My attempt here is to look at the classificatory method in Aristotle’s definition of tragedy. I will therefore restrict myself to the first few chapters of the Poetics. What I find remarkable about Aristotle’s method of classification is its organic nature. He begins his discussion by distinguishing the various fine arts. For this he institutes a threefold framework of imitation: medium, object and manner of imitation. This allows him to obtain a definition of tragedy that is particular, distinct from other arts and from other genres of poetry. When he goes onto discuss the constituent parts of tragedy, again he works with this framework; this time he obtains the six parts of a tragedy based again on the threefold notion of imitation. Thus plot, character and thought are objects of imitation, song and diction are the medium of imitation and spectacle is the manner of imitation. Thus at both the level of the general (arts) and the particular (tragedy) his framework for analysis is constant. This approach in Aristotle strengthens his claim about poetry that it sees the universal in the particular; his own theory follows that precept.
When he discusses the structure of the plot he points that it is a whole and complete, and has magnitude. These again are categories that depend on each other: a whole has a beginning, middle and end. The relation between these three leads us to the notion of completeness or unity. Further, by complete he also means what has an appropriate end: i.e. the action being imitated should reach the appropriate conclusion. This leads us back to the notion of whole as well as his notion of the function of tragedy. By magnitude he means the size, and how long a tragedy should be depends not on anything but the action being imitated and its end. So whatever length is required take the tragedy to its conclusion is the right magnitude. Here again, notions of whole, unity and the function are invoked. Thus the categories that he uses interilluminate each other.
When we attend to the interrelationship between the various categories in Aristotle we notice that they all are linked seamlessly. Discussion of one makes it necessary to invoke the other categories. Then again, his principle of probability and necessity work with the notions of unity, whole and the function. The imitated action he says should be probable and necessary: that means that it should be contextually realistic and that it should be included only if necessary to achieve the function, only if fits into the focus of the tragedy. Thus whatever angle we approach his theory we notice that his categories are integrated: supporting each other and interilluminating each other. This illustrates one of his prescriptions for the plot of a tragedy: that it should be well knit, organic.