In these papers the author traces the history of the concept of schizophrenia and secondarily, of neurosis, from Hippocrates to contemporary writers. His concern is focused on whether schizophrenia is a disease entity which may have neurotic symptoms as secondary or accessory phenomena, as held by Bleuler, or whether hysteria and obsessional neuroses are symptom complexes capable of transformation into schizophrenic states. In reviewing twenty-five carefully detailed cases from the literature and the records of Maudsley Hospital, the author concludes that transformation from neurosis to psychosis does occur and the processes of change can be observed. Thus, we are dealing with reaction sets rather than disease processes.