Although formal thought disorders, due to their outstanding ability of revealing underlying cognitive changes in schizophrenic psychoses, are amongst the most widely examined psychopathological phenomena in these psychoses, agreement with regard to the diagnostic significance is lacking. Contradictory findings in psychopathological research concerning these complex phenomena might be one possible reason. Since the quest for establishing a characteristic schizophrenic thought pathology was unsuccessful, it seems obviously advisable to abandon the hypothesis of a uniform kind of schizophrenic thought disorder. Therefore, a more detailed classification based on exact clinical observation seems desirable. Karl Leonard has developed a differentiation of formal thought disorder with diagnostic significance, which possibly allows for more convincing results. The phenomenology of formal thought disorder along the lines of Leonard is presented according to his theoretical views, and further research perspectives are shown that support this differentiation and illustrate the pathogenetic background with regard to linguistic methods.