What makes Henry James' The Turn of the Screw timeless?

I suppose one of the central aspects of this story that
makes it timeless is the way that, although it is obviously based in one context, it
contains elements and themes that are universal and are more about the human condition
rather than anything else. This means that even for us, who are separated from the
actual time of writing the story by so many years, can identify these elements and
themes, and because we can relate to them and apply them to our lives, the story
maintains its relevance and timeless quality.

There are so
many to choose from, but for me I would have to pick the ending as being the perfect
demonstration of this. Note how throughout the story, the narrator, the governess, is
actually presented as an unreliable narrator. We never know if she really can see the
ghosts that she tells us about, or if she is making them up, psychologically creating a
reason for her employer, the attractive ward of Miles and Flora, to come and rescue her.
It is clear from their first meeting that she is deeply impacted by his character, but
that she lacks enough perspicacity to realise her own feelings. Note what happens at the
end to continue this theme of ambiguity as the governess and Miles are on the bed
together and Peter Quint is supposedly haunting them:

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I caught him, yes, I held him--it may be
imagined with what a passion; but at the end of a minute I began to feel what it truly
was that I held. We were alone with the quiet day, and his little heart, dispossessed,
had stopped.

From her point
of view, the governess believes that Miles has died out of the shock of being haunted,
but the way in which she holds him with such a "passion" suggests that she actually
might have suffocated him. The ambiguity of the writing means we never actually know for
sure, but are left with these doubts. A timeless classic.