"We grow accustomed to the Dark—" begins with a more literal representation of darkness. The speaker recognizes that all of us have the capacity to adjust our eyes to darkness when light disappears. She uses the example of a neighbor holding up a lamp to say goodbye; when that light is taken away, the speaker's vision is a bit "uncertain" as she tries to find her footing again.
In lines 9–10, Darkness becomes a extended metaphor. The speaker compares darkness to an "Evening of the Brain," when the light of the moon cannot be seen. The juxtaposition of these images of darkness and light, particularly when examined within the context of the "Brain," become metaphors for despair and hope. These large "Darknesses" are times when humans face the uncertainties inherent in loss, fear, and grief.
The speaker believes that the "Bravest" people continue to "grope" through those moments of mental darkness, which sometimes results in unexpected pain. A metaphorical "Tree," an obstacle that is unforeseen and unmoving, sometimes appears out of nowhere in the darkness, causing an abrupt change of course. Yet these brave souls continue on, determined to continue on a path forward even though light, or hope, is sometimes nonexistent. Living in a state of mental darkness requires courage, and finding one's way isn't easy. Yet with brave efforts, those who find themselves living in mental darkness can find a way to walk "almost straight" into an unknown future through directly confronting these "Evenings of the Brain."