Navigation Main History Quiz Categorization Common Illnesses in Teens Cause Cures Psychiatric Hospitals Myths	... Introduction ... A mental illness is a disorder in the brain that causes a disruption in someone's thinking, feeling, moods, and ability to relate to others. It is often thought to be the same as insanity but there is a difference. Insanity is a legal term that sprouts from mental illness. There is not enough information about the brain to fully understand mental illnesses.