Sarah had a secret. It was this: someone was sending her written messages, in a very peculiar medium. She didn’t know who they were, or where they lived. But one thing was certain: they were very creative in finding hiding places. And they knew a lot about her.

She kept finding these postcard-sized bits of bark. If she was digging in the garden, she’d find one under a stone: if she was changing her bed, she’d find one under her pillow: if she was reading a new book, she’d find one between pages 50 and 51. She had a pair of favourite boots whose soles were getting paper-thin: and one day she found that two pieces of bark had been slipped inside, to keep her feet dry. It was all very unnerving.

Sarah had read about the Vindolanda tablets, private notes written in colloquial Latin by members of the Romano-British garrison in the North. The utterances had been written on oak bark with carbon-based ink. Perhaps these were something like that. Sarah recalled that the originals had been hard to decipher, and had to be steeped in acid and then photographed in ultra-violet light in order to release the messages they bore.

Accordingly, she tried all sorts of vinegar - none would do the trick - until finally in desperation she tried her own urine. Cold would not do the trick: it had to be warm and fresh, seemingly. It was rather undignified peeing on her messages, but she hoped it might be construed as research. Anyhow, once they were dry, she put them under an ultra-violet lamp, and waited to see what happened.

The little missives slowly swam into view. They were clearly written for her eyes only: “do you remember the time in Greece when you nearly drowned?”: “did you really fall in love on Victoria Station?”: “I know where your blue Rigby and Peller brassiere is”: “why didn’t you finish that poem?”. Then there were more abstract utterances: “do your tears taste of disappointment, or triumph?”: “what is really worth being sorry about?”: “do the dead ever watch you?”: “what did your father’s voice sound like?”

The writer knew her well, but he or she also knew other people. They knew how Helen felt, watching the wooden horse trundle through the gates of Troy. They knew how Napoleon felt on Elba. They knew how George Eliot felt when she lay down her pen after finishing Middlemarch. They knew how the pilot of the Enola Gay felt as he dropped the bomb on Hiroshima. The tablets said “imagine this, Sarah. Imagine this. You know this. I am sure you do.”

She was now seriously rattled. Her secret scribe was making impossible demands, stretching her competence, her empathy and recall. It was crucial to find out who he or she was, to reproach them and to confront them. Sarah gathered together her Vindolanda tablets (there were nearly 400 of them) and went into the wood. A friend had introduced her to the idea of “forest bathing”: walking among the trees, listening to them, living instinctively for once. Sarah walked for hours in the silence, and then she heard a faint sawing and a tapping sound. From a distance, she saw a woman stripping bark from various trees, little pieces about the size of a postcard. The woman turned round. It was herself. A little taller, a little thinner, a little greyer, but it was her all right.

Sarah lay her Vindolanda tablets on the ground: “why did you send me these?” And the other Sarah said quietly: ”to test you, to tease you, to push you, to pinch you, to take away the muffle and the baffle, to lift the veil. You can know this. You can see this. You can hear this. Just lose your fear, and imagine. Imagine.”

They walked towards each other, and suddenly there was only one Sarah, not two. She walked away, and everything was bright. She left the Vindolanda tablets where they were. No need of them now. She knew how to imagine at last.