The fact that we’re all different people with each a different view on the world.
The perspective that any person has of him-/herself is one from the inside out, while the perspective that a person has on others, is one from the outside in.
The consequence of that is that everyone’s world is unique and different.
To illustrate: my world differs from the world of all other people, since I may be present in theirs, as one of the other people, while as such I will never be present in my own.
Hope that makes sense.