7. 
In 7, Aristotle addresses the proper magnitude of Tragedy as it relates to its characterization of an imitation of an action which is a whole. Tragedy: definition: tragedy is ‘an imitation of an action that is complete in itself, as a whole of some magnitude.’
A whole is ‘that which has beginning, middle and an end.’ 
The most important element of tragedy is the Plot.
A well-constructed plot must have a certain magnitude. Like all wholes, to be beautiful, it must have a determinate magnitude, because if it is too big or too small, this does not please the senses.

8.
In 8, Aristotle describes what makes for the unity of the plot. This consists in the representation of a single action. A single action is not unified in virtue of being of a single person, but that all of the elements make sense for the wholeness of the thing. This means that all things must make a perceptive difference by their presence and absence –no ‘arbitrary’ or superficial elements which are rationally unaccounted for, whose existence makes no difference. Then Aristotle introduces the distinction between poetry and history.

Aristotle, Poetics, 8-9:
Now here is the real issue, the crux of the matter for our purposes. Aristotle says that history is about what has happened while poetry is about what could happen, what is possible ‘as being probable or necessary’. He must mean as that which can happen in accordance with certain laws and order that make human sense, a certain teleological necessity, for then he establishes poetry as consisting of statements ‘of the nature rather of universals’, and history of ‘singulars’. What he means is that what happens in history has a great deal of contingency integrated, while poetry, especially Tragedy, deals with unities in accordance to universal laws of actions.
This is important for several reasons. History can or cannot be like poetry, in the sense that history can also be about the probable, because what happens can be what could happen as well. So there is a sense in which history might imitate action, but this is not essential to history, while it is essential to poetry. What would make it so that history could be about the probable in accordance with the universal? Well, if history is an imitation of a unified action, just like poetry is. We can make poetry of out history in two ways. The first way would be in the sense of the postmodernist conception of historical work, like White. That is: by the argument that all history is narrative, and narrative involves a plot, so every historical narration involves the element of emplotment, of a construction in accordance with criteria of unity. This is all well and good, but it does not claim that history itself, i.e., what has actually happens, the historical data which the historian works upon or the matter of history, it does not claim this is in itself unified and constituted in accordance with universality. Of course the post-modernists will doubt that there is such a thing as the ‘raw’ data of history, but the point still stands: we need not ascribe necessity to the historical matter itself, but simply claim that the form in which it must be conveyed, namely narrative, is inherently emploted, thus it is like poetry in that it involves action-like criteria of unity and order, in accordance with the prejudices or decisions of the historian. Yet there is a second way it could be Poetry-like: if it is designed to be so –if there are human choices being made so that history, the matter itself for the historical writing, produces certain outcomes in accordance to a plan. History can be inherently emploted if history has been designed. How could history be inherently designed, or designed in its matter, not merely in the form of its being retorativelly told in the historical narrative? Here too we have two choices, given that the design of an action requires the purposive decision of a maker. It can be designed by an intelligence outside history, or it can be designed by an intelligence within history. The first we might call the Providential view of history, the second we might call the Conspiratorial view of history.
	To summarize, two are the manners in which history itself can be poetic. History can be said to be poetic in the postmodernist sense that all history is narrative of history, and all narrative is emplotted. So it can be poetic due to its necessary form of being done qua the work of the historian working with certain prejudices and structural or methodological decisions. This is the position of White. Second, history can be poetic in the sense that it is inherently unified in a sense that displays the unfolding of a whole (action). In this second option, it can be said to be inherently poetic in two ways: either because of the providential design of an external architect, or because of the internal design of human beings as conscious architects of history. But, since not all human beings are conscious architects of history, then some use others for the sake of their plan. Thus, the second option is the conspiratorial view of historical poiesis. 
Aristotle will provide a normative theory of what makes for a good Tragic plot. If you are an architect of history, you will use such information to determine how to have the best effects for the ones who follow the plot unconsciously, so that history is structured to fit your outcomes.