Horizon

What was it --I paused to think --what was it that so unnerved me in the contemplation of the House of Usher? It was a mystery all insoluble; nor could I grapple with the shadowy fancies that crowded upon me as I pondered. I was forced to fall back upon the unsatisfactory conclusion, that while, beyond doubt, there are combinations of very simple natural objects which have the power of thus affecting us, still the analysis of this power lies among considerations beyond our depth. It was possible, I reflected, that a mere different arrangement of the particulars of the scene, of the details of the picture, would be sufficient to modify, or perhaps to annihilate its capacity for sorrowful impression; and, acting upon this idea, I reined my horse to the precipitous brink of a black and lurid tarn that lay in unruffled lustre by the dwelling, and gazed down --but with a shudder even more thrilling than before --upon the remodelled and inverted images of the gray sedge, and the ghastly tree-stems, and the vacant and eye-like windows.

I think this story is all about the difficulty of correct interpretation, the hermaneutic circle, that sort of thing.

Actually, isn't most horror about that?

When I think of true horror I think of the struggle with internal monsters. Which I'm guessing have their source in that difficulty of correct interpretation.

Well, as I recall pretty much everything in The Fall of the House of Usher hinges on the narrator's inability to correctly interpret what's going on around him, starting with the fact that he can't account for the effect the house has on him.

I haven't read a lot of them for a while, but a lot of Poe's stories play with this kind of uncertainty. Things are often very ambiguous (or sometimes vague) and the consequences of guessing wrong are generally gruesome. I'm thinking of The Black Cat, The Telltale Heart, pretty much any of his more famous stories that come to mind at the moment.

And, I don't know, seems to me that kind of uncertainty is a big part of what scares us, regardless of whether it's internal or not.

Uncertainty is the key word. The unknown has it's terrors but it's when we are uncertain about the known that real horror arises. And I think this is strictly an internal matter. There is no ghoul waiting in the boughs of that tree over there but my imagination wills him into existence.

Not sure if this supports anything, but I read The Imp of the Perverse this morning and think this passage explains a lot about Poe's work:

We stand upon the brink of a precipice. We peer into the abyss- we grow sick and dizzy. Our first impulse is to shrink from the danger. Unaccountably we remain. By slow degrees our sickness and dizziness and horror become merged in a cloud of unnamable feeling. By gradations, still more imperceptible, this cloud assumes shape, as did the vapor from the bottle out of which arose the genius in the Arabian Nights. But out of this our cloud upon the precipice's edge, there grows into palpability, a shape, far more terrible than any genius or any demon of a tale, and yet it is but a thought, although a fearful one, and one which chills the very marrow of our bones with the fierceness of the delight of its horror. It is merely the idea of what would be our sensations during the sweeping precipitancy of a fall from such a height. And this fall- this rushing annihilation- for the very reason that it involves that one most ghastly and loathsome of all the most ghastly and loathsome images of death and suffering which have ever presented themselves to our imagination- for this very cause do we now the most vividly desire it. And because our reason violently deters us from the brink, therefore do we the most impetuously approach it. There is no passion in nature so demoniacally impatient, as that of him who, shuddering upon the edge of a precipice, thus meditates a Plunge. To indulge, for a moment, in any attempt at thought, is to be inevitably lost; for reflection but urges us to forbear, and therefore it is, I say, that we cannot. If there be no friendly arm to check us, or if we fail in a sudden effort to prostrate ourselves backward from the abyss, we plunge, and are destroyed.

Imperceptible is an important word too, i.e., the impossibility of accounting for or understanding things because whatever mysterious mechanisms they are based upon are by nature hidden from us. Poe seems obsessed with the idea that human reason cannot grasp anything important; the object of this grasping is not necessarily internal, though the anxiety obviously is.