It is argued that the substantial heuristics used by scientists offer the most adequate explanations of theoretical developments in science. In contrast to formal or procedural heuristics (such as generate and test), substantial heuristics are the implications for problem-solving which can be derived from the theoretical principles on the basis of which scientists work (such as conservation notions). They provide directives indicating where to find problems (such as violations of conservation principles) and where to search for solutions. Reconstruction of the substantial heuristics in a particular domain not only leads to a deeper historical understanding, but also enables theoretical psychologists themselves to make a contribution towards theory development. This general thesis is illustrated by references to the work of Freud. To a considerable extent, the birth and growth of psychoanalysis can be understood from Freud's psychological transformation of biomedical heuristics. The focus is on contributions of the traditional aetiological search scheme. This scheme requires the specification of external noxious influences and constitutional weak points. Freud did so, again and again.