Aristotle's Poetics is one of the earliest works of literary theory in the western tradition. Book I deals with tragedy (i.e., tragic drama) (chs. 6-22) and epic poetry (chs. 23-26), while Book II, which has been lost, dealt with comedy. The discussion of tragedy is of particular interest, and while many of Aristotle's claims are open to dispute, some of the terms he uses to make them have become established in the vocabulary of literary theory.
These terms are explained individually on their own pages in AWE. This page outlines the context in which they were originally used by Aristotle in the Poetics.
Aristotle opens his discussion of tragedy with a definition: 'tragedy,' he says, 'is an imitation (mimesis) of a serious and complete action of some magnitude ... By means of pity and fear it achieves the purification (katharsis) of such emotions' (ch. 6, 1449b22-28). Two of the terms in this definition - mimesis and catharsis - have generated a vast expository literature and are still used by literary theorists.
Having listed the different 'elements' of a tragedy - plot, character, speech, etc. (ch. 6, 1449b28-1450a14) - Aristotle argues that the most significant 'element' is plot (ch. 6, 1450a16-39), and so the greater part of his discussion (chs. 7-11, 13-14, and 16-18) is devoted to considering what are the features of a good tragic plot.
In chs. 7 and 8 Aristotle insists that a good plot must have a certain unity: it must have an appropriate starting point and an appropriate conclusion, and 'the parts should be combined in such a way as to make a difference to and disturb the whole if one part is moved from its position or taken away altogether' (ch. 8, 1451a32-34). Put succinctly, the plot must be a 'single whole' and, as such, must have 'a beginning, a middle, and an end' (ch. 7, 1450b26-27). Aristotle also comments on the length of the plot, saying that it must be neither so short that it does not allow for the occurrence of significant events nor so long that the audience cannot easily hold it in their minds (ch. 7, 1450b34-1451a15). Aristotle's claims in chs. 7 and 8 are the source of a theory particularly associated with certain Italian and French authors of the sixteenth and seventeenth centuries, the doctrine of the so-called classical or Aristotelian unities (i.e.,.the unity of action, the unity of place, and the unity of time).
In ch. 10 Aristotle draws a distinction between simple (ἅπλους, haplous) and complex (πεπλεγμένος, peplegmenos) plots and expresses a preference for the latter. In a simple plot the change in the proptagonist's fortunes comes about without a peripeteia (i.e., a sudden unexpected reversal of circumstances, as when an action does not have the consequences the agent intended or the spectators had expected) or an anagnorisis (i.e., a recognition by the protagonist of a crucial mistaken belief on his part, as when in Sophocles' Oedipus Tyrannus Oedipus comes to realise that the man he has killed was his father and the woman he has married is his mother). A complex plot, by contrast, involves either a peripeteia or an anagnorisis or both. Peripeteia and anagnorisis are discussed in ch. 11, and different kinds of anagnorisis are distinguished in ch. 16.
In ch. 13 Aristotle argues that the purpose of tragedy sets limits to the appropriate content of a good tragic plot. Since the purpose of tragedy is to arouse pity and fear in the audience, and we feel pity for those who meet with ill fortune without deserving it, and are most likely to feel fear for others when they are like ourselves (1452b30-1453a7), it follows that a good tragedy will be about someone who 'is in an intermediate position, who is not outstandingly virtuous or just, and who meets with misfortune not because of any wickedness or vice, but because of some mistake (hamartia, ἁμαρτία) that he makes' (1453a7-10). Aristotle's claims about the tragic protagonist and the cause of his or her misfortunes have influenced several later theories of tragedy - sometimes through a mistranslation of the word hamartia (ἁμαρτία) at 1453a10, which has been taken to mean not 'mistake' but 'flaw'.
In the final chapter on plot (ch. 18) Aristotle introduces, but does not develop, a distinction between different parts of the plot: the earlier part up to the point at which the protagonist's fortunes change and a tragic outcome is unavoidable - some of this part of the plot may lie outside the play itself - and the later part from the point at which a tragic outcome is unavoidable to the end of the play (1455b26-29). Aristotle calls these two parts of the plot the 'desis' (literally 'binding') and the 'lusis' (literally, 'loosing') (1455b24), terms perhaps best translated as 'complication' and 'resolution'. The application of this distinction to particular plays is sometimes not at all straightforward, but the terms 'desis' and 'lusis' are still very occasionally used in literary theory.
Clearly, although Aristotle introduces all these terms in the context of tragic drama, at least some of them will have application in the discussion of other literary forms.