This paper deals with the question of how the history of psychology might be brought to bear on theoretical psychology. The theoretical problem that is addressed in particular is the systematization rather than unification of psychology. It is argued that essential to any system is an organizing ontological perspective. Next, this concept of perspective is clarified by going into a modern interpretation of the phenomenological concept of essence as founding definition of a knowledge domain. Finally, the possible fruitfulness for systematization of founding definitions is suggested by an illustration from the history of social psychology.