The Ninth Configuration (1980)

To figure out whether the inmates of a psychiatric institution for Vietnam War veterans feign insanity or not, unorthodox psychiatrist Colonel Kane arrives at a converted castle in New England. And while struggling with his inner demons, the metaphysical enquiries of psychotic former astronaut Captain Cutshaw intrigue Kane, triggering a feverish recurring nightmare. But as Kane and Cutshaw engage in intense theological debates over evil and the existence of God, the troubled scientist finds himself at a dead-end. As a result, only a reckless plan can determine the root of the soldiers' complex mental breakdowns. However, can Colonel Kane provide proof of an afterlife?