From this he falls to thinking of all the paintings he knows which represent actual windows and the view beyond ... The effect of the frame within the frame is potent, almost magical. What is it about windows? The comfort and safety of the known world set beside the promise and excitement of a world beyond."
From Motherland by William Nicholson.
Painting: A Window in St. John's Wood, Harold Knight, c. 1932.
There's more Nicholson/Knight here.