Try this exercise: close your left eye. Notice that big thing obscuring the left side of your vision? Yes, it’s your nose! Now, close just your right eye. You can see your nose on the right! No big surprise there.
OK, now open both eyes – where’s the nose?! Sure, if you focus your vision right in front of you can make it out, vaguely. But you never really notice, as you are looking out on the world, that your nose is right there in the middle of your field of vision.
Where did it go? What you see with both eyes is the combination of what you see with each eye individually, isn’t it? Yet the nose gets magically subtracted out somewhere. Your brain does that, automatically. It recognizes that the nose isn’t anything important (it’s always going to be there, or at least, it’s not a problem until it ISN’T there), so it simply doesn’t process the nose into the visual image you are aware of.
This process is repeated in countless other ways. Your eyes “see” everything around you; all they do is receive light waves in a certain frequency range. Your brain determines what is significant enough to reach your level of conscious awareness. Why do you notice things the second time you see a movie that you were unaware of the first time? You “saw” exactly the same movie! But you weren’t focused on the same details, the first time, so it was as if they never entered your field of vision.