What cognitive models of human imagination exist? How does the human imagine things that don't exist?

Suppose I ask anyone to imagine himself climbing a mountain. He can't imagine that until he has some experience about mountaineering whether through a movie, documentary or some reading material and Wikipedia page on Imagination says that

...without knowledge, imagination can not be developed...

If he has this experience he would be able to imagine himself climbing a mountain. Are there any theories that explain this?