There’s a saying that might appear familiar to readers of Terry Pratchett: “You can’t cross the same river twice”. If it wasn’t for the clue in his books that the obvious interpretation is not the intended one then I don’t think I’d have worked it out. I’m too literal a thinker to have realized, unaided, that the river here refers to the water rather than its course. I’m not going to take the metaphor too far and start considering the water cycle through evaporation and rainfall by which the same water molecules could be crossed again. For the purposes of this metaphor, once the water has flowed downstream it must be considered “lost” as different water flows down, replacing it. The river crossing in this instance is an event in space-time: the time coordinate is key to understanding.

So far, so what? Is any of this relevant? Well,… yes. You see, I sometimes get nostalgic. I recall places in terms of past events and some of those events evoke memories of happiness. I feel a longing to experience a particular event again that going back to the same place in the present can’t satisfy: it’s not the same river.

This unsatisfied longing to go back to an event – a situation – in the past can be so overwhelmingly powerful that I feel a profound sense of loss because there is no way to return, wind back the clock. This can be a problem when I find myself in the same place doing the same thing, whether it’s socializing, playing darts or watching a band. Association triggers memories of the historical event and I can be left feeling that there is something missing, some vital piece of the puzzle that would complete the picture.

So I’m left with my memories, replaying the experiences, going back in my mind instead of in time. Maybe I can’t cross that river again, but it will always be there.