The sun and not the sun

Imagine all your life
you were blinded
and kept in a room.

Suddenly the cloak
is pulled off your head
and there you sit,
in front of a perfect mirror
slanted toward you.

Through a hole in the ceiling.
the sun shines down
onto the mirror
and directly into your eyes.

You hear someone say:
  This is the sun.
  This is not the sun.
  It is both the sun and not the sun.
  It is neither “the sun” nor “not the sun”.

What are they trying to say?