RS Crane (Languages of Crit. & the Structure of Poetry), again, emphasizes how Aristotle's Poetics differs in approach from most contemporary crit., even that which uses Aristotelian terminology.
In Winters, Ransom, Empson, Brooks & Warren, et al., the difference is clear right at the beginning, with their various opening definitions of poetry. As Crane shows, these critics define poetry as a branch of discourse. Aristotle defines tragic & epic poetry as a branch of artistic imitation. The poem is an "artificial whole" or "imitation". The imaginative wholeness of the poem evokes, by analogy, the wholeness of a particular action (beginning, middle, end). Aristotle doesn't try to define all poetry; he focuses on examples of a particular kind. Within this kind, the poem is a kind of imaginative simulacrum (mimesis) of an action. This is its substance ("plot is the soul of the poem"); language is the medium (there are other kinds of imitations - visual, musical - which use different media).
The relations between this particular kind of poetry (imitative : tragic/dramatic, epic) and other kinds (didactic, lyric) are not dealt with by Aristotle. But a criticism which takes as its subject the poem as aesthetic whole, rather than the poem as form of discourse, or merely verbal structure, is a criticism which is marking out a different approach to poetry as a whole. The notion of the poem as some form of relation or balance - in Aristotle's terms, a mean between extremes - a balance, that is, between logos & mythos, between medium and theme, between discourse and subject - a relation aiming at a fusion, an aesthetic wholeness, a complete impression, a fruition or fulness (Aristotle's proper "magnitude")... - this whole idea raises new perspectives, new plateaus, for critical reception of various kinds of poetry.