Freud's revolution may be viewed as the discovery of a way of locating in the mind objective entities which can be studied like physical things. If Freud's is representative of scientific revolutions, perhaps what Thomas Kuhn has described as a change of paradigm might generally consist of the demonstration of new entities. This particular revolution occurred in the setting of a prevalent concern about the entities underlying all of the sciences. Because of his genius for structural thought, Freud was able to respond satisfactorily to a challenge that all the sciences were facing. It is that common challenge rather than a popular exemplary model, such as mechanics or hydraulics, that shaped Freud's theory.