"Sir Isaac Newton, renowned inventor of the milled-edge coin and the catflap!"
"The what?" said Richard.
"The catflap! A device of the utmost cunning, perspicuity and invention. It is a door within a door, you see, a..."
"Yes," said Richard, "there was also the small matter of gravity."
"Gravity," said Dirk with a slightly dismissive shrug, "yes, there was that as well, I suppose. Though that, of course, was merely a discovery. It was there to be discovered." …
"You see?" he said dropping his cigarette butt, "They even keep it on at weekends. Someone was bound to notice sooner or later. But the catflap … ah, there is a very different matter. Invention, pure creative invention. It is a door within a door, you see."

Dirk Gently's Holistic Detective Agency (1987)