I grew up with the knowledge that signs and portents were everywhere.

A lone raven, perched unexpectedly nearby. The hoot of an owl. The colour of the sky as daylight fades. The pattern of tea leaves in the bottom of a cup.

My grandmother believed very much in the invisible world. Her own grandmother made sure of that; the old woman had the sight. She knew of the power of salt, and of images, and of the placement and arrangement of things. Meaning – and often, warning – was to be found by those who knew where to look. And how to.

We’d catch glimpses from time to time. Shapes in the dark. Reflections in glass; there one moment, and gone the next. Or a familiar face, where its owner couldn’t possibly be. Perhaps we’d see a relative standing in the hallway at night, then they’d be gone. We knew it was time to call, or go around and visit – just to be sure. We knew that, one day, we’d see them there again, and we’d call, but the phone would no longer be answered.

When a family member died, we knew to close our eyes tightly that night, and not open them until morning, because we knew we’d see the familiar face by the foot of the bed – visiting one last time.

We knew that timepieces had a habit of stopping when their owner’s life ended, and that those objects then gained power, as long as they weren’t repaired.

We knew about the wrong people. We only ever crossed paths with a handful. My grandmother would see them from across the street, or from a passing car; strangers, never met before. Ordinary in appearance, just like us. But she would turn us away. She saw something we couldn’t.

We knew about the wild places, out beyond the cities and towns, where the land is still as it’s been for centuries; we knew about the things that live there. The ones from the trees, and the ones that whisper in the wind. We knew that, when alone and near water, the sound of a horse is something to fear and turn immediately away from. There are things that can look like other things, and they aren’t to be seen by our eyes.

There are empty places that are nevertheless inhabited.

All cultures have their myths and legends; few more so than ours. There are spirits and beings for every inch of the natural world. Some explain away carelessness with imagined thievery. Some keep children from real danger, by augmenting it with the fantastic. Some just add colour to a slate-grey sky. And in the modern world – so recent, so fragile, and so small – it all seems to just be superstition. The primitive imaginings of earlier times, channelled into a living fiction, passed down by the indulgent or the credulous.

It seems that way. Until you turn your head too quickly one evening, and catch a glimpse of someone – or something – that couldn’t be there. Until you find yourself outside the realm of sane streetlights and the ever-present rumble of traffic. Past the wi-fi signal’s last gasp, far out beyond the markers of civilisation’s domain. On a hill, perhaps, in the true darkness of night, where even the silent carven stones of millennia ago feel too new, and like an intrusion.

An ancient agreement is in place.

We will mostly stay indoors during the deepest hours of the night. In the event that we do need to venture out, we may – but we’ll stay near to our own domain, and we’ll go quickly and quietly. Being respectful, and we’ll tell ourselves that it’s simply for each other. But every child knows the truth instinctively.

There are other things.

They were already old when our kind barely knew how to stand upright. Things of the water, and the land, and the air, cohabiting. And sometimes, when their attention is drawn, watching us. They let us go about our business – for the most part. They stay beyond the boundaries of our settlements, and rarely walk afield outside the hours of darkness.

But they do check up on us, once in a while. The agreement must be upheld.

So it may well be that, one lengthening night as we’re out on business that can’t wait for the sun to rise – perhaps near flowing water, say – we’ll see something. A shape that’s not of our own world. A face that’s not from us, but is instantly familiar to our blood, and the hair on the backs of our necks.

Avert your eyes, our ancestors will whisper insistently, and we’ll obey without question. We’ll wait for a moment, because in that moment we may not be able to move. The feeling will pass soon enough. The other face has seen our kind before, on nights like this down through aeons. It’s only here to remind us that we’re bound to each other.

It will turn, and move away; we’ll know this, even though we won’t see it go. We’ll sense it in our hearts and bones. And we’ll be faced with a choice: leave, and continue towards home, or give chase, stepping further into the hidden world that brushed up against us unexpectedly.

It’s no choice at all.

We made our decision generations ago. The bloodlines of those who chose to give chase never had a chance to continue. They vanished into the flowing water, or the dark and rolling hills, or up into the chill air under a cold moon.

We learned, and we’ve never forgotten the lesson. So we’ll make the right choice.

We’ll leave it alone.

I’m sitting in my armchair, and the park outside is silent. I look out of our living room windows and see sodium lamps casting murky, orange pools of light in a broken chain along a path that’s deserted.

Shapes flit through the sky, too fast to see clearly. The ravens watch from bare trees, black against dark. From time to time, they speak urgently about matters we’re not privy to. The river is swollen from the rains, and it flows quickly. Sometimes, it sounds like hooves.

I close the curtains.

My grandmother would approve. She was a kind, hardworking and pragmatic woman, who ran around the back garden kicking a ball for the dog when she was in her seventies. She knew how to laugh at the ridiculousness of things, but she also knew there were some things not to be laughed at. Not to be spoken of, or even seen.

It’s been some years since she died – I’ve glimpsed her since – and perhaps she now belongs more to those silent places than to the human world she guided us through. I don’t doubt that she’s stood at the foot of the bed, or passed through a nearby crowd, or looked out from the edge of a mirror, just to make sure we haven’t forgotten.

I took her warnings seriously.

They worked their way into my mind, and I probably channel them when I write about things that couldn’t be. It’s a very natural thing to do when you grew up in a world that sat on the boundary of the familiar and safe, and the strange and frightening. It’s easy when it’s already all around you – out beyond the last streetlights, or down by the river, or pressed up against the glass of the window just across from you.

You could look, of course. You could wait for nightfall, then walk over, take a deep breath, and draw back the curtain. It’s up to you.

But I wouldn’t.