“The most merciful thing in the world,” wrote H.P. Lovecraft, “is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far.”

If Lovecraft’s mind was particularly vexed in terms of its contents — all that racism bouncing around in there, along with many-tentacled things and strange, impossible architecture! — he was right in the respect that the things we fear most are those that we are unable to picture.

When you survey the pantheon of great literary monsters, one thing becomes clear: the more terrifying the monster, the less we know about what it looks like. The scariest monsters are the ones who remain the most mysterious. The more fear becomes the object of language, the more language itself starts to break down.

Lewis Carroll’s The Jabberwocky is perhaps the most famous example of the failings of conventional language in describing the indescribable — the poem is famous for Carroll’s neologisms (Slithy toves! Galumphing! O frabjous day!), but the titular monster remains shapeless. None of Carroll’s nonsense words are deployed in the service of describing it; all we get is “the jaws that bite, the claws that catch” and the fact that when it appears, the creature has “eyes of flame.” And yet, and yet — we all have our own idea of what the Jabberwock looks like, right? (Even those of us, like your correspondent, who have aphantasia.)

Beowulf‘s Grendel is another classic monster whose actual physical description remains elusive — part of what makes him so terrifying is that he only walks at night, and as such, one only ever catches the most fleeting view of him. We hear that he’s horrible, and possibly a giant, but we never get a detailed account of his appearance — instead, we get a recounting of his deeds, which most notably include slaughtering 30 of Beowulf’s kinsmen for partying too loudly, and on his return, tearing a hapless warrior limb from limb and eating him whole. When he’s prone to such acts, who cares what he looks like? (So it also goes, incidentally, for Grendel’s mother, who’s been depicted as everything from a seductress to a hideous swamp-hag, which only goes to show that human society has never been short on ways to demonize women.)

What we’re really dealing with here is the power of the imagination — if you don’t know what the Jabberwocky or Grendel looks like, you’ll imagine something that’s as frightening as the text in question suggests that the monster is supposed to be. Perhaps the most iconic example of this is Stephen King’s IT, a monster that quite literally takes the form of whatever its victim fears most. (Its default form is a sinister clown, which seems fair considering that pretty much everyone is terrified of clowns.) For the majority of King’s novel, the terrifying monster is whatever you want it to be, or perhaps more accurately, whatever you don’t want it to be.

It’s notable that It rather loses its power to shock when it finally comes into the light. In both the book and the 1990 film adaptation, the appearance of the creature in its final form — a giant spider — is somewhat anticlimactic, because for all that a giant spider is terrifying, it’s still a lot less terrifying than a sort of formless shadow that promises to take the form of your worst nightmares. In the book, for what it’s worth, it’s implied that the monster’s actual form is something called “the deadlights,” a sort of hideous endless orange light that drives anyone who sees it insane. This is very much in the Lovecraftian tradition of truths so terrible that the human mind can’t even comprehend them, let alone describe them (which, of course, also saves the fiction writer the trouble of having to come up with suitably terrifying prose.)

Even Lovecraft himself was at his best when he was eschewing trying to describe all the terrifying tentacle-toting bat-winged creatures that were squirming around in his brain. The shoggoths and Cthulhu and Brown Jenkin and the rest are disconcerting, mainly because they give an insight into the mind of their creator, but perhaps Lovecraft’s most intriguing monster is the invisible creature of The Dunwich Horror, “something as big as a house that one could not see, but that had all the vicious malevolence of a daemon.” It’s only rendered visible for a moment at the story’s end, and for all that Lovecraft piles on the description at that point — “Bigger’n a barn… all made o’ squirmin’ ropes… ten or twenty maouths or trunks a-stickin’ aout all along the sides, big as stovepipes, an’ all a-tossin’ an’ openin’ an’ shuttin’…” — it’s the monster’s mundane aspect, its human face, that makes it most horrifying.

Which brings us to the final, least definable monster: us. Me! You, hypocrite lecteur, mon semblable — mon frère! The monsters that frighten us most are those that make us look within ourselves, which is why Robert Louis Stevenson’s tale of Dr. Jekyll and Mr. Hyde has proven such an enduring one. The metaphor is almost too obvious — the monster within, all our most alarming thoughts and desires and imaginings made flesh — but it works precisely because it’s so brilliantly simple. The minute you realize what Mr. Hyde is (and it doesn’t take long, given that Stevenson isn’t really interested in making his story a mystery, as such), you start to think of the Mr. Hyde within yourself, and what it might be like if he got out to play.

The whole notion of a monster — a creature whose entire existence is predicated on its ability to frighten — means that the truly universal monster is a rare one, because fears, just like hidden hopes and desires, vary from person to person and from culture to culture. (It’s not for nothing that mirrors often play a central role in fairytales and folk tales.) The most enduring monsters, then, are those who invite us to define them, and thus invite them into our nightmares for years to come. Happy Halloween!