It springs from a saying coined in 82AD by Juvenal, a Roman satirist - rara avis in terris nigroque simillima cygno. ("A rare bird in the lands, and very like a black swan.") It means, quite simply, something that doesn't exist. For absolutely centuries, this was stated as a bald truth, by those who had a mind to discuss such things. Europe's swans were strictly white.
Imagine the surprise of Europe, then, when in 1697 Dutch explorers in western Australia discovered that very thing: black swans.
So now, the term Black Swan means something that we thought was impossible, but wasn't. It is something that was beyond our field of vision that has just sprung into the light.
The author Iain M. Banks, one of my favoured science fiction writers, described this in the Culture novel Excession as an Outside Context Problem. It is something that we can't possibly have seen coming because our worldview does not include its possibility. To understand how that would work is very difficult - getting outside of our own minds is hard. They're our minds after all.
How we relate to the world, and how we understand it, is always put through a lens of our own experiences. When told to imagine our reaction to something, well, it's just been described to us by someone with a generally similar experience of the world. That's how hard it is to describe an Outside Context Problem, but the example Iain M. Banks uses is this:
The usual example given to illustrate an Outside Context Problem was imagining you were a tribe on a largish, fertile island; you'd tamed the land, invented the wheel or writing or whatever, the neighbors were cooperative or enslaved but at any rate peaceful and you were busy raising temples to yourself with all the excess productive capacity you had, you were in a position of near-absolute power and control which your hallowed ancestors could hardly have dreamed of and the whole situation was just running along nicely like a canoe on wet grass... when suddenly this bristling lump of iron appears sailless and trailing steam in the bay and these guys carrying long funny-looking sticks come ashore and announce you've just been discovered, you're all subjects of the Emperor now, he's keen on presents called tax and these bright-eyed holy men would like a word with your priests.
Sometimes, though? Sometimes, we're the ones that place the object outside of our context.
See, in a case of a world or society where nobody ever talks or communicates (every kid with a mobile phone, if you ask a certain subsect of a certain generation) - that is next to impossible to imagine. How would anything even work? I'm communicating the idea to you, that is what reinforces how alien that idea is. That isn't a scenario that we made impossible, though; we could only develop into the species we are if we communicated, social evolution being just as important to who and what we are as physical evolution.
Now, though - now imagine a world without any kind of vehicle that isn't horsedrawn or man-powered.
That, we did to ourselves. We made that internal combustion engine, and we put it everywhere, and we made it necessary to our day-to-day lives in certain areas, and we so firmly embedded it into our culture and society that it is actually fairly hard to picture a world without them - or a world in which even the concept of them never existed.
The thing is, there are people alive today for whom that is a reality; people for whom the modern world is outside of context. Five generations ago, the notion of most people in England owning a mobile phone would be a totally alien one. Now place yourself in the shoes of the shocked, and advance the world past you. What lays in wait for us? Can we even predict, outside of speculative fiction?
This, right here, is why so many problems we face - as a race, as a whole - seem impossible to solve.
We've painted ourselves into so many corners, and then thrown away the brushes, and then forgotten that paint actually dries. The problem is A - but the solution B is impossible, because C. We accept all of these points because we just...can't not. That's why solutions are often dismissed as radical, because we just... don't think they can happen. Beyond a certain point it is preprogrammed into us. It's too far a jump, and thus it's not something we can grasp.
Even after having said all that...we still found our black swans. It took over sixteen centuries for us to prove Juvenal wrong, but it still happened. All we can do is keep educating ourselves, keep learning the positive changes that CAN happen - and dreaming of the ones that we don't think can, yet. Total nuclear disarmament - a dismantling of the primary capital-based system - a real and lasting green revolution.
Sooner or later, they may well become a reality.