Sarah knew that the principle of change determines everything. The flower in bud will shed its leaves eventually: the rosy face will wrinkle in time: the things you desire will become humdrum. That sounds worse than it is. She had learned that there is great pleasure to be gained from impermanence. You can seize the minute, knowing that its pleasures are fleeting. The blackbird running across the lawn, the smile of a lover, the mood that makes everything golden: so long as you didn’t expect them to last forever, these intense moments could constitute your own little museum, and to thumb through its catalogue could remind you of your life’s high points. That was a kind of wisdom.

Well and good. Sarah sat in the garden on the first day of summer. There was a slight breeze and the clouds were scudding across the sky, tumbling and changing shape as they were blown along. Fancifully she saw them become dogs, tennis rackets, trees. They were subject to the laws of physics. All except one. It was a large fluffy white cloud, and it did not move or change its shape at all. Now that was strange, she thought. What was the thing which never changed, the thing which never swayed with the prevailing wind? Well, death perhaps. That certainly was a constant, it presumably came to all and, whether you faced it with equanimity or fortitude, it would have you for breakfast.

But it wasn’t just that. There was another lesson to be learned here. The cloud hung low over the landscape, and Sarah walked towards it. Hesitating for a moment, she took a small run at it, and then she was inside. Inside the thing which didn’t change. At once she was wet through with driving rain, pounded with hail, buffeted by powerful gusts which took her breath away. Pain, confusion, discomfort assailed her. There was nothing to be gained from this. Summoning up all her strength, she ran through to the cloud’s other side, and found herself lying panting on the grass.

Sarah sat and thought hard, and made a little joke to herself about the Cloud of Unknowing. All of a sudden it came to her. The things which never change are the things we will never be able to see or report back on. The dark side of the moon, for example. We can never see it, unless we are Gods or astronauts, and so it stands like a fortress in our imagination. Or the workings of the ventricles of our own hearts: we can normally never see those. Or the reason why we are here, on this blue planet in deep space. Or the secret workings of the unconscious, as it goes about its tasks of blame and healing. Or exactly how the child feels as it tumbles out of the maternal body. These things could not be known or seen, and so they could remain as they were: as symbols almost. The imagination could not reconstruct them. It could speculate, but could never know for sure.

She sat back on her heels. There was, after all, comfort in knowing that some things could never be fathomed. She looked steadily at the cloud. It had performed its task of transforming her mind. Now, in the sunlight, something was happening. The cloud was not dispersing, but solidifying. It was turning into stone.

There it stood, like a tower with no windows or doors. Sarah realised that it was a monolith. Like a stone circle or one of the great sea henges, it stood there as a testament to silence and permanence. The tides, the winds would not humble it. Sarah bent down and at her feet she found a little model of the monolith: rugged, impassive and cold. It was a maquette. She slipped it into her pocket. It would always remind her that some things never change.