"Nature tends towards certain ends; when it fails to achieve those objectives, art and science intervene. Man, as part of nature, also has certain ends in view: health, gregarious life in the State, happiness, virtue, justice, etc. When he fails in the achievement of those objectives, the art of tragedy intervenes. This correction of man's actions is what Aristotle calls catharsis.
Tragedy, in all its qualitative and quantitative aspects, exists as a function of the effect it seeks, catharsis. All the unities of tragedy are structured around this concept. It is the centre, the essence, the purpose of the tragic system. Unfortunately, it is also the most controversial concept. Catharsis is correction: what does it correct? Catharsis is purification: what does it purify?"
(Augusto Boal 1993, p.27)
Boal, Augusto. 1993 Theatre of the Oppressed, London, UK: Pluto Press.