Why can't I see the blind spot in my own eye?

The optic nerve has to leave the eyeball somewhere, and where it does — the optic disc — there is no room for light-detecting cells. The result is a real hole in the retina of each eye, sitting about 15 degrees off-centre from where you are looking.

You can catch it. Close one eye, fix the other on a mark on a page, and slowly move a second mark toward and away from you. At a certain distance the second mark simply vanishes. Keep going and it comes back.

The part that should unsettle you is what happens next. Your brain does not leave a gap or a smudge. It fills the space with whatever is around it — aim your blind spot at the middle of a yellow ring and you will see a solid yellow disc. Your visual system is not reporting the world. It is producing a best guess, continuously, and it is very good at hiding the seams.