Tragedy is an imitation of action with the following characteristics: it is serious, complete, of significant magnitude, depicted with rhythmic language and/or song, in the form of action (not narrative), and produces a 'purgation' of pity and fear in the audience (also known as catharsis).
Since tragedy is the imitation of action, it is chiefly concerned with the lives of men, and thus presents a stage for character and thought. Character - the qualities ascribed to a certain man - and thought, according to Aristotle, are the two causes from which actions spring. These elements also determine the success of a given action. Plot, then, is arrangements of incidents (successes or failures) that result from character and thought giving way to action.
With the above in mind, Aristotle lays out the six parts that define a tragedy:
c. diction (rhythmic language)
Plot is the most important part of a tragedy for a number of reasons. First, the result of a man's actions determines his success or failure, and hence his happiness, so it is action which is paramount - not character, which doesn't necessarily affect every action. Second, without action, there cannot be a tragedy - but there can be a tragedy without character. Thirdly, diction, song, and thought - even elegantly combined - cannot replicate the action of life without plot.
Plot, then, is the 'soul of a tragedy,' and character comes second. Rounding out his rankings: thought, meaning what a character says in a given circumstance, followed by diction, song, and spectacle.
Aristotle goes on to describe the elements of plot, which include completeness, magnitude, unity, determinate structure, and universality. Completeness refers to the necessity of a tragedy to have a beginning, middle, and end. A 'beginning' is defined as an origin, by which something naturally comes to be. An 'end,' meanwhile, follows another incident by necessity, but has nothing necessarily following it. The 'middle' follows something just as something must follow it.
'Magnitude' refers simply to length -- the tragedy must be of a 'length which can be easily embraced by the memory.' That said, Aristotle believes that the longer a tragedy, the more beautiful it can be, provided it maintains its beginning, middle, and end. And in the sequence of these three acts, the tragedy will present a change 'from bad fortune to good, or from good fortune to bad.'
'Unity' refers to the centering of all the plot's action around a common theme or idea.
'Determinate structure' refers to the fact that the plot all hinges on a sequence of causal, imitative events, so if one were to remove even one part of the plot, the entire tragedy 'will be disjointed and disturbed.' More simply, every part of a good plot is necessary.
'Universality' refers to the necessity of a given character to speak or act according to how all or most humans would react in a given situation, 'according to the law of probability or necessity.'
Aristotle ends this discussion of plot elements by pointing his out his particular disdain for 'episodic' plots - plots in which episodes succeed one another 'without probably or necessary sequence' (like a weekly sitcom, for instance). These episodic dramas stretch plot 'beyond their capacity,' and hence are inorganic.
Aristotle highlights the primacy of action in this section as the key to an artist's imitation. Indeed, because action initiates a chain of causal events, it is the single most important driver of plot. Though an astute reader might ask 'But what causes action?', Aristotle quickly responds by arguing that ultimately the things that drive action - character and thought - aren't nearly as important as the action itself. For plot is the simple arrangement of incidents in causal chains, and in this plot alone we can find satisfaction, even if it is not clearly motivated with character or thought.
That said, the best of tragedies maintain the primacy of plot while also inlaying the drama with character, diction (rhythmic language), thought, spectacle, and song. Character here refers to the attributes either ascribed or clearly evident in a given man - virtues which ultimately define a tragic hero's flaw and the source of his redemption. Thought, meanwhile, refers to the ideas of a given character, conveyed by speech. Though thought illuminates character, it is not necessary for it - indeed, a silent hero still would have a clearly delineated character, and perhaps an even clearer one than a loquacious character. Again, Aristotle's thesis is proved - that it is action that is paramount, regardless of motivation or underlying cause.
'Unity' is another concept which may confuse the reader, since Aristotle does not spend much time explicating it. Unity refers to the ability of the best plots to revolve around an axis, a theme which 'unites' all the action. A unified drama will have a 'spine' - a central idea which motivates all the action, characters, thoughts, diction, and spectacle.
'Determinate structure' follows from unity -- if the action revolves around a central spine, it creates a full skeleton of plot. But remove one bone, and the entire body of action becomes unstable, since every bone radiates from the central spine and is thus fully necessary. The test, Aristotle says, is to see if there is any part of the plot which can be removed without missing it. If this is true, then it must be excised. A true drama never wanders from its central spine for fear of losing its unity.
'Universality', meanwhile, is slightly more vague, but appeals to our common sense. Aristotle simply states that a character must act in accordance with human nature - either through probability, i.e. what 'most of us' would do, or through necessity, i.e. what we are 'forced' to do. An action cannot seem arbitrary - otherwise not only will it violate the determinate structure and break unity, but it will also irritate an audience that sees no basis for the action in human behavior.