It is argued that an interactionistic philosophy which on the one hand acknowledges that the physical world, the psychic world, and the mental world each has its own character and on /the other hand claims that these worlds are coupled through intimate interactions and cooperative relationships, broadens essentially the domain of applicability of synergetic principles and methodology, which in the past have been used successfully mainly in the physical sciences, including chemistry and biology. Structures, processes and organizations in each of those three domains arise not only from cooperative interactions within these domains but also from such interactions between entities of different domains. Language is a particularly important mediator between the three worlds in having physical as well as psychic and mental aspects.In the second part it is shown that synergetics as a theory for the dynamic, self-organizing development of structures finds a rich and very natural field of application in developmental psychology. We emphasize here the work of Jean Piaget as a fruitful guideline. Developmental psychology is also very suitable to demonstrate the intimate intertwining and synergy of the physical, psychic, and mental worlds.