"Turned . . . to stone"—these are the only words in the poem that speak in the past tense. Even so, they do not name something gone by, something no longer present. They name something that persists and that has already persisted. It is only in turning to stone that the threshold presences at all

The threshold is the ground-beam that bears the doorway as a whole. It sustains the middle in which the two, the outside and the inside, penetrate each other. The threshold bears the between. What goes out and goes in, in the between, is joined in the between's dependability. The dependability of the middle must never yield either way. The settling of the between needs something that can endure, and is in this sense hard. The threshold, as the settlement of the between, is hard because pain has petrified it. But the pain that became appropriated to stone did not harden into the threshold in order to congeal there. The pain presences unflagging in the threshold, as pain.

But what is pain? Pain rends. It is the rift. But it does not tear apart into dispersive fragments. Pain indeed tears asunder, it separates, yet so that at the same time it draws everything to itself, gathers it to itself. Its rending, as a separating that gathers, is at the same time that drawing which, like the pen-drawing of a plan or sketch, draws and joins together what is held apart in separation. Pain is the joining agent in the rending that divides and gathers. Pain is the joining of the rift. The joining is the threshold. It settles the between, the middle of the two that are separated in it. Pain joins the rift of the difference. Pain is the dif-ference itself.

		Pain has turned the threshold to stone.

The verse calls the dif-ference, but it neither thinks it specifically nor does it call its nature by this name. The verse calls the separation of the between, the gathering middle, in whose intimacy the bearing of things and the granting of world pervade one another.

Martin Heidegger (GA 7) Language

GA 12 p. 23-24