Notice that Machen said that if you encountered a singing rose you would be overwhelmed with horror rather than fear. Fear is a primitive emotional response-an instinctive reaction to perceived danger that we share with other mammals. But horror is a uniquely human state of mind that depends on sophisticated cognitive capacities of a sort that only human beings possess.
To see how horror works, look closely at Machen's examples. The idea of a singing rose has two components, neither of which is unnerving all on its own, but which result in something that's deeply disturbing when they're combined. Roses aren't horrifying. And singing isn't horrifying either. But marry the two components in a single thing-a singing rose-and the result is something horrifying.
A singing rose is an unnatural entity because it's a fusion of two incompatible kinds of thing into a single entity-human and plant. It is disorienting and threatening because it transgresses natural boundaries. Machen's other examples-talking dogs, growing stones, and blossoming pebbles-also violate what we take to be the boundaries that demarcate kinds of things from one another. Singing roses, talking dogs, and budding pebbles all seem, impossibly, to belong to two mutually exclusive categories at once.
Halloween, too, is replete with such images of unnatural boundary violations such as leering pumpkins, walking skeletons, and green-complexioned women who fly through the air on broomsticks. Of course, none of these things distress us because we know that they are all pretense. The ghosts and monsters that roam our streets, trick-or-treat bags in hand, are mere simulacra of horror. But if such beings were real-or if we believed they were real, as many people did in the past-then our emotional response would be different. We would be paralyzed with horror.
We get a glimpse of what it would be like to live in such a demon-haunted world by immersing ourselves in horror movies. But these movies introduce an additional element to the mix. Monsters, fiends, and demons populate horror fiction. These are not just ghastly, category-defying entities-they are also deadly and malevolent. For example, zombies contravene boundaries by being both alive and dead, and they also want to kill you and feast on your brain. Werewolves are both wolf and man, and they like nothing better than ripping you apart with their fangs. They're frightening because of their dangerousness and horrific, because of their contradictory character. [...] such things are metaphysically threatening. The threats posed by physically dangerous things can, at least in principle, be managed. You can avoid stepping on a rattlesnake or take cover when a deranged shooter opens fire. But metaphysically threatening things endanger our entire conception of the structure of reality, and jeopardize the edifice of assumptions and expectations that we count on to provide a measure of security in an uncertain world. If roses can sing, anything can happen. If roses can sing, there is no place that's safe.
"Natural boundaries", that's the key. Philosophers talk of natural kinds (Smith is a philosopher), that is, the kinds of things that make up the world we know. These kinds are organized into a conceptual structure that has come to be known as the Great Chain of Being, which is structured into the grammar of thought and language. The concept of is closely bound up with the Great Chain. Such ontological cognition has been one of my recurring interests over the years.
