As imitative arts Aristotle mentions poetry, dancing, music, and painting. They differ, he says, in their medium, objects, and manner. Poetry, dancing, and music he classifies together because they use the similar media of rhythm, language, or harmony either singly or combined. Music, for instance, uses both rhythm and harmony, dancing uses rhythm alone, and poetry uses language alone. Aristotle by this does not, as might seem, exclude rhythm and harmony from poetry. Indeed, he states explicitly that most forms of poetry do use all of the media mentioned: rhythm, tune, and meter. He is only insisting that imitation in unmetrical language is still poetry; that meter is not the characteristic element of poetic.[16] It is important to recognize that in classifying poetry with music and dancing, Aristotle is insisting that the common element in these arts is movement. Movement is characteristic of poetry, as color and form are characteristic of painting and sculpture. Thus in discussing the plot of tragedy, which he holds to be the highest and most characteristic form of poetry, Aristotle urges the necessity of unity and magnitude, both of which he defines in terms not of space relations, but of movement. For instance, to possess unity a plot must have a beginning, a middle and an end.
A beginning is that which does not itself follow anything by causal necessity, but after which something naturally is or comes to be. An end, on the contrary, is that which itself naturally follows some other thing, either by necessity, or as a rule, but has nothing following it. A middle is that which follows something as some other thing follows it.[17]
Furthermore, the magnitude which this dramatic movement should possess is also discussed not in terms of bulk, but of length.
As, therefore, in the case of animate bodies and organisms, a certain magnitude is necessary, and a magnitude which may be easily embraced in one view; so in the plot, a certain length is necessary, and a length which can easily be embraced by the memory.[18]
It is noteworthy that to Aristotle the characteristic movement of poetic depends on the dramatic unity and progression of a dramatic action, a plot. In the _Rhetoric_ he shows that the arrangement of the movement of a speech is governed by entirely different considerations. The unity of rhetoric is not dramatic, but logical. The order of the parts of a speech is determined not by a plot, but by the needs of presentation to an audience. For instance, a statement of the case is given first, and then the proof is marshalled.