If a tree falls in the forest and there's no-one there to observe it, is it real? George Berkeley (March 12 1685 to January 14 1753) said "Esse est percipi" ("to be is to be perceived") which fits nicely with quantum mechanics, where observing an experiment can change the outcome. Ronald Knox wrote this pair of limericks to sum up Berkeley's view:
There was a young man who said "God,
I find it exceedingly odd,
That the willow oak tree
Continues to be
When there's no one about in the Quad."
"Dear Sir, your astonishment's odd,
For I'm always about in the Quad;
And that's why the tree
Continues to be.
Signed Yours faithfully, GOD."