No live organism can continue for long to exist sanely under conditions of absolute reality; even larks and katydids are supposed, by some, to dream. Hill House, not sane, stood by itself against its hills, holding darkness within […] (1.1)
Hill House unfortunately lacks access to that lovely subconscious release we all know as dreams. Has this given the place a surplus of fear and madness, the perfect house un-warming gift?
I am making these directions so detailed because it is inadvisable to stop in Hillsdale to ask your way. The people there are rude to strangers and openly hostile to anyone inquiring about Hill House. (1.46)
It's the classic "people fear what they don't understand" theme. Whether it's a house or somebody unknown driving through town, it's the same fear reaction.
No human eye can isolate the unhappy coincidence of line and place which suggests evil in the face of a house, and yet somehow a maniac juxtaposition, a badly turned angle, some chance meeting of roof and sky, turned Hill House into a place of despair, more frightening because the face of Hill House seemed awake, with a watchfulness from the blank windows and a touch of glee in the eyebrow of a cornice. (2.1)
It's completely irrational, but we totally understand where Eleanor is coming from here. Sometimes you look at the front of a house, and it just gives you the heebie-jeebies. Hill House, not surprisingly, is one such house.
[…]—Theodora caught at Eleanor's thought, and answered her. "Don't be so afraid all the time," she said and reached out to touch Eleanor's check with one finger. "We never know where our courage is coming from." (2.101)
Theodora hits upon one of Eleanor's major character flaws. She's always afraid, and mostly, she's afraid of herself.
"[…] People," the doctor said sadly, "are always so anxious to get things out into the open where they can put a name to them, even a meaningless name, so long as it has something of a scientific ring." (3.93)
The novel suggests that our ability to catalogue, categorize, and specialize give us a certain type of fear-cancelling power. This might prove true at a distance, but we'd have to put this one to the test in a dark forest filled with lions and tigers and—oh, we don't know—bears or something.
"We have grown to trust blindly in our senses of balance and reason, and I can see where the mind might fight wildly to preserve its own familiar stable patterns against all evidence that it was leaning sideways." [Dr. Montague] turned away. "We have marvels still before us" […] (4.98)
Remember the fear of the unknown we talked about earlier? Here, Dr. Montague explains what he feels is the key to that fear: the unknown.
The intelligent thing to do, perhaps, was to walk over and open the door; that, perhaps, would belong with the doctor's views of pure scientific inquiry. (4.269)
Unfortunately, reason and scientific inquiry go by the wayside when some unknown entity is banging on your door in a wild, terrifying fury.
[…] so that each of them seemed always waiting for a cry for help from one of the others; intelligence and understanding are really no protection at all, [Eleanor] thought. (5.131)
Just to drive the point home: Hill House is beyond intelligence and understanding. It just exists, and it is terrifying. If you connect this theme to other themes related to Hill House itself—like "The Home" or "Family"—it can really bring you to some interesting conclusions.
"Fear," the doctor said, "is the relinquishment of logic, the willing relinquishment of reasonable patterns. We yield to it or we fight it, but we cannot meet it halfway." (5.167)
We like the idea of fear being a willing relinquishment. It's almost as if the book is saying that we only fear the things we want to fear. That's neat. Does the novel itself support Dr. Montague's claim?
"Walled up alive." Eleanor began to laugh again at their stone faces. "Walled up alive," she said. "I want to stay here." (9.78)
Eleanor first feared arriving at Hill House, but now she understands Hill House and wants to stay. Well, she certainly feels like she understands it. Whether or not she truly does understand is debatable.