Aristotle suggests that it is human nature to write and appreciate poetry. We are by nature imitative creatures that learn and excel by imitating others, and we naturally take delight in works of imitation. As evidence of the claim that we delight in imitation, he points out that we are fascinated by representations of dead bodies or disgusting animals even though the things themselves would repel us. Aristotle suggests that we can also learn by examining representations and imitations of things and that learning is one of the greatest pleasures there is. Rhythm and harmony also come naturally to us, so that poetry gradually evolved out of our improvisations with these media.
As poetry evolved, a sharp division developed between serious writers who would write about noble characters in lofty hymns and panegyrics, and meaner writers who would write about ignoble characters in demeaning invectives. Tragedy and comedy are later developments that are the grandest representation of their respective traditions: tragedy of the lofty tradition and comedy of the mean tradition.
Aristotle stops short of saying that tragedy has achieved its complete and finished form. He lists four innovations in the development from improvised dithyrambs toward the tragedies of his day. Dithyrambs were sung in honor of Dionysus, god of wine, by a chorus of around fifty men and boys, often accompanied by a narrator. Aeschylus is responsible for the first innovation, reducing the number of the chorus and introducing a second actor on stage, which made dialogue the central focus of the poem. Second, Sophocles added a third actor and also introduced background scenery. Third, tragedy developed an air of seriousness, and the meter changed from a trochaic rhythm, which is more suitable for dancing, to an iambic rhythm, which is closer to the natural rhythms of conversational speech. Fourth, tragedy developed a plurality of episodes, or acts.
Next, Aristotle elaborates on what he means when he says that comedy deals with people worse than us ourselves, saying that comedy deals with the ridiculous. He defines the ridiculous as a kind of ugliness that does no harm to anybody else. Aristotle is able only to give a very sketchy account of the origins of comedy, because it was not generally treated with the same respect as tragedy and so there are fewer records of the innovations that led to its present form.
While both tragedy and epic poetry deal with lofty subjects in a grand style of verse, Aristotle notes three significant differences between the two genres. First, tragedy is told in a dramatic, rather than narrative, form, and employs several different kinds of verse while epic poetry employs only one. Second, the action of a tragedy is usually confined to a single day, and so the tragedy itself is usually much shorter than an epic poem. Third, while tragedy has all the elements that are characteristic of epic poetry, it also has some additional elements that are unique to it alone.
Aristotle further elaborates on the value of the mimetic arts with his assertion that we are naturally imitative creatures who delight in imitation. Aristotle relates this claim to our ability to learn and reason: we exercise our reason when seeing something as an imitation of something else. It takes a certain level of recognition to see a bunch of men dancing and singing in masks as imitations of characters from ancient myths, to see stylized gestures as imitations of real action, or to see the emotional intensity generated both by actors and audience as an imitation of the emotional intensity that would have been felt if the action on stage were transpiring in real life. Aristotle defines humans as rational animals, suggesting that our rationality is what distinguishes us from other creatures. If the ability to recognize an imitation and understand what it is meant to represent requires reasoning, then we are delighting in that very faculty that makes us human.