However, this immediately leads to obvious absurdities. Since the vast majority of the physical universe is not being observed, it must not exist. In fact, when you leave an empty room, it pops out of existence -- at least until someone re-enters it.
But Berkeley apparently formulated his ontology (philosophy of being) with this problem in mind, because he used it to argue for the existence of God. Since it will always be more rational to believe that the universe exists even when we are not observing it, this proves that the universe as a whole is always being observed. There is, in other words, a Cosmic Observer. When we leave a room and it has no human observer, it is still observed by God.
This is a pretty ingenious argument. Of course, the whole thing hinges on Berkeley's dubious ontology that "to be" means "to be observed" (although, this does bear a striking resemblance to the Copenhagen interpretation of quantum mechanics).
All of this is just background to two limericks which I had forgotten about but just rediscovered. The first one goes like this:
There was once a man who said, ‘God
Must think it exceedingly odd
If he finds that this tree
Continues to be
When there’s no one about in the quad.’
This then led to the second limerick:
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.
I just thought that was kind of funny.