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Suppose you have two intelligent agents interacting, with each one one's behavior based on their prediction of what the other will do. For example, suppose you have two mind readers playing rock, paper, scissors. It is impossible to predict what the other will do. Alternatively, suppose someone attaches a machine to my brain that reads my mind and tells what I will be doing for the next 10 minutes. I can always improvise by making a deviation from the predicted behavior.

It would seem that there are limits to how well a person's behavior can be predicted. Can there truly be determinism without predictability? Without being able to set up experiments to successfully test the prediction of a person's behavior, doesn't determinism become scientifically meaningless?

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    Chaos theory looks at dynamical systems that exhibit/assume determinism and yet also exhibit unpredictability under any finite precision.
    – Galen
    Apr 23, 2022 at 21:49
  • Consider that predictability is relative, depending on who makes the prediction. Determinism implies full predictability under 2 conditions: perfect knowledge of the situation, and a perfect theory of reality. Only both together allow for perfect predictions. Alas, we will never have any of those but for very trivial cases. As an exemple, think about drawing the top card from a shuffled deck: the card you are going to draw is already fully determined, but because you have only partial information the outcome is unpredictable to you.
    – armand
    Apr 24, 2022 at 8:27
  • Does this answer your question? Is the debate on free will over?
    – tkruse
    Apr 24, 2022 at 10:55
  • You may be interested in determinism vs prediction
    – Nikos M.
    Jul 5, 2023 at 11:22
  • @armand uncertainty is a result of having more than one possible outcome and not a result of imperfect knowledge of the underlying mechanism of the only possible outcome (as would be the case if determinism is true). This is true both in science and in everyday life. Science rarely has an exact mechanism and infinite precision, yet some events are known to be certain (when this happens) because they are the only possible outcomes to expect and there is no uncertainty in these cases. In determinism for every given state there is only one possible outcome thus there can be no uncertainty ever.
    – Nikos M.
    Jul 7, 2023 at 14:10

4 Answers 4

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In general determinism also leads to predictability, but in special cases this is not the case.

This was discussed also here Is the debate on free will over?

As i have said there, it is logically impossible to predict a deterministic system from within when that prediction is used by a so called Counter that will act the opposite of a prediction. This is known as Scrivens paradox.

In the linked answer that is demonstrated with a very simple setup, your example is more complex but still similar.

In general, apart from practical issues in gathering perfect knowledge and computing fast enough ahead of time, there are other known issues that can cause even simple deterministic systems to not be predictable by computation. Such systems do not become indeterministic though, and can be theoretically perfectly predictable from the outside.

However, even a system that is not deterministic can allow to make perfect predictions in many cases. More importantly, within a non-deterministic system, elements can still remain perfectly determined by past events, and sometimes elements can even be perfectly predictable, though not the system as a whole.

So while a perfectly deterministic universe would settle the question of free will, a universe with indeterminism still allows deterministic models of the mind to be true. In simpler terms, even if quantum mechanics generated a lot of random noise, this might not render neuronal activity less deterministic, the same way small waves on a lake do not prevent to steer a boat.

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  • No system could be partially deterministic. In a deterministic system there is nothing indeterministic and vice versa. Apr 24, 2022 at 11:16
  • A rock is not a system. Apr 24, 2022 at 20:44
  • There are no deterministic systems in reality. Nothing to sit on. In an imaginary deterministic system there could be no humans to sit on anything. Sitting is an act of free will, you have to decide where to sit and when. Apr 25, 2022 at 2:51
  • Do you have any reference for those claims?
    – tkruse
    Apr 25, 2022 at 2:58
  • Look up the definition of determinism. See, if you can find anything like that in reality. Experiment, try to sit somewhere without deciding where and when. Apr 25, 2022 at 3:51
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Let's suppose an atom is deterministic in the sense that you can logically/mathematically predict it's behavior. Using calculus you can approximate a continuum of a gazillion atoms, though it won't be an exact prediction. Still, the model works because of the deterministic behavior of a single atom. If you require exact predictability, then I imagine you'd need a computer the size of the universe to simulate/predict the entire universe. Does that mean that the universe is not deterministic?

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Determinism is scientifically and logically meaningless when studying human behaviour. Determinism specifically assumes that there is no such thing as human behaviour (=free will). There is only a causal mechanism with a fixed but unpredictable future. A deterministic system is constantly "predicting" its own future as fast as physically possible.

Determinism is meaningful only in classical physics, as a simplified model of reality. A very practical tool that makes understanding physics much easier and calculations simpler.

Besides, your example of two mind-readers is meaningless speculation on something that has never happened and probably will never happen.

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  • Determinism assumes no such thing. Determinism is open to compatibilism and incompatibilism. Compatibilism explicitly contradicts what you say. So your claims directly contradict sources like en.wikipedia.org/wiki/Compatibilism : "Compatibilism is the belief that free will and determinism are mutually compatible", and you are obviously pushing your own philosophy, in violation of the rules of this site. See : philosophy.stackexchange.com/help/on-topic: "Please note that this site is not a personal blog or a pulpit for you to express your own personal philosophical beliefs."
    – tkruse
    Apr 26, 2022 at 0:58
  • Compatibilism is both an illogical and a completely useless idea. Free will and determinism cannot be reconciled without redefining both beyond recognition. Besides, there is no determinism in reality to be compatible with. Apr 26, 2022 at 3:20
  • Again, pushing your opinion.
    – tkruse
    Apr 26, 2022 at 4:13
  • Not an opinion. The definition of determinism does not allow any kind of free will by any definition. Apr 26, 2022 at 5:26
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No and all the comments/answers suggesting this to not be the case are wrong or are confusing predictability in principle with predictability in practicality.

If it is deterministic, then one can in principle predict X. Otherwise, it is not deterministic. However, can it be practically impossible to determine something despite determinism being true? Yes.

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  • Where's the in principle possibility of predicting light-speed particles coming in from distant space (so called space invaders)?
    – J Kusin
    Feb 3 at 19:51

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