It is argued, with the help of three casuistic illustrations, that, if one wants to be able to make statements about prognosis and treatment approach, it is impossible to confine oneself only to systems which emanate from one level of assessment of the psychic phenomena. Differentiated diagnostics make use of several levels of measurement in which the starting point consists of various principles of elucidation. In regard to this, psychological tests can play an important part. Only the interaction between symptomatology and to a lesser or greater extent stable personality characteristics, in which the emphasis falls on the individual differences, makes it possible to be able to make predictions about results of treatment strategies. Furthermore, a theoretic explanation is given about the hierarchic structuring and the extent of abstraction of psychic phenomena.