The poem turns on a twist in the familiar saying about missing the forest for the trees. Here the speaker asks who will manage to see the whole forest, and then narrows the view to one "melancholy tree": the brain. The question hangs in the air without an answer, which gives the opening a quiet, uneasy tone.
The last two lines are the most puzzling. The brain "goes to the light / with a thorn," as if it reaches toward something bright and hopeful while carrying a sharp pain it can't shed. Judging by the imagery, the speaker seems to be thinking about how sadness narrows our attention, so that we see only our own troubled mind and not the wider world around it. Still, the movement toward light adds a thread of hope. The mood is wistful and reflective, a little lonely, but not entirely without comfort. The poem is short enough that its gaps invite you to fill them in yourself.