Aristotle classifies Greek words in an esoteric discussion of 'simple' and 'compound' terms, and the reader can sift through a majority of this analysis and focus instead on his definition of a few key literary terms.
First is 'metaphor,' or the use of 'transference' to link two unlike things. 'Life's setting sun,' for instance, does not hedge or qualify its comparison with 'like' or 'as' (that would be a simile), or create primacy around one term (as in an analogy). Instead, a metaphor simply links two objects with the understanding that the reader will find the unity of concept that connects them.
Aristotle points out that the best poetry uses only 'current and proper words,' meaning the contemporary lexicon. When an author resorts to 'lofty' or esoteric language, he alienates the reader. Indeed, a metaphor, says Aristotle, only truly works when it uses ordinary words; if one were to use 'strange' or 'raised' words for a metaphor or other literary device, it simply collapses into jargon.
And yet, Aristotle also permits the good poet to lengthen, contract, and alter words to fit his purpose. By playing with ordinary words, the poet creates 'distinct' language, but at the same time ensures that the reader will maintain clarity. By playing with accepted or ordinary words, the poet can engage the reader at the highest level. (One can think of Shakespeare here, and the way he so often uses recognizable words in extraordinary ways to achieve his rhythms and images.)
Aristotle next proceeds to a discussion of the epic form - which employs a single meter, a dramatic plot, unity, and all the other features of a tragedy. (As mentioned before, a proper epic maintains all the elements of a tragedy, since tragedy evolved from the epic form.) An epic does not portray a single action, but rather a single 'period,' thus often charting the course of many characters over the course of many events.
Epic poetry falls into the same categories as tragedy: simple, complex, ethical or pathetic. Also like tragedy, it requires reversals, recognitions, scenes of suffering, and artistic thought and diction. There are a few differences between tragedy and epic, however.
First, an epic poem, however, will not use song or spectacle to achieve its cathartic effect. Second, epics often cannot be presented at a single sitting, whereas tragedies are usually capable of being brought within a single view. Epic poetry, after all, is not confined to the stage - and thus, many events and characters can be presented simultaneously because of its narrative form. Finally, the 'heroic measure' of epic poetry is hexameter, where tragedy often uses other forms of meter to achieve the rhythms of different characters' speech.
Aristotle points out that the poet should take as little part as possible in the actual story of an epic - meaning limited first-person narration, and no personal appearances in scenes if possible. At the same time, 'wonderment,' created by absurdity or irrational events for the purposes of indulging the reader's pleasure, is allowed in an epic poem - even moreso than in a tragedy. An absurd event or moment can pass more unnoticed in an epic poem, simply because it is not being dramatized onstage.
That said, Aristotle notes that a tragic plot cannot have 'irrational parts.' There must be likelihood, no matter how seemingly impossible the circumstances - as long as we trust that given the initial incident, the plot follows logically and probably, then the poet is in the realm of good drama. But if we believe neither the inciting incident, nor the chain of events that follows, the poem is simply absurd, and thus summarily dismissed.
Chapters 21 and 22 of The Poetics offer a complex discussion of language types and tropes that a reader unversed in the scripts of ancient Greek can quickly sift through. He does, however, stop to offer a clear and concise discussion of the use of metaphor - a device of figurative language that is frequently misunderstood.
A metaphor is not simply a comparison of objects - but rather the use of two unlike things in proximity in order to illustrate a larger, unified concept. The second term in the metaphor, then, is subsumed by the first - in the case, for instance, of 'life's setting sun,' the sun becomes part of a fictional trope illustrating the ebb and flow of life. A good metaphor forces us to actually position the objects together in an imaginary, but completely probable relationship. And furthermore, a metaphor - unlike a simile or analogy, which establishes the primacy of the first term - can easily be reversed. After hearing the metaphor of 'life's setting sun,' we can then look at the sun and see in its rise and fall the course of life. Metaphors, then, are more fluid than the rest of figurative language, and hence Aristotle focuses on them as the primary device of the good poet.
Metaphor also replicates the human instinct to find connections through imitation. As one critic notes, "The metaphorical system of the myth imitates life with the organization of metaphors and the story line. Metaphors imitate it statically and the story dynamically" (Hermeneutics, 1). As Aristotle pointed out earlier in the Poetics, we cannot help but imitate as a device for learning. The metaphor offers the clearest device for imitation while also maintaining enough idiosyncrasy for the author to engage the reader in his own imaginative world.
The difference between epic poetry and tragedy may confuse some readers, but it can be boiled down simply to the fact that epic poetry unfolds in a narrative form, as in the Iliad, while a tragedy depends on staging for its cathartic effect (Oedipus). The brief distinction that Aristotle makes between the two forms on the basis of spectacle has wider implications, perhaps, then he gives it. Epic poetry has no need for spectacle because it gains its design from a large span of time. Tragedy, however, is limited in its time frame - usually to a single day - and thus the spectacle of suffering and horror is necessary for catharsis.
Finally, Aristotle points out that despite the invention required for tragedy and epic, both forms depend on likelihood and probability. A tragedy depends on probability even more than the epic, in that the events are dramatized, forcing us to filter all the events onstage through our own experience and classify it as either 'rational' or 'absurd.' An epic poem, however, has more room for maneuvering, since the oral tradition allows a balancing of the irrational with the pleasures of indulgence - as long as the reader can imagine some world where the fiction could be real, then he will continue to find the story engaging. If, however, he finds that the action is 'false,' then the causal chain of plot is broken, and often the work can never recover.