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This chapter considers evidence from both perspectives and argues for a critical appraisal of the role of cognition in psychotic illness. The psychiatric pioneers of schizophrenia research considered a variety of cognitive problems in their clinical case descriptions, but these efforts were limited by the questionable validity of interviews and subjective data and observations as well as by sampling biases. The cognitive impairment reliably occurs at very high rates in schizophrenia, typically approaching 75% of the patient population, which equals or exceeds the prevalence of impairment in many neurological disorders. The possibility of preserved cognition in a significant minority of people with schizophrenia has not been resolved and this challenges the assertion that cognitive impairment is a truly defining characteristic of the illness. The majority of patients with Parkinson's disease eventually develop cognitive deficits, but psychosis is much less common and largely a by-product of medication.