40 years old! The ending still ambiguous.
One of the many great things about this movie, is the open for interpretation ending.
Experts on this movie have theories but they remain just that, theories.
I never thought this movie was that ambiguous.
The ghosts were real. They let Jack out of the storage closet. When Jack dies at the end of the movie, he joined the other ghosts as the caretaker. Like Grady said, he was the caretaker. He was always the caretaker.
Here are the reasons supporting the psychosis / cabin fever theory, and what we are seeing are the delusions of cabin fever, not ghosts.
1. The delusions are never seen by two characters at once.
2. The viewer never sees the delusions without a character present. The viewer only sees what the character sees, nothing more.
3. The delusions are specific to each character, no character has contact with another characters delusions.
4. Cabin fever is real psychiatric condition. People without adaquate social contact can eventually develop it.
5. The photo in the ending shot was seen by Jack soon after the staff left; and along with the murder story by a previous cartaker, provided the framework for his psychosis, which the viewer is invited into.
6. The Grady delusion is never seen alone to the viewer unlocking the door. We HEAR the delusion of the unlocking along with Jack. Grady's "ghost" is not even in the scene, implying that we are hearing Jack's delusion.
I find those arguments to be pretty weak. It's clearly established that Danny has genuine psychic powers and he sees ghosts himself, and he sees the ghosts first and most clearly, at least until the hotel really starts to get its hooks into Jack. IMO it makes a lot more sense that his abilities were showing him something real than that all three of them were having delusions due to cabin fever.
IIRC Kubrick said in an interview that Jack being let out of the storage closet confirmed that the ghosts weren't imaginary, which just confirmed what I already thought.
I find it very interesting that the ghosts and Danny's abilities can never be proven real to the viewer.
There is no actual proof that the ghosts are real and not delusions.
All that is needed is one of the conditions to be violated above.
Why do these conditions 1,2,3 and 6 exist? Surely that is not just a coincidence that they do in this film. But put in deliberately to provide doubt for viewers who don't believe in ghosts.