Aristotle suggests that the best kinds of plot are complex plots that arouse fear and pity. He thus concludes that three kinds of plot should be avoided. First, we should avoid plots that show a good man going from happiness to misery, since such events seem more odious than fearful or pitiable. Second, we should avoid plots that show a bad man going from misery to happiness, since this arouses neither pity nor fear and appeals to none of our emotions. Third, we should avoid plots that show a bad man going from happiness to misery, since it will also not arouse the feelings of pity or fear. We feel pity for undeserved misfortune (and a bad man deserves his misfortune), and we feel fear if the person we pity is something like ourselves.
Aristotle concludes that the best kind of plot involves the misfortune of someone who is neither particularly good nor particularly bad and whose downfall does not result from some unpleasantness or vice, but rather from hamartia—an error in judgment. A good plot, then, consists of the following four elements: (1) It must focus around one single issue; (2) the hero must go from fortune to misfortune, rather than vice versa; (3) the misfortune must result from hamartia; and (4) the hero should be at least of intermediate worth, and if not, he must be better—never worse—than the average person. This explains why tragedies tend to focus around a few families (there are many tragedies about the families of Oedipus and Orestes among others): they must be upstanding families that suffer great misfortune from an error in judgment rather than a vice. Only second-rate plots that pander too much to public taste focus on a double issue where the good fare well and the bad fare poorly.
Pity and fear—which Aristotle calls the "pleasures" of tragedy—are better if they result from the plot itself rather than the spectacle. A story like that of Oedipus should be able to arouse pity and fear even if it is told without any acting at all. The poet who relies on spectacle is relying on outside help, whereas the poet who relies only on his own plot is fully responsible for his creation.
We feel pity most when friends or family harm one another, rather than when unpleasantness takes place between enemies or those who are indifferent to one another. The deed may be done knowingly—as when Medea kills her children—or unknowingly—as when Oedipus kills his father. A third alternative is that one character plans to kill another, but then discovers the family connection between them in time to refrain from the killing.
Thus, the deed can either be done or not done, and it can take place in either ignorance or knowledge. Aristotle suggests that the best kind of plot is of the third alternative, where anagnorisis allows a harmful deed to be avoided. The second best case is where the deed is done in ignorance. And the third best is the case where the deed is done with full knowledge. Worst is the case where there is full knowledge throughout, and the premeditated deed is only refrained from at the moment of action. This scenario is not tragic because of the absence of suffering, and it is odious besides. Still, Aristotle acknowledges that it has been used to good effect, as with the case of Haemon and Creon in Antigone.
The Greek word hamartia translates pretty directly as "error" or "shortcoming" without any necessary overtones of guilt or moral failure. Our modern conception of tragedy and the "tragic flaw" of the hero usually involves the concept of hubris, or overweening pride, that leads to disaster. Macbeth, for instance, has the arrogance to think he can overstep the laws of God and state and ultimately pays dearly for this arrogance. Macbeth is a tragic hero with a clear tragic flaw: his downfall results from a moral failing and can be seen as divine retribution proportional to his guilt. But Macbeth also contains heavy Christian overtones that would of course be found nowhere in Greek tragedy. An understanding of Aristotle's concept of hamartia—and indeed an understanding of Greek tragedy in general—relies on an understanding of the ethics and cosmology of the ancient Greeks.