Berkeley's tree in the quad

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."