Nowra aims to convey three main ideas in Cosi: 1) the inhumanity of the 20th century Australian mental health system, 2) the insanity of war and the need to protest it, and 3) theatre as a form of political protest and art that enriches lives. He depicts the mistreatment of the mentally ill to condemn a society that labels and institutionalizes people, subjecting them to inhumane conditions. Nowra also uses the structures of setting, character, and a play-within-a-play to blur lines between sanity and insanity, and explore ideas of betrayal, loyalty, and the power of illusions over reality.