Almost imperceptible differences in the similar (and similarities in difference) are what Perloff in her new book calls the "infrathin." She gives a paradigmatic example from Wallace Stevens: an unseen "cat running over the snow almost inaudibly." Or take Duchamp's readymade snow shovel, titled "In Advance of a Broken Arm," which Perloff mentions in this new book: the shovel anticipates the snow and the broken arm; this is infrathin as metonymy or, indeed, as sleight (slip) of hand. Moreover, as with all Duchamp's readymades, the difference between art and not-art is infrathin. (Perloff cites Stevens's "The Snow Man": "Nothing that is not there and the nothing that is.")

Poetry is not made up (only) of big ideas or themes or grand contrasts—good and evil, love and hate, night and day, life and death—but (also) of microtonal shifts, as with the difference between close, micro, and infra. Over...