Professor David Lewis explains that the symptoms of schizophrenia are typically defined as either positive or negative.

Schizophrenia is characterized by a set of symptoms. The symptoms that tend to get individuals to clinical attention are what we call the positive symptoms, which include delusions, holding a fixed false belief in the face of contrary evidence, hallucinations, which are having sensory experiences like hearing voices in the absence of an external stimulus or someone speaking. And then there are the negative symptoms, which basically can be thought of as a loss of normal function - a loss of motivation, a loss of the ability to express emotion, a loss of the ability to generate speech, a loss of the ability to experience pleasure.

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