Heraclitus, the pre-Socratic philosopher, left us to interpret and apply his often-cryptic aphorisms. One of the fragments that survives from his writing declares, “All is flux, nothing stays still”, a phrase that Plato quoted in the Cratylus. Who could think otherwise? Everything, of course, changes, which is why you can’t step into the same river twice, another thought attributed to Heraclitus: it’s not just that the water in the river flows by so the river is different, but that you’ll be different too. When you dip your toe back in to the water you’ll be different physiologically because cells will have changed, you’ll be older even if just by a few seconds, and perhaps you’ll even be wiser, though maybe you won’t. You are subtly different already from the person who started to read this sentence, and I am different from who I was when I began writing it.

The human predicament is, then, one of inescapable change. The biggest inescapable change, is, of course, our own deaths, the point at which we cease to be living beings and start to become compost or ashes, or possibly get cryogenically frozen. This is perhaps why religious thinkers believe that there is either an unchanging God outside of our everyday world, or some stable essential element at our core, some soul-stuff that is eternal and unchanging. If only there were something unchanging. But Heraclitus was right, it’s all flux – there is nothing permanent, nothing fixed, nothing stable.

But it is not just the religious who strive after something they can’t have: permanence. Those who seek reputation, power, or wealth, apart from when they are just showing off, seem to want something lasting in a world of change. To which I respond, “Don’t count on it. Remember Ozymandias.”

And yet, despite preventable and unpreventable change staring us all in the face (perhaps almost literally when we look in the mirror), many of us act much of the time as if things will stay more or less the same. Change occurs, yes. But ageing is usually quite a slow process. Character change is slow too. Let’s not get too worried. Society changes, but not so quickly. My circumstances will change as I age, but let’s not think too hard about that now. The climate will change, but not too much, and probably not catastrophically in my lifetime or my children’s. Or so we like to think. But we are probably wrong to do so. Perhaps, then, we all ought to be more sensitive to how change is all around us and within us, and openly acknowledge it. Perhaps that was part of Heraclitus’s wisdom, that we need reminding of something that is in a sense obvious: change will be a feature of all our lives.

Far harder to take account of, however, is unexpected change, what Thomas Hardy described as “the persistence of the unforeseen”. Unexpected change is, by definition, unexpected. By ‘unexpected change’ I mean that the type and scale of change is unexpected. It can be dramatic, as with the social transformation brought about by a new technological development, such as with digital technology and the internet. It could be the result of something just highly unlikely happening, such as buying a ticket in the lottery and winning a huge prize when the odds are stacked so highly against it for any individual. (Anyone who buys a lottery ticket and expects to win simply doesn’t understand the odds.)

If, through a combination of factors, climate change accelerates and becomes irreversible, passing a tipping point into global climate disaster within a decade, that won’t be an unexpected type of change for many of us, just an unexpected acceleration of a process that already looks difficult to stop. But if little green super-intelligent aliens with five arms arrive from another galaxy, decide that they like the taste of human flesh, and then farm us as factory farmers now farm cattle, serving human beings on wholemeal sourdough toast as a delicacy, then that would be an unexpected change in how we live. I don’t think many people will have seen that coming.

In 2002, Donald Rumsfeld, who at the time was US Secretary of Defense, gave a news briefing in which he distinguished between facts that we know (‘known knowns’), and things that we know we don’t know (‘known unknowns’). He then went on to draw attention to the existence of ‘unknown unknowns’ – the things we don’t even know we don’t know, adding, “if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones”.

Rumsfeld was mocked at the time for what sounded like pseudo-profundity. Yet closer scrutiny of what he said has led many – including me – to revise their opinion and see his observation as accurate and important. Unknown unknowns are the source of unexpected change. It is highly unlikely, to say the least, that you and I will end up on toast as a snack for a little green five-armed alien from another galaxy. But strange things do happen. And when they do, we don’t see them coming, and it is hard to believe that they have happened. There is little that we can do to prepare for unexpected change – you can’t prepare for something you can’t foresee. I suppose the best we can do is to recognise that such change has happened when it does, and change ourselves quickly if we want to have a chance of staying afloat.