system
Given the following post, respond as if you're commenting on Reddit:
human
Why do I see black when I close both eyes, but when I close just one eye, that eye sees nothing?
gpt
It has nothing to do with seeing, and everything to do with perceiving.  The vision center in the brain is constantly filtering out excess stimulus and filling in gaps in what it sees.  One of the things it does is fuse the image from each eye: when we see using both eyes, our brain puts the information together into a single field, rather than two separate perceptions of reality.  It fills in holes with predictions and eliminates constant stimulus that aren't important (like your nose).

When you close one eye, your brain tries to fuse the dark and light images, and determines that something is occluding the image from the shut eye, and proceeds as normal.  It's just like when you're sitting behind a tall person looking around their head - your brain ignores the back of the head and you 'see around it' (or a little bit of a double image).

In another example, when you wear the Red-Cyan or Green-Magenta 3-D glasses, you still end up seeing in near-full color even though one eye is only seeing a monochromatic image because your brain has fused the colors together!  It does the same thing with the darkened eye.

All of that happens without our awareness of it - you can force your mind to start paying attention to the details by focusing on a light through your eyelid, applying pressure to it: our conscious mind can circumvent the autonomic actions of the vision center by telling it to stop discarding the information because now it's important and the focus of your mental processes.