This paper illustrates how heuristics affect theory construction through the use of a case study of the early development of psychoanalysis. Freud borrowed the heuristic starting points of his research program for the neuroses directly from traditional and recent biomedical thought, but not without adapting these biomedical search schemes for the purpose of theory construction at a psychological level. He abandoned the requirement of a specific pathological anatomical change for each disease and introduced the requirement to look for a specific psychic pathological mechanism for each psychoneurosis. This new criterion led him to a mechanization of the mind and the construction of a general theory of the psychic apparatus.