You see a family in the park and you naturally list the members as "Mom, Dad, son, and daughter". But from whose perspective is this?
"Mom" and "Dad" are identifiers as seen from the perspective of the children, while "son" and "daughter" are identifiers as seen from the perspective of the parents. It's as if we shifted perspective midway through the list!
A fixed perspective would be, for example, something like "me, my sister, my mom, and my dad".
Or maybe the perspective is fixed. Not fixed at any particular person, but rather fixed at the center of gravity of the family, floating somewhere between parents and children. Like some sort of averaged perspective.
Then, it gets more complicated when extended family members come along: "Oh, here comes Grandpa and Auntie!"
So now we've got Mom, Dad, son, daughter, Grandpa, and Auntie. That's two identifiers from the perspective of the parents, and four identifiers from the perspective of the children. But why shouldn't it be from the perspective of the mother, since she is now closest to the center of gravity of the family? She would describe the whole thing as "me, my husband, my father, my sister, and my children". That gives a completely different sense from "Mom, Dad, son, daughter, Grandpa, and Auntie".
(And don't even think about how Auntie views the whole thing: "me, my father, my sister, my brother-in-law, my niece, and my nephew".)
As our list grows, it becomes more and more a view from nowhere.
Shifting perspective, center of gravity, view from nowhere... What the heck am I talking about?