“What would your feelings be,” asks Ambrose in Arthur Machen’s novel The House of Souls, “… if your cat or your dog began to talk to you, and to dispute with you in human accents?” He goes on:

You would be overwhelmed with horror. I am sure of it. And if the roses in your garden sang a weird song, you would go mad. And suppose the stones in the road began to swell and grow before your eyes, and if the pebble that you noticed at night had shot out stony blossoms in the morning?[

Machen’s examples are disturbing, but it’s not immediately obvious why. It’s not that they’re frightening, at least not in the ordinary sense of the word. Normally, we’re scared of things because we think they pose a physical danger to us, but singing roses don’t pose any such hazard, so why is the thought of them so nightmarish?

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.

Here’s why.

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.

These reflections on horror would be little more than an amusing pastime were it not for the fact that, although monsters are not real, people really do think of other people as monsters. My main area of research is the phenomenon of dehumanization, the strange, almost delusional state of mind in which one group of people conceives of another group of people as less than human. In its most toxic form, the victims of dehumanization are seen not only as physically menacing—as rapists, murderers, or terrorists—but also as both human beings and subhuman animals in a single package. They are therefore believed to present a metaphysical threat as well as a physical one.

It’s a cruel paradox that even though the dehumanized are often the most marginalized and vulnerable members of a population, they are seen by their tormentors as overwhelmingly dangerous and endowed with terrible, monstrous powers. During the middle ages, European Christians endowed Jews with horns and tails, and considered them to be malevolent beings with a penchant for cannibalizing Christian children. The image of the demonic Jew was reiterated well into the twentieth century, as evidenced by the themes in Nazi propaganda. Likewise, the thousands of Black men who were subjected to unimaginable violence and cruelty in the American south were routinely portrayed as monsters, fiends, and violent, predatory apes by White Americans, an image that sadly persists to this day. And in the xenophobic frenzy that has been enflamed by the rhetoric Trump administration, and in the xenophobic media frenzy Latin Americans fleeing violence and poverty in their countries of origin are morphed into demons in human form. That is true horror.

All this being said, it’s still not clear why it is that boundary-violating entities seem so terribly ominous. I think that the correct answer is that 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.

David Livingstone Smith is Professor of Philosophy at the University of New England in Biddeford, Maine. His forthcoming book On Inhumanity: Dehumanization and How to Resist It (Oxford University Press) is available for pre-order. More from David at https://www.davidlivingstonesmith.com

Read more from our Halloween Issue:

The Dark Side – Stephen de Wijze

The Return of the Living Dead – Slavoj Žižek

The Search for Extraterrrestrial Life - Lynn J. Rothschild

Dressed To Kill - Shahidha Bari

Rest In Peace - Diane Purkiss

Why We Should Care About Our Corpses - Thomas Laqueur