The direct impact of phenomenological thinking on psychological science has not been great (as we have seen in Chapter 2). There has certainly been indirect influence, especially through the migration of German Gestalt psychologists to the United States during the 1930s. There has also been a definite effect of existential phenomenology—the engagement of phenomenology with the elements of Kierkegaard's existentialist thinking, engineered by Heidegger. This had an undeniable role in the establishment of humanistic psychology, some kinds of counseling, and (perhaps more peripherally) on psychiatry. But the authors of this book are united in their view that the low impact of phenomenological thinking on psychology is much to be regretted. Psychologists have in very large measure chosen a natural science model of research somewhat in line with the positivist tendency (though not always fully, technically positivist, since the criterion of truth is not always that the "facts" should be observable and the relationships between observables). It is, however, true to say that psychological science generally assumes—without reflecting overmuch on the assumption—that there is one real world which has determinate characteristics, and the purpose of science is to model this world in its theories. These theories will show how certain variables interrelate, especially how they relate to each other in a cause-and-effect fashion. Mathematical formulations of the relationships between variables are to be sought if at all possible.