A clinical study of changes in mental function, subjectively experienced by a group of 40 young schizophrenic patients, is reported. Various phenomena related to disturbances in attention, perception, memory, motility and speech are described and discussed, with special reference to early diagnosis. It is suggested that these phenomena may be subjectively experienced by the patients long before signs of established disease appear overtly. It is argued that the clinical data presented support the view that schizophrenia is an organic psychosis, and also that the impairment in speech in this disease is aphasic in nature. It is suggested that the alteration in the perceptual and psychomotor functions in schizophrenia results in a flooding of consciousness with sensory data to a degree beyond the limits of normal experience. Blocking phenomena are regarded as transient disturbances in consciousness which develop in association with a failure to exclude irrelevant stimulation from internal and external sources. A clinical comparison is made between the latter phenomenon and epileptic disturbances of consciousness, the important differences being noted. Various emotional changes, catatonic symptoms and the development of delusions are discussed in relation to particular defects in perception and cognition. Finally, the tentative suggestion is made that the clinical approach outlined might be utilized, in conjunction with experimental psychological methods, for the purpose of classifying schizophrenic illnesses in a more specific manner.