He couldn’t be real, I thought. Clearly he was nothing more than another hallucination. A good one, to be sure, and very vivid, but not real.

But it was frightening to know I’d lost control of my own senses so completely. I could only stand there, frozen, not quite certain what to do. And I would always in my mind replay the moment when, incredibly, he turned. And saw me, too.

His eyes were not brown. They were of some lighter colour, clear in their intensity against his suntanned face.

Uneasy, I stepped backwards, and he raised one hand to calm me.

‘Do not fear me. I’ll not harm you.’ Something struck me as familiar in the cadence of his voice – a deep voice, rough around the edges.

But I was afraid. It made no sense for me to be, I knew, because he wasn’t real, he wasn’t there. Pure instinct made me take another backwards step, and then another.

‘Wait,’ he said, and moved as if to follow, only something very curious began to happen then. As I moved backwards and away from him, he started to dissolve and fade, becoming like a shadow I could almost see, but not.

I thought I saw him stretch his hand towards me as though trying to reach out, to stop my leaving. And then all at once he was no longer there on the hillside, and in the same heartbeat I wasn’t there, either.

Instead I was back on the coast path again, standing all by myself at the place where it rounded a cleft in the cliffside a little too closely, and watching the water boil white on the black rocks below.

Uncle George’s study, where I’d worked on the computer this past week, had been my favourite room for hiding in when we’d played hide-and-seek as children. From behind the green chair I’d been able to see all three doors – the one directly opposite that led into the corridor, and both doors that connected with the rooms to either side. The old desk had a kneehole space that had been just the size for me to crouch in, and the long and heavy drapes that hung to each side of the window were the perfect length for someone small to hide behind.

I couldn’t hide there now, but it was maybe no coincidence that I had sought my refuge here this afternoon. I’d been here an hour, having raced up through the fields in record time to find, to my relief, that there was no one in the house yet – they were all still out attending to their work. Which meant nobody was here to ask me why I was now surfing through the Internet for articles on mental illness and hallucinations.

What disturbed me wasn’t simply that I’d slipped out of reality so quickly and so easily, but that I could have done it so completely, and that my own sense of place and time had been so badly skewed that in my mind I could believe I’d climbed the hill and seen the man in brown, when actually I’d never left the coast path.

I could no longer blame the sleeping pills – even though the articles I’d read did say withdrawal from the drugs could have the same effect as taking them, I’d only taken two pills to begin with and I’d gone a week without them, so I doubted I was going through withdrawal. And in case the cause was stress, I’d tried removing any source of that as well, even going so far as to take off my wristwatch and put it away in the drawer with my mobile phone, both former trappings of my tightly scheduled life.

I’d done everything I could do, but this obviously wasn’t something I should try to diagnose myself. I’d have to find a local doctor and explain what had been happening and see if there was any way to treat it. And I had to find out how I ought to handle any episodes I might have in the meantime.

Curled in the green chair, I scrolled through the pages and carried on reading:

Sane people know they are hallucinating. Those with mental illnesses do not.

Well that, at least, was one small bit of comfort. I’d been very well aware, the whole time I’d been interacting with the strange man on the hill, that he had not been real. And yet, it had all felt so real. The strong wind and the sun’s warmth and the hardness of the ground beneath my feet …

Hallucinations can involve a multitude of senses – touch and taste and smell as well as sight and hearing – to the point where it can be all but impossible to distinguish the imagined from the real. A glass of juice, the writer said, will taste like juice, and silk will feel like silk.

I searched on for further points on ‘Seeing People’, and discovered it was common to imagine human figures when hallucinating, and to hear them speak, and just as common to be able to have conversations in which they responded exactly as real people would. The articles assured me that, so long as these imagined people didn’t try to tell me to do questionable things, like kill somebody, they were harmless.

Our studies have shown, one psychiatrist said, that while ignoring these apparitions appears to have little effect, success can sometimes be achieved by simply telling them to go away.

Moreover …

I broke off my reading as a movement caught the corner of my eye.

I glanced up, expecting Mark or Susan, only there was no one in the doorway to the corridor. No one … and yet the doorway was not empty.

The figure of a man was passing. I could see him very faintly, like a grey transparent shadow, moving past as though he’d meant to walk right by the room. But at the doorway’s edge he stopped and leant back slightly, looking in as though he’d just seen something that he too considered out of place.

I closed my eyes and held my breath, not moving. Counted heartbeats. One. Two. Three. It isn’t real, I told myself. My brain cells were misfiring, as the articles had just explained. There wasn’t really anybody there.