The major argument which has been presented is that the concept of schizophrenia was introduced and has been developed and used in a way which bears little resemblance to the methods of construct formation used in medical and other empirical sciences. Reports of hearing voices in the absence of external stimuli are said to be one of the most common or typical symptoms of schizophrenia. Studies of the ‘normal’ population suggest that between 10 and 50 per cent of people have experienced auditory or visual hallucinations. The role of suggestion, emphasised by Seashore, has been supported by subsequent research. A number of writers have noted the possible contribution of structured external stimulation, and the extent to which it engages the attention of the subject, to reports of auditory and visual hallucinations. A similar confusion is evident in the posing of some questions about hallucinatory experiences.