Aristotle attributes much of art to the intellect's amazing ability to recognize patterns and the human tendency to imitate. Humans clearly take pleasure in discovering likenesses, and according to Aristotle, much of our compulsive need to create art comes from this pleasure. However, imitation is not the only purpose of art. To take it a step further, Aristotle believed that part of the artistic exercise was not only to capture what we see, but to make it more extraordinary by removing its imperfections.
According to Aristotle, art is an attempt to grasp at universal truths in individual happenstances. Aristotle took a particular interest in tragedy through art, which he described as an imitation of action. It creates a treatment for the more unbearable passions we hold in our minds. Aristotle referred to this phenomenon as catharsis.
When we talk about Aristotle's theory of art, we are actually discussing his treatments of "those things pertaining to the Muses" or, in practice, music, poetry, sculpture and painting. These are discussed in several of Aristotle's extant works, Poetics, Politics, Nicomachean Ethics, and Rhetoric.
All arts, for Aristotle (though music is somewhat of an exception), are forms of imitation or mimesis. Visual arts imitate by means of paint or stone and verbal arts by means of words. We derive natural pleasure from imitation. The arts can benefit the polis because they function to train the emotions. In the case of tragedy, for example:
...through pity and fear effecting the proper purgation of such emotions.