The speaker describes having two kinds of sight. The first eye opens naturally, drawn by the sun's warmth, and all it does is stare. It seems to stand for simple, instinctive seeing, the way a body takes in the world before it starts thinking about it. The second eye is different. It is "ripped open" by an apothecary and held in place with toothpicks, systems and words, as if learning and language were forced on the speaker rather than chosen. The image is painful and a little grotesque, like a medical procedure done without consent.
The speaker admits to barely knowing this second eye is there, except that it aches when they think too much. Then the ending turns bleak: the first eye is blind, and there is no other. Judging by the lines, the speaker feels that neither way of seeing can be trusted. The mood is stark, uneasy and lonely, with a dry, almost clinical tone that makes the sense of loss hit harder.