My daughter is eighteen months old. Her favorite word is "dog". She loves dogs. Whenever she sees one, she gets excited, points, and calls out "Dog! Dog! Dog!"
Sometimes she does this whenever she sees a cat, or a rabbit, or even a squirrel. This is quite understandable: she's generalizing furry, four-legged animals as dogs. Sometimes, however, she does this to people, which was a bit worrisome.
And sometimes she does this when there's nothing to point at. She'll point in the general direction of trees and brush and say "Dog," when there is no dog, or any other animal, present. This vexed me until last night, when I suddenly realized what was going on.
In the pilot episode of Star Trek: Deep Space Nine, Benjamin Sisko encounters the beings that the Bajorans call "the Prophets" and worship as gods. They have difficulty communicating with each other until Sisko realizes that he experiences time differently than they do. He lives in linear time -- that is, he experiences one moment of time, and cannot move to the past or the future, except in succession. The Prophets, however, experience all points in time simultaneously; they can be in the past, present, and future, all at once.
So, why does my little girl point at nothing and say "Dog"? It's because she, as a very bright and talented child, is not limited to linear time. She experiences the past, present, and future simultaneously. When she points at what I see as nothing and says "Dog", there was in the past, or will be in the future, a dog there.