From the London paper Independent, 2004:
But when the spell holds, it holds just as strongly in deserted places, where there's no figure at all. The New York street shown in 'Early Sunday Morning' is entirely unpeopled. There is no one in it, no one looking at it. The bright illuminated pharmacy window in 'Drug Store', with its sign saying "Prescriptions Drugs Ex-Lax", is blazing away on a street corner through the night, advertising itself - to nobody.
"When we were at school," Hopper once remembered, "we debated what a room looked like when there was no one to see it, nobody looking in, even." People often talk about that kind of thing. They don't often find a way to paint it, as Edward Hopper found, for a short stretch of his career, painting a transfixing blank, a world from which human perspective has been withdrawn."