Mimesis is a Greek term that means imitation. The first step in understanding Aristotle's account of mimesis is remembering that he spent many years studying at Plato's Academy. In Platonic thought, the things we encounter via our senses, the phenomena, are imitations of ideal forms. Art (whether poetry or painting), in imitating the phenomena, is thus merely an imitation of an imitation. Plato also divides imitation by medium (words, paint, marble, etc.). He further divides the verbal techniques of imitation into pure imitation or mimesis, in which an actor impersonates a character on stage, and diegesis, or narration, in which a narrator speaks in the third person about events. Epic is a mixed form, using both impersonation and narration when performed by a rhapsode. Plato tends to condemn imitation as degrading, because (1) impersonation can inculcate bad or non-rational habits and (2) because it focuses attention on mere phenomena.
Aristotle accepts the Platonic distinction between mimesis and diegesis, but finds both valuable as modes of training and educating emotions. Ontologically, he does not believe in separated forms, but argues that forms inhere in phenomena, and thus the only way to understand concepts or qualities is as they are embodied and thus advocates rather than objects to close study of appearances.
For Aristotle, mimesis is a natural human activity. He agrees with Plato that children learn by imitation. While Plato worried that people observing villains or despicable characters in poetry would imitate them, Aristotle believes that bad examples teach people how not to behave just as good examples teach people how to behave.
Aristotle believed that mimesis could be defined as the replication of nature. In Greek thought, the concept of mimesis was very important because they believed that art was a search for imitating the beauty of reality, and this concept of imitation is very important in the search for true art that reflects reality accurately. For Aristotle, mimesis did not just simply involve imitation but also equally appealed to mathematical principles in search of perfection.
Aristotle, linked to the concept of mimesis, wrote about the "four causes" in nature. The first was a "formal cause" which is like a blueprint, and the second is the material, which focuses on what an object is made out of. The third is the agent which is the artist that made the object. The fourth and final cause is the good, or the purpose of the object. It is a natural human inclination, Aristotle argued, to try and reflect the beauty and perfection of reality that we see around us in poetic form.
Aristotle also believed that mimesis is the key to the cathartic response that he thought was so important in tragedy. Through watching tragic events occur on stage, we experience "simulated repersentation" which enables us to engage in a kind of mimetic roleplay, allowing us to experience the same emotions that are reflected on the stage and thus be purged of them.