if a tree falls in the forest and nobody hears it...

I just heard an interesting discussion on philosophybites.com about different "levels of reality" and perception -- why quantum mechanics can describe a world equivalent and yet completely different from ours.

john-campbell-on-berkeleys-puzzle

Over a century later Berkeley's thought experiment was summarized in a limerick by Ronald Knox and an anonymous reply:

There was a young man who said "God
Must find it exceedingly odd
To think that the tree
Should continue to be
When there's no one about in the quad."
"Dear Sir: Your astonishment's odd;
I am always about in the quad.
And that's why the tree
Will continue to be
Since observed by, Yours faithfully, God."

In reference to Berkeley's philosophy, Dr. Samuel Johnson kicked a heavy stone and exclaimed, "I refute it thus!" A philosophical empiricist might reply that the only thing that Dr. Johnson knew about the stone was what he saw with his eyes, felt with his foot, and heard with his ears. That is, the existence of the stone consisted exclusively of Dr. Johnson's perceptions. What the stone really consisted of (given that such a question can in fact be asked sensibly) could be entirely different in construction to what was perceived - it existed, ultimately, as an idea in his mind, nothing more and nothing less.

see also http://en.wikipedia.org/wiki/George_Berkeley