An onlooker looks at a mountain, but the left half of it is gone. It’s not really gone—other folks looking from his location will say that there’s an entire, beautiful mountain there—but the onlooker will insist that he can only see the right half.
This onlooker has Hemispatial Neglect, a disorder characterized by lack of awareness of one side of space. It’s not the same as being blind; someone with Hemispatial Neglect can still have very good vision. It’s not even like you’re closing one eye. In that case, you’ll still be able to see pretty much the same thing as when both your eyes are opened, though the image is slightly shifted.