What is the theme of Aristotle’s Poetics ?

The Poetics is Aristotle's attempt to explain the basic problems of art. He both defines art and offers criteria for determining the quality of a given artwork. The Poetics stands in opposition to the theory of art propounded by Aristotle's teacher, Plato. In his Republic, Plato argues that "poetry is a representation of mere appearances and is thus misleading and morally suspect" (Critical, 1). In the poetics, Aristotle, Plato's student, attempts to refute his teacher by exploring what unites all poetry: its imitative nature and its ability to bring an audience into its specific plot while preserving a unity of purpose and theme. The tone of the Poetics reflects its argumentative spirit as Aristotle attempts both to explain the "anatomy" of poetry and to justify its value to human society.

Gradesaver has an entire page dedicated to the themes of Aristotle's Poetics. I have provided the direct link below.

http://www.gradesaver.com/aristotles-poetics/study-guide/themes