this definition.