Aristotle, from Poetics:
Tragedy is mimesis [representation] of an action that is serious, complete, and possessing serious implications, in embellished lexis [speech or diction], each kind of which is used separately in the different parts, in the mode of action and not narrated, and bringing about through pity and fear the catharsis1 of such emotions. By embellished, I mean having rhythm and melody, and by separately in different parts I mean that some parts of a play are carried on solely in meter while others are sung.
The Constituent Parts of Tragedy
Since the mimesis is carried out in the dramatic mode2 by the personages themselves, it necessarily follows, first, that the arrangement of opsis [spectacle] will be a part of tragedy, and next, that melos [melody] and lexis will be parts, since these are the media in which they bring about the imitation. By lexis I mean precisely the composition of the verses, by melos only that which is perfectly obvious. And since tragedy is mimesis of an action and is enacted by men in action, these persons must necessarily possess certain qualities of ethos [character3] and dianoia [thought or theme], since these are the basis for our ascribing qualities to the actions themselves ethos and dianoia are two natural causes of actions and it is in their actions that men universally meet with success or failure. The imitation of the action is the mythos [plot]. By mythos I mean the sequence of events. Ethos is what allows us to say that people are of a particular quality. And dianoia is present in everything in their utterances that aims to prove a point or that expresses an opinion. Necessarily, therefore, tragedy as a whole, considered as a special form, consists of six constituent elements, namely mythos, ethos, lexis, dianoia, opsis, and melos. Of these elements, lexis and melos are the media through which they stimulate the imagination, opsis is the manner, and mythos, ethos, and dianoia are the objects they imitate. These are the only parts. So then drama employs these six forms, not just some of them, for every play has each of these elements. But of all of these, the most important is mythos.
The Relationship between Mythos and Ethos
Tragedy is not mimesis of men but of actions and of life. Happiness and unhappiness are found in action, and the end we aim at is a kind of activity, not a quality. In accordance with their ethos men are of a particular quality, while in accordance with their actions they are fortunate or unfortunate. Consequently, a play’s characters do not engage in action for the purpose of displaying their ethos; rather, they take on the their ethos for the sake of their actions. Thus, what happens that is, mythos is the end or purpose for which a tragedy exists, and that purpose is the most important thing of all. What is more, tragedy could not exist without action, but it could without characterization. . . .
Essential Qualities of Mythos
Now that the parts are established and we have determined that mythos is the most important element of tragedy, let’s next discuss what qualities are essential to mythos. I have argued that tragedy is mimesis of an action that is complete in itself and possessing serious implications, for a thing may be a whole yet result in no implications worth mentioning. Now a thing is a whole if it has a beginning, a middle, and an end. A beginning is that which does not come necessarily after something else, but after which another thing naturally exists or comes to be. An end, on the contrary, is that which naturally comes after something else either as its necessary sequel or as its usual and hence likely sequel but has nothing after it. A middle is that which both comes after something else and has another thing following it. A well-constructed mythos, therefore, will neither begin at some chance point nor end at some chance point, but will observe the principles here stated.
Contrary to what some people think, a mythos that centers on one man is not necessarily enough to unified. Many things indeed an endless number of things happen to any one man, some of which do not go together to form a unity, and similarly among the actions one man performs are many that do not go together to produce a single, unified action. Those poets who have composed a Heracleid, a Theseid, and other such poems4 all have erred, therefore, by evidently assuming that since Heracles was one man, their plot was bound to be unified. . . .
The poet’s function is not to report things that have happened, but rather to tell of things that might happen, things that are possibilities by virtue of being in themselves inevitable or probable. Thus the difference between the historian and the poet is not that the historian employs prose and the poet verse the work of Herodotus5 could be put into verse, and it would be no less a history with verses than without them. Rather, the difference is that one tells of things that have been and the other of such things as might be. Poetry, therefore, is a more philosophical and a higher thing than history, in that poetry tends to express universals, while history conveys particulars. A universal is what a certain kind of person will say or do either probably or necessarily in a particular set of circumstances; this is the universal that poetry aims for (with the addition of names for the persons). On the other hand, an example of a particular is what Alcibiades6 did or had done to him. . . .
Simple and Complex Forms of Mythos
Mythos can be simple or complex; indeed, the actions depicted on stage can be differentiated in this way to begin with. Assuming the action to be continuous and unified, as already defined, I call that action simple in which a change of fortune takes place without peripeteia [reversal] or anagnorisis [recognition], and that action complex in which the change of fortune involves one or both. These events ought to be so rooted in the mythos that they follow from the preceding events as their inevitable or at least most likely outcome; for the difference between following from and merely following after is vast. . . .
Peripeteia7 refers to a change from one state of affairs to its exact opposite, and this too should happen in accord with probability or necessity. For example, in Oedipus, the messenger comes to cheer Oedipus by relieving him of fear with regard to his mother, but by revealing his true identity, does just the opposite of this. . . .
Recognition is a change from ignorance to knowledge, leading either to friendship or to hostility on the part of those persons who are marked for good fortune or bad.8 The best form of recognition is that which is accompanied by a reversal, as in the example from Oedipus. . . .
Among simple types of mythos, the episodic form is the worst. I call any episodic any mythos in which the episodes follow one another in a sequence divorced from inevitability or probability. Mythos of this kind is constructed by bad poets on their own account, and by good poets on account of the actors. Because they are composing entries for a competitive exhibition, they stretch the plot beyond what it can bear and are compelled to dislocate the natural order. . . .
Next in order after the points I have just dealt with, we must specify what one should aim at and what avoid in the construction of mythos, and what produces the effect appropriate to tragedy.
Because in the finest kind of tragedy the structure should be complex and not simple, and because it should also be mimesis of terrible and piteous events (that being the special mark of this type of mimesis), the first rule is that good men should not be shown passing from prosperity to misfortune, for this does not inspire either pity or fear, but only revulsion. Nor should evil men be shown rising from misfortune to prosperity, for this is the least tragic mythos of all it lacks every requirement, in that it neither elicits sympathy nor stirs pity or fear. And again, neither should an extremely wicked man be seen falling from prosperity into misfortune, for although such a plot might indeed call forth human sympathy, it would not elicit pity or fear, since we feel the first for a person whose misfortune is undeserved and the second for someone like ourselves. We are left with the man whose place is between these extremes. He is not pre-eminent in virtue and justice but does not fall into misfortune through vice or depravity. He falls because of hamartia9, like Oedipus and Thyestes and other renowned and prosperous men from similar families.
Therefore, the mythos that achieves excellence will necessarily be single in outcome and not, as some contend, double10, and will consist of a change of fortune, not from misfortune to prosperity, but the opposite from prosperity to misfortune, occasioned not by depravity, but by hamartia on the part of one who is either such as I have described or better than this. (What actually has taken place confirms this, for though at first the poets accepted whatever myths came to hand, today the finest tragedies are founded upon the stories of only a few houses, for example, those of Alcmeon, Oedipus, Orestes, Meleager, Thyestes, Telephus, and such others who have chanced to suffer terrible things or to do them.) So, then, tragedy employing this construction is the finest kind from an artistic point of view. And consequently, people who charge that this is what Euripides does in his tragedies and that most of his plays have unhappy endings are wrong. For this is in fact the right procedure, as I have said; and the best proof is that on the stage and in the dramatic contests, plays of this kind seem the most tragic, provided they are well done, and Euripides, even if in everything else his management is faulty, is the most tragic of the poets. . . .
In the characters and the mythos alike, one must strive for that which is either necessary or probable, so that whatever a character of any kind says or does may be the sort of thing such a character would certainly or most likely say or do, and the events depicted may follow one after another either inevitably or almost so. Obviously, then, the tragedy’s resolution should arise from the mythos itself and not be brought about from the machine11, as it is in Medea and in the embarkation scene in the Iliad 8. The machine is to be used for matters lying outside the drama, either antecedents of the action which a human being cannot know, or things subsequent to the action that have to be prophesied and announced, for we accept that the gods see everything. Within the events of the plot itself, however, nothing must be unreasonable, or if it is, it should be kept outside the play proper, as is done in the Oedipus of Sophocles.