Why did the slogan "Mental diseases are brain diseases" become popular in the nineteenth century?

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Many theorists in the nineteenth century believed that mental disorders were caused by disturbances in brain function. The view that mental diseases were brain diseases was popularized during this time period, as it fit with the growing emphasis on scientific and medical explanations for various phenomena. This view was also supported by advances in neurology and psychology, which led to a greater understanding of how the brain works and how it can be affected by mental illness.

Why Did the Slogan "Mental Diseases Are Brain Diseases" Become