2.8. The Contents of the Tragic Action
It remains to define those terms which are to serve as the basis of the first classification of plots into simple and complex. The criterion is the presence or absence of reversal (peripeteia ) and discovery (anagnorisis ). Reversal and recognition are aspects of the action where form and content are indistinguishable.
Reversal of the situation is a change by which the action veers round to its opposite, subject always to our rule of probability or necessity. . . .
Recognition, as the name indicates, is a change from ignorance to knowledge, producing love or hate between the persons destined by the poet for good or bad fortune. The best form of recognition is coincident with a reversal of the situation, as in the Oedipus. (Poetics 54, XI.1-2)
Reversal is related to the fortune of the characters, and recognition is relative to their identity. In reversal, the intention of the character produces results opposite to the desired ones; whereas there is not a specific intention in the case of recognition. Reversal is intimately connected with the requirement set by Aristotle for the best tragic plots, which involve a passage from happiness to unhappiness. Reversal has also a suggestion of sudden change from good to bad or bad to good. The key moment of reversal is the turning point (metabasis ) where the downfall of the protagonist begins. It may be that the two meanings are not consciously divided by Aristotle. It is important to note that the reversal must be both intelligible and paradoxical. This is the key to Aristotle's conception of plot as integrating pleasure and instruction.
The definition of discovery is related to the requirement that the tragic action must involve friends or next of kin, because only these relationships can bring about the greatest suffering on scene, and only they are capable of causing pity and fear, which are the objective proper to tragedy. There is a potential for irony here: the character's deeds or words escape his intention, as he is not in knowledge of all the facts, and are charged with another meaning by the situation. At the turning-point, "action is confronted by its own unintended outcome." So, both peripeteia and anagnorisis are developments and complements of the basic ignorance or error (hamartia ) of the character.
Another basic content of the tragic action, indeed, the one which is popularly associated with the concept of tragedy, is calamity (pathos). The best effects of pity and fear are obtained by the means of scenes of pathos, of physical or moral suffering on the part of the characters: "The scene of suffering is a destructive or painful action, such as death on the stage, bodily agony, wounds, and the like" (Poetics 54-55, XI.6)
The conditions set by Aristotle on these key elements of tragedy are strict: they must not be gratuitous, and the unity of the plot must be preserved. Reversal and recognition "should arise from the internal structure of the plot, so that what follows should be the necessary or probable result of the preceding action. It makes all the difference whether any given event is a case of propter hoc [causal relationship] or post hoc [mere succession]" (Poetics 54, X.3). Succession does not equal causality; post hoc non ergo propter hoc . The disaster of the tragical hero must be a logical disaster. Aristotle bans supernatural or irrational solutions from the tragedy.
It is therefore evident that the unraveling of the plot, no less than the complication, must arise out of the plot itself, it must not be brought about by the deus ex machina. (Poetics 57, XV.7)
Here and elsewhere Aristotle is not describing actual Greek practice, but rather selecting what he thinks is the best. In this way he defines his own ideal of poetry, one in which there is no place for the irrational except in peripheral areas of the work, and everything in the plot, except maybe the occasion of the plot itself, is subject to the play of human intentions, expectations and agency.
The formal requirements of unity, causality, and the element of surprise and point of view represented by recognition and reversal are integrated in Aristotle's remark that pity and fear are heightened when things happen unexpectedly as well as logically, for then they will be more remarkable than if they seem merely mechanical or accidental: "The tragic wonder will then be greater than if they happened of themselves or by accident; for even coincidences are most striking when they have an air of design" (Poetics 54, IX.12).
 Stephen Halliwell, Aristotle's Poetics 208.