Schizophrenia has been conceptualized as a biologically caused mental disease. However, its causes and pathophysiology have not yet been clearly defined. Unlike physical diseases that are defined using specific biomarkers, mental disorders, such as schizophrenia, are usually diagnosed using the diagnostic criteria established by the consensus of a professional committee. The Diagnostic and Statistical Manual of Mental Disorders (DSM) published by the American Psychiatric Association is widely used as the authoritative guide for the diagnosis of mental disorders, including schizophrenia. If the cause and pathophysiology of a disease are uncertain, a diagnostic concept should be considered valid only when the course, prognosis, or treatment response is generally predictable and consistent. However, patients with schizophrenia who are diagnosed based on the criteria defined by the DSM frequently display varied disease courses and outcomes. This strongly suggests that the current diagnostic concept of schizophrenia in the DSM fails to meet the standards for a valid diagnosis. Although this limitation has been recognized since the DSM-III was published, many psychiatrists today mistakenly believe that the diagnostic concept in the DSM looks at schizophrenia as an explicit disease rather than as a concept. In this review article, we analyze the historical changes in the concepts and diagnostic criteria of schizophrenia. We believe that this will help us to better understand the nature of the current diagnostic approach and also improve our understanding of how the DSM should be used in research and everyday clinical practice.