Sentence:  What is the cause of insanity? Nobody can answer such a sweeping question as that, but we know that certain diseases, such as syphilis, break down and destroy the brain cells and result in insanity. In fact, about one-half of all mental diseases can be attributed to such physical causes as brain lesions, alcohol, toxins, and injuries. But the other half—and this is the appalling part of the story—the other half of the people who go <WSD>insane</WSD> apparently have nothing organically wrong with their brain cells. In post-mortem examinations, when their brain tissues are studied under the highest-powered microscopes, they are found to be apparently just as healthy as yours and mine. Why do these people go <WSD>insane</WSD>? 
Meaning of the word marked with <WSD> tags:  Exhibiting unsoundness or disorder of mind; not sane; mad