Second, we might ask to what extent katharsis is the purpose of tragedy, and to what extent it is an occasional effect of tragedy. The question of in what way art may be good for us is a very difficult question to answer. The best art (and this applies to Greek tragedy) is not didactic: it does not try to tell us outright how we ought or ought not to behave. At the same time, there is definitely a lot we can learn from a subtle appreciation of art. The value of art, on the whole, seems to stem more from its ability to arouse emotion and awareness on an abstract, general level, rather than to teach us particular truths. Oedipus Rex is valuable because it engenders a certain state of mind, not because it teaches us to avoid marrying older women whose family histories are uncertain.
Though katharsis may be an important effect of tragedy, it is hardly the reason for which poets write tragedies. If that were so, poets would be little more than emotional therapists. Again, Aristotle is writing as an observer more than as a theorist. He has observed that tragedy has a cathartic effect on its viewers, but he is not trying to enunciate this as the end goal of all tragedy.
The other important concept we encounter in this chapter is that of mythos. While "plot" is a pretty good translation of this word in reference to tragedy, mythos can be applied to sculpture, music, or any other art form. The mythos of a piece of art is the way it is structured and organized in order to make a coherent statement. Thus, when Aristotle speaks about the "plot" of a tragedy, he is not just referring to who did what to whom, but is speaking about how the events in the story come together to bring out deeper, general themes.
Plot, then, is central to a tragedy, because that is where, if at all, its value lies. If character were central to tragedy, we would be watching Oedipus Rex in order to learn something about Oedipus, about what makes him tick, or how he reacts in different situations. The character of Oedipus in itself is uninteresting: why should we care about the personality of someone who never existed? The value of Oedipus lies in what we can learn about ourselves and our world from observing his fate. What we learn from a tragedy—the effect it has on us—results from the way it is structured to draw our minds toward general truths and ideas; that is, from its mythos.