O what a life is the eye! what a strange and inscrutable essence!
Him, that is utterly blind, nor glimpses the fire that warms him;
Him that never beheld the swelling breast of his mother;
Him that smiled in his gladness as a babe that smiles in its slumber;
Even for him it exists! It moves and stirs in its prison!
Lives with a separate life: and—“Is it a spirit!” he murmurs:
“Sure, it has thoughts of its own, and to see is only a language!”

Based on the title and what happens in the poem, my guess is that this poem is about how he has a new appreciation for his sight, he was blind for a while but it has now returned. It could be truth or metaphor, it could be that he truly lost the ability to see the world around us and then got it back or it could be that he lost the ability to see the truth and now he feels that it has returned.