"...the failure of the poem to reach the objective right margin of the page is for me one of the almost definitional ways poetry makes absence felt as a presence." (Ben Lerner)
"[Poetry is] lineated. It’s just that. A three-hour class on what is a prose poem is? A waste of time. That doesn’t mean it can’t be prose, or that prose can’t be poetry—but for all practical, speaking purposes, it’s right-flush margin or it’s lineated. It’s so simple. What is all this postmodern complicated bullshit?" (Mary Ruefle, HT Andrew Shields)
It can't be lineation. I'm not sure what Ben Lerner means by "the objective right margin" (the edge of the page? i.e., not the imaginary line where the text "returns" to the left margin.) In any case, there are too many examples of poetry, as distinct from prose, that does not depend on lineation. (At least lineation conceived of as a line return that stops short of the right margin.)
There's a difference between what Tony Tost did in Invisible Bride and what he did in Johnny Cash's American Recordings. And also a difference between the non-lineated passages of Ben Lerner's Angle of Yaw and what he did in Leaving the Atocha Station. That difference is the prose/poetry distinction, though all these works have right-flush margins.
Or here's an easy case: the introduction to Waldrop's Curves to the Apple vs. the actual pieces in The Reproduction of Profiles. Again, all right-flush margins but no question about what is prose and what is poetry. Much of Kate Greenstreet's "56 Days" has right-flush margins and is clearly poetry, not prose. Or compare Lisa Robertson's Seven Walks with Nilling: all the margins are flush but the former is poetry the latter is not. (The distinction is probably most subtle and most interesting in Nilling's "Time in the Codex" and "Lastingness", which might easily be confused with prose poems if it did not say "prose essays" on the cover.)
More and more I think Robertson is right that poetry is writing that "innovates its receivers". Prose is writing that does not. (This is also what Merleau-Ponty probably meant when he talked about "founding a new universality" with a "poetry of human relations".)
Another great example: Wittgenstein's Philosophical Investigations vs. Heidegger's Being and Time. Wittgenstein himself said philosophy should be written like poetry. And I think if we take a moment to seriously consider the "implied reader" in those two books, one of them is clearly being innovated while the other is merely being "convened", if you will, i.e., addressed as conventional academic subject.
The difference between poetry and prose is one thing. The difference between poetry and philosophy is another. As a contrast to prose, real philosophy is on the same
side page as poetry. That's an important clue to what poetry is.