After reading Aristotle's "Poetics," I felt a severe sense of shame for not having read it much, much sooner. As a student of literature, I found that many of the concepts upon which my evaluation of literature are based, whether I picked them up in classes or through amateur theorization, are founded in the "Poetics". The "Poetics," which the Penguin editor Malcolm Heath explains in his outstanding introduction/explication, is probably comprised of lecture notes, and not intentionally meant for public consumption, nonetheless stands as the standard against which literary criticism is gauged. This is amazing, as the work itself is hardly 50 pages long.
Aristotle begins by talking about the origins of art in imitation: Artists convey their sense of the world through imitating what they see and feel around them. This is accomplished both in visual art, and for a more thorough understanding of human events, in poetry. Aristotle goes on to explain the history of literature: how encomium(praises) and invective(curses) give rise respectively to epic and lampoons. These then pave the way for tragedy and comedy. In terms of these basic steps, in the later part of the "Poetics," Aristotle gives definitions to parts of speech, to wit, nouns, verbs, etc., and how they are used in different forms of speech, and in various contexts within the genres he outlines.
Spending the greater part of the work on an investigation of tragedy, Aristotle examines the component parts of what he takes to be the best kinds of tragedies. In terms of quality, the work must be complete, showing the causal relation of events and the causal reactions of characters to those events. It should have a plot wherein a character or characters experience a reversal of fortune or a recognition that leads to the conclusion of that plot.
Plot is essential to Aristotle, and, to appropriate Heath's translation, 'universalizes' the "Poetics" to encompass even those prose works for which Aristotle himself admits to have no definition. We can apply his standards to short stories, novels, and so on. Aristotle's notions of unity, completeness, and magnitude are the conventions to which and against which all Western literature and criticism can be seen to either conform to or struggle against. Without Aristotle's strict definitions of tragedy, comedy, unity, and so on, I can scarcely imagine how we would have notions of mock-tragedy, tragi-comedy, or even the modern or post-modern literary forms. In short, the "Poetics" is absolutely crucial reading for anyone who reads anything.