What does the last 2 lines in poem mean?

In the poem, the speaker watches as a man who has lost everything sits by a stream. The man is still, and does not move "when the flies come." The last two lines of the poem imply that if there were some way for the man to feel again – whether it be the movement of flies, or the feeling of the water – then he might move.

As it is, the man is "so tired" that he cannot find the energy to feel. He is so far from himself, or so trapped in the depth of his suffering that he no longer feels what is happening to him.