In working on something else, I was reminded of the following passage from a letter from H. P. Lovecraft to Robert E. Howard, dated 4 December 1930:
I agree with what you say about suggestion as the highest form of horror-presentation. The basis of all true cosmic horror is violation of the order of nature, and the profoundest violations are always the least concrete and describable. In Machen, the subtlest story -- The White People -- is undoubtedly the greatest, even though it hasn't the tangible, visible terrors of The Great God Pan or The White Powder.[...] Of course, there is such a thing as excessive indefiniteness, especially among novices who do not really understand how to handle cosmic suggestion. Crude writers use the old trick of calling a hidden horror "too monstrous to describe", merely as an excuse for not forming any clear picture of the alleged horror themselves. But the skilled author who knows what he is doing can often hint a thing much better than it can be told. Drawing the line between concrete description and trans-dimensional suggestion is a very ticklish job. In the greatest horror-tale ever written -- Blackwood's Willows -- absolutely nothing takes open and visible form.[...]
-- Selected Letters III, p. 174
Now, I will admit up front that this is very much my own view of the matter, and one of the reasons I so often have trouble with many modern horror writers: because I feel that their tendency to pile on more and more visible, concrete horror actually shows a paucity of imagination and an excess of authorial sadism and a relish for the merely mundane physical level of horror, rather than the more difficult, and more genuinely terrifying, metaphysical terror which -- even if only for a moment, and for the duration of whatever tale it may be -- causes us to question or doubt our conceptions of what the universe and the nature of reality may be really like. No matter how unnerving a horror of the first type -- whether it be a serial killer, a ghoul, a zombie, a werewolf, or a particularly violent ghost -- may be, it remains more a physical threat or menace, rather than a "spiritual" something which genuinely unsettles a person's view of the fundamentals of existence.
After all, death is a thing we all face, sooner or later; but very little in life actually challenges us on the ground of our perceptions of existence, and it is this, I think, which constitutes truly good "horror" (or, to use the term I prefer, terror) literature whose impact lasts beyond the closing of the book. True, you can have tales which use the first as powerful symbols of the latter; Ambrose Bierce did that sort of thing quite well, perhaps never more effectively than in "The Death of Halpin Frayser", with its terrifying implications. But so few writers of today (not none, by any means, but very few, it seems to me) seem capable of doing this, let alone going straight for the basic underlying terror of that "violation of the order of nature" in its purest form, with all its metaphysical implications. It is here, I think, that the best "horror" verges on the realm of great poetry: the realm of the sublime.
At any rate, I thought I'd bring this quotation (and my thoughts) in for discussion, as I'd be interested in hearing what others think on this one....