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When we say that something is predictable we are saying that it belongs to a class of, say, events, that have certain properties that allow them to be predicted to some precision and accuracy. This implies that there is a class of events that is the opposite: unpredictable.
What does it mean for something to be unpredictable? What different flavors of unpredictability should we be thinking about?
One possible answer is that the unpredictable is random, and you can even go so far as to define randomness in terms of unpredictability. 1 This is the approach taken by Eagle, A., 2005. Randomness is unpredictability. The British Journal for the Philosophy of Science. In this interpretation, unpredictability essentially means that something is entirely random and there is nothing I can know about the event today that will allow me to move the probability meaningfully in any way away from the “tossing coin”-state all events start out in.
But this way of thinking about it is filled with assumptions already: is it right to say that all events start out in a 50/50-state of probability? The way this is sometimes defended is to say that when we know nothing about the probability of an event we should start with the assumption that it is equally likely that the event occurs as it is that it does not. But why is 50/50 a good starting position? The mathematical argument is that it gives us the largest chance of being right if we know nothing else — we will be right half of the time, which, for at least binary events, is as good as it gets. If we seek the initial probability for events that have more states, we should just have it be 1/n, assuming that all states are equally likely (say drawing a specific card from a deck of cards).
But it seems as if this assumption is tricky in that we need to know the set of possible outcomes. If we do not even know that, if our model of the world is incomplete to such a degree that we do not know what kind of deck of cards it is (it could be, say, the tarot) – then we are in deeper waters. What initial probability should you ascribe to something if you do not know how many cards are in the deck, or even what those cards are? Say I ask you what the probability of drawing an Orc is – what should you say? Should you approximate the number of cards in the deck, and assume that the Orc is one of the Tarot? If indeed this is the Tarot and not Magic the Gathering?
Here unpredictability seems to inch closer to Knightian uncertainty – where no numeric approximation makes sense at all. The answer to the question how probable we think a certain event in this framing is should simply be: I don’t know.
So are the unpredictable things those that we know nothing about?
Then unpredictability seems to be more than randomness – it seems to be a state in which we have no access to a model of the world in which we can enumerate states, outcomes and alternatives.
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Another way to approach this is to speak of limits, as we noted in an earlier working note. What are the unpredictability limits? We could argue that they occur in space and time — we cannot predict that which happens so fast that the formulation of a prediction is not possible.
This is interesting, because it assumes that a prediction cannot be immediate – or at least not concurrent with the event itself. A prediction needs to come before, and it will consume some small amount of time to produce, so we should expect that there is this small lag built in to predictions — anything that happens faster than the time it takes to predict it is unpredictable in a very real way.
This is not just a fun point, either – for very complex systems that are irreducible to simpler models, the time requirement makes them unpredictable in this way for a large set of cases. We can think about this as a special case of algorithmic complexity, perhaps, where the size of the algorithm for predicting a system at some accuracy runs optimally in the time it takes for the system to reach the State s+1 which is the state after the state S we tried to predict.
Complexity boundaries also exists in the future – some complex systems, such as the weather, have an upper complexity boundary beyond which they are unpredictable.2 In the case of weather this seems to be around 14 days – see https://phys.org/news/2024-02-limits-weather-future.html, but, of course, this does not apply to climate forecasts, because they forecast something fundamentally different — and at a different resolution.
Other limits exist in space. We cannot predict things that are beyond the visible universe in any meaningful sense, since they are spatially inaccessible to us.
Computational limits seem more practical, but for some things we could predict the computational requirement may be such that there is no energy to produce such computations.
Et cetera. The notion of limits of predictability is an interesting one, and it probably also has recursive elements – when more than X predictors are trying to predict a phenomenon that is self-reflexive, the resultant complexity may make something initial predictable quite unpredictable.
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Unpredictability is also ontologically interesting. Is it a feature of reality or an epistemological limitation? The same holds for uncertainty, and this too is interesting in a “philosophy of physics”-way. Here, obviously, we find the heated debate about things like Heisenberg’s uncertainty principle.
A lot to return to.
Footnotes and references
- 1This is the approach taken by Eagle, A., 2005. Randomness is unpredictability. The British Journal for the Philosophy of Science.
- 2In the case of weather this seems to be around 14 days – see https://phys.org/news/2024-02-limits-weather-future.html, but, of course, this does not apply to climate forecasts, because they forecast something fundamentally different — and at a different resolution.
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