Aristotle proposes to approach poetry from a scientific viewpoint, examining the constituent parts of poetry and drawing conclusions from those observations. First, he lists the different kinds of poetry: epic poetry, tragedy, comedy, dithyrambic poetry, and most flute-playing and lyre-playing. Next, he remarks that all of these kinds of poetry are mimetic, or imitative, but that there are significant differences between them.
The first kind of distinction is the means they employ. Just as a painter employs paint and a sculptor employs stone, the poet employs language, rhythm, and harmony, either singly or in combinations. For instance, flute-playing and lyre-playing employ rhythm and harmony, while dance employs only rhythm. He also addresses the question of non-poetic language, arguing that poetry is essentially mimetic, whether it is in verse or in prose. Thus, Homer is a poet, while Empedocles, a philosopher who wrote in verse, is not. While Empedocles writes in verse, his writing is not mimetic, and so it is not poetry. In tragedy, comedy, and other kinds of poetry, rhythm, language, and harmony are all used. In some cases, as in lyric poetry, all three are used together, while in other cases, as in comedy or tragedy, the different parts come in to play at different times.
The second distinction is the objects that are imitated. All poetry represents actions with agents who are either better than us, worse than us, or quite like us. For instance, tragedy and epic poetry deal with characters who are better than us, while comedy and parody deal with characters who are worse than us.
The final distinction is with the manner of representation: the poet either speaks directly in narrative or assumes the characters of people in the narrative and speaks through them. For instance, many poets tell straight narratives while Homer alternates between narrative and accounts of speeches given by characters in his narrative. In tragedy and comedy, the poet speaks exclusively through assumed characters.
The very first paragraph of the Poetics gives us a hint as to how we should approach the work: it is meant to be descriptive rather than prescriptive. That is, Aristotle is not so much interested in arguing that poetry or tragedy should be one thing or another. Rather, he wants to look at past examples of poetry—tragedy in particular—and by dissecting them and examining their constituent parts to arrive at some general sense of what poetry is and how it works.
This is the same scientific method that Aristotle employs so successfully in examining natural phenomena: careful observation followed by tentative theories to explain the observations. The immediate and pressing question, then, is whether Aristotle is right in applying his scientific method to poetry. Physical phenomena are subject to unchanging, natural laws, and presumably a careful study of the phenomena matched with a little insight might uncover what these natural laws are. Aristotle seems to be proceeding with the assumption that the same is true for poetry: its growth and development has been guided by unchanging, natural laws, and the Poetics seeks to uncover these laws.