Can there be determinism without predictability?

Question

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?

Answer 1

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?

Answer 2

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.

Answer 3

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.