Darkness and light depend on each other in this poem, and the speaker keeps turning that idea over. It begins with darkness that seems to come from nowhere, almost like a thought taking shape, with its "four corners" and its "wrinkles pressed gently out." There's a quiet physical presence here, even in the tears, which the speaker seems to accept rather than push away.
Light is described in similar terms: it flows, shimmers, and touches both soul and body. Then comes the central question: if you had never known light, how could darkness mean anything? The last stanza answers it. A shadow only exists because of light, and brightness only matters against the "grayscale of feeling."
The mood is calm, reflective, and tender. The speaker doesn't treat sadness as something to escape. Instead, the poem suggests that pain and joy give each other shape, and that most of life happens somewhere in between.