This is from an essay called “Here Today,” in Hayden Carruth’s Effluences from the Sacred Caves. In my early years as a poet, I learned more about craft from reading this book than almost anywhere else:
“[W]hen poems gain in fluency and intelligibility, and hence in meaning, from being printed as prose, it is because the line has ceased to function…and when the line has ceased to function it is because the language has become too dull to sustain the measure. This, incidentally, is the right way to say it: language sustaining the measure, not the other way around, as many have thought–or hoped.”
It’s worth noting that he’s not talking about any specific measure, a term he is using in its musical sense, just the idea that for a line to function as poetry, it ought to have one.