In short, it is perceiving the world in a way that differs from that of 99% of human populations around the globe: Patients with schizophrenia hear (or see) things that are not there (outside their brain, that is), while they may have difficulties (to varying degrees) with some things that are normal for almost everyone else (e.g. perceiving the emotion in the face of someone they meet, or remembering where they parked their bicycle).
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