That is the short answer. Here is the longer answer:
Isaac Newton (1642–1727), best known not for cat doors but for his work in mathematics, optics, and that thing called gravity, had rooms at Cambridge University’s Trinity College. In 1802 (you will notice this is some 75 years after Newton’s death), the cat-door story first appeared. This particular story was originally used to illustrate examples of the supposed incompetence of English people. Supposedly Sir Isaac wanted a convenient way to let his cat and her kitten in and out of his room without disturbing his work. So, genius that he was, he cut two holes in the door: a larger one for the cat and a smaller one for the kitten. The gist of this story is that Sir Isaac was actually sort of an idiot because he could not see that the kitten would just follow its mother through the larger door. Supposedly there are (or were) two plugged-up holes of appropriate size in the door to Newton’s room.
Now, I am here to tell you that Isaac Newton was not an idiot. Also, we do not even know if he actually had a cat. Also, even if there actually are (or were) two plugged-up holes in Newton’s door, that does not mean Newton himself made them. And finally, even if Newton did create a cat door, he was not the first to do so, and so we cannot say he “invented” the concept. Consider this snippet from Chaucer’s “The Miller’s Tale,” written in the late 1300s:
A hole he found, low down upon a board,
Through which the house cat had been wont to creep…
Sounds like a cat door to me.
This 15th-century French door with a cat door
cut into it offers proof that Isaac Newton
did not “invent” the cat door.
Public domain photo
Unported (CC BY-SA 3.0)],
via The Walters Art Museum.
And then there is the photo at right, which shows a hole, presumably for cats, cut into a 15th-century door from France.
So, Isaac Newton may have been a brilliant fellow, and maybe he was nuts about cats (though there is no evidence for this), but he did not invent the cat door. Sorry, Internet.
[She of Little Talent urges me to tell you that we went to The Straight Dope for the answer to this question. I must also tell you how disappointed old SoLT was to learn that the Newton Cat Door myth is not true.]