3) necessity and probability
Key to the arrangement of the events of a tragedy is the notion of necessity and probability. When the play begins, we are admitted to an unknown world. The early exposition provides us with information out of which the subsequent events will grow. The events that follow should, then, have a causal link with what precedes them so that each event in the plot's sequence should appear to grow organically out of what went before it. This continues until the sequence reaches its powerful conclusion. The magnetic energy that draws us from one event to the next is called "probability" or "necessity."
Necessity, which can be considered as the highest level (100%) of probability, plays a significant role in ancient Greek mythology. We remember that in the Myth of Er, which concludes Plato's Republic, we are presented with a vision of the afterlife in which the souls of the departed are prepared for a subsequent rebirth. The nature of that next cycle of life is woven by three fates, Lachesis, Clotho, and Atropos, the daughters of Ananke, or Necessity (ἀνάγκη), who rules over them. What is fated is, by definition, what must be. As probability increases, it follows the path of a hyperbola, moving ever closer to the asymptote that defines its trajectory. That asymptote is the unbending line of necessity.
The nature of the causal links between the events says something profound about the rules of human motivation assumed in the play (economic, social, psychological, etc.). A history of western drama could be written entirely from a study of what is assumed to have motivated the events of a given drama from one moment to the next.
Aristotle's insistence on the governing rules of necessity and probability excludes any irrationality within the structure of the plot itself.
Aristotle's discomfort with irrationality in the structure of the sequence of event appears to run deep. It probably originates, in part, from the Socratic dictum, conveyed by Aristotle's teacher, Plato, that no person knowingly does evil. As a result of this conviction, Aristotle makes an essential distinction between the kinds of ignorance that can induce a good person to do an evil deed unknowingly (unjust acts -ἀδικήματα vs. mistakes - ἁμαρτήματα) in both the Nicomachean Ethics and the Poetics. Behind this conviction is the seemingly naïve belief that evil can be overcome by proper education.
On a more immediate level, however, Aristotle's concern is for the emotional experience of the audience of a tragedy. His insistence on the logical development of a plot is based on a sense that if the events at any given moment in a tragedy appear to come about naturally as a probable result of what preceded them, the audience will have the impression that the drama is not an artificial construct, but rather has an organic life of its own, which the audience is privileged to witness. If the motivating energy that propels the drama from one event to another is consistent with the logic of the world of the drama, the audience will be carried along by the emotional experience of the drama, and totally unaware of the existence of the hand that wrote the script. On the other hand, if the drama were to move in a seemingly arbitrary, unpredictable, or irrational manner, the audience's attention would be drawn away from the flow of events to the apparently inscrutable mind of the play's creator. The audience would not lose itself in an emotional experience occasioned by events that appears to live on their own, but would, instead, become aware of the seemingly arbitrary twists and turns of a plot imposed by the play's author. In a truly effective drama, by comparison, the audience should lose the sense that the play even has a creator.
Given Aristotle's confidence in the sequencing of events governed by probability or necessity, one can understand how the inclusion of central characters that are irrational or insane would be problematic. In this case, the dramatist would be obliged to make the logic of the character's irrationality visible to and experienced by his or her audience. This factor may explain why a dramatization that depicts Medea slaughtering her children was unacceptable to Aristotle, but understandable to a contemporary audience. If we have a sense that there are psychological motivations that can compel Medea to such extreme action, then we can also believe that that action does have a probable cause in what went before it. In other words, we can experience a method to her madness. If, as appears to be the case in the time of the Homeric myths that are staged in Greek tragedy, such a psychology did not exist, then her action appears to be entirely arbitrary. This position is argued in A.E.R. Dodds' book, The Greeks and the Irrational. In it, he states that the madness of Homeric heroes was considered a state of mind imposed on mortals by the gods. While the golden era of ancient Greek tragedy came in a later age and accompanied a flowering of rational self-consciousness, there remains in the tragedies a persistent sense of the imposition of the gods on human behavior. This is particularly visible in the recurrent reference to dreams as communications from the gods, rather than expressions of the inner self, as we tend to understand them in a post-Freudian era. "
Aristotle favors those events which are most astonishing because they happen contrary to the audience's expectations. At the same time, he insists that while they are unexpected, they must adhere to the causal logic of the play. This apparent contradiction results in surprisingly interesting situations in which the audience is astonished by an unexpected turn of events, and still has to concede that it could have foreseen the surprise if they had not been led to develop contrary expectations.
