I've never posted on here but I am interested in philosophy. I think a lot about free will / determinism / compatibilism. I always felt like I have some degree of free will. I know free will is unfalsifiable.

I'm posting here because my friends are all determinists. They bring up a lot of good points and I haven't found any arguments against them. My Google searches end up with someone on the internet repeating my friends' arguments and saying the debate is over. We are all meat machines.

I think I've come up with something. I only took one formal philosophy class a while ago so I don't know if this is solid or not. Also, if there are other arguments I can use, I would be happy to hear about them!


  • Assume the universe and everything in it is deterministic
  • This implies that if we had enough information, we could predict what happens in the future.


  • Suppose there is a machine that has perfect information. It prints out what you are going to do after you read the paper.
  • You do the opposite of what it prints on the paper
  • The printer was incorrect and could not predict what happens next with perfect information, which contradicts the premise.

Therefore, the universe is not deterministic.

EDIT: I'm reading other posts on here and I don't know a lot of philosophy jargon

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    The debate isn't over. There are just a lot of people with very strong opinions who aren't willing to engage the debate properly. Sad state of affairs... Apr 7, 2022 at 3:40
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    The position either for or against libertarian free will is unfalsifiable. Lacking a time machine, there is no experiment that could demonstrate once and for all if an individual could have chosen to do otherwise in the exact same situation, for the exact same situation happens only once. All we have is conjectures, like the observation that psychotropes or neural damage modify an individual's behavior, etc... That's why compatibilism moved on to refine the concept of free will into something that can actually be discussed.
    – armand
    Apr 7, 2022 at 3:58
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    – tkruse
    Apr 8, 2022 at 7:02
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    “We must believe in free will, we have no choice.” — Isaac Bashevis Singer Apr 8, 2022 at 22:05
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    It looks like the debate is over - in the past people might have discussed philosophy, but now they just close discussions. Problem solved! Apr 14, 2022 at 22:59

6 Answers 6


The idea of the mentioned paradox is not new, online articles discuss this referencing a paper from 1965 by Michael Scriven ("An essential unpredictability in human behaviour.") and is called "Scrivens paradox". It has already several answers.

It's not merely practically impossible to build a perfect predictor for such cases. The existence of a so called "Counter", something that would use the prediction to do the opposite, makes it logically impossible to both calculate the future truly and reveal it ahead of time.

That alone refutes the idea of disproving determinism by logic alone.

The intuition that in a deterministic universe perfect knowledge means predictability can be found as Laplace's demon, and it is also refuted, though artificial systems can be both fully deterministic and predictable from the outside, some even from the inside.

As a side-note: even with perfect knowledge in a purely deterministic universe, not all futures are necessarily computable. E.g. see Undecidable_problem. But that's just yet another reason such a machine cannot be built, not the main reason such a prediction is impossible.

Mainstream philosophy holds that likely our universe is not 100% deterministic, but free will would still be possible even if it was 100% deterministic (Compatibilism), so any source of random does not matter for the discussion. A third alternative, something that is neither determined nor random but "intentional" is regarded as philosophically nonsensical by mainstream philosophy, but still upheld by religions.

Mini-Universe Example

Even without considering the issue of predictability not being granted in determinism due to undecidability, a simple example can show the logical problem of predicting a future that is influenced by the prediction:

enter image description here

We can imagine a closed system of a rock dropping down a tube, a gate that can be open or shut, and a computer predicting the future, and another computer acting as the Anti, like in the image above. Since the gate is open, the predictor predicts the rock will drop to position A. The Anti, seeing the prediction, shuts the gate, so the rock wont drop to A but be stuck at B. The predictor could anticipate that, and predict B, but that would just make the Anti choose to keep the gate open, so that prediction would also fail.

This system has no humans, no "free will" in a sense breaking determinism, no random, no quantum effects or other aspects preventing perfect knowledge. Yet a true prediction is impossible for the predictor machine to both produce and reveal.

If such a device was built, it would be perfectly predictable from the outside, it would be possible to truly predict that either the rock ends up where predicted or not, but the predictor in the system could not logically predict that.

The Anti has free will in the sense that it can prevent each prediction, but it's so simple that we don't even need a computer to build it, it can be a purely mechanical device, same as the predictor.

Special circumstances

This is different in fatalism and predeterminism, in which a machine could be built that predicts the future and the human cannot do the opposite, prevented by the determined future from doing the opposite.

But that's not the same as determinism, and not a popular modern belief. It's a misconception of determinism by dualists, sometimes used as straw man to dismiss monism or physicalism.

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    Without free will no-one can design or build any machines. Apr 9, 2022 at 7:56
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    In determinism there is nothing "randomly appearing". In determinism nothing happens intentionally (=no free will) and nothing happens unintentionally (=no randomness). In determinism nothing happens. Apr 10, 2022 at 3:12
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    I think its more a need to defend sound and valid argumentation
    – armand
    Apr 11, 2022 at 4:41
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    "I have no arguments." Yeah, we saw that... "I have only the definitions." You don't even. Where are they? You never state them or give any reference so how could we know? And should you provide them, if you provide no argument or reference to justify they are pertinent, we are in right to discard them right away. Definitions have to be agreed upon. By using your own definitions only you agrees with, you might as well be stuttering gibberish. What's the point? This is very fundamental.
    – armand
    Apr 11, 2022 at 23:07
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    1 your definition is wrong, uncertainty and inaccuracy are possible even under determinism. When we don't have perfect information and perfect modelization (which is always the case) there is uncertainty, however deterministic is the world. 2 your statements have nothing to do with what you dare call a definition: belief has nothing to do with determinism because people can hold a fact to be true even if they have no choice. Of course, those rebuttals have been given to you dozens of times and you never came with a counter or changed your position, so I am not holding my breath.
    – armand
    Apr 12, 2022 at 3:41

Problem with the Proof:
Step 2 of the proof ("You do the opposite of what it prints on the paper") assumes that it is possible for you to not do what the computer said. In other words, it assumes you have free will. By assuming the goal of the proof, you committed a fallacy called Begging the Question.

Where to go from here
I good place to start is always the Stanford Encyclopedia of Philosophy and the Internet Encyclopedia of Philosophy. Both sources are peer-reviewed and do not take sides, so they are very trustworthy. They can get rather technical at times, but they usually lay things out well enough for a general audience to follow.

  • I don't agree that the argument is begging the question. Everyone would agree that we have the illusion of free will. No one thinks that the subject might read: you will now raise your right arm, and then decide not to do it, yet despite his intention, the right arm goes up. What the argument is doing is suggesting a conflict between determinism and the illusion of free will that determinists claim we have. Apr 7, 2022 at 4:25
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    the idea behind the oracle printer argument is that on getting to know the prediction the subject will be determined to change their behavior, as in a conscious effort to prove the oracle wrong, and concludes that therefore the oracle defeats its own prediction, leading to a contradiction. The argument fails because, going by the infinite knowledge premise of the thought experiment, the oracle can include this reaction into its prediction, and then the reaction to this update, and so on. It makes the prediction potentially infinite, but logically there is no contradiction.
    – armand
    Apr 7, 2022 at 9:59
  • @armand - But the prediction might be about whether some concrete action will happen within some set time of receiving the prediction, like whether the person will press a certain button within 10 minutes of receiving it. And the person could have the intention to press the button if the prediction said they wouldn't press it, and not press it if the prediction said they would. Even a very simple deterministic machine could be designed to thwart a prediction of this kind, but as I said in my answer, this doesn't actually conflict with determinism.
    – Hypnosifl
    Apr 8, 2022 at 14:18
  • @Hypnosifl but the machine wouldn't make this prediction in the first place because it knows it's not going to happen. The world is supposed to be deterministic, and the machine has perfect information, so it knows the subject will read the prediction and it knows the subject is in a contrarian mood, so it can feed back the effect of the prediction into its computation. Of course this feedback loop makes the computation much more complex but not impossible. Even humans can anticipate other's reaction to what they say.
    – armand
    Apr 8, 2022 at 22:03
  • @armand - "but the machine wouldn't make this prediction in the first place because it knows it's not going to happen" It might calculate in advance the deterministic behavior of the person given all possible signals (or other causal inputs) under the machine's control, and find that for every set of external inputs, the person would do something different than what was indicated by the signal supposed to represent the "prediction". This isn't a failure of prediction, because e.g. it can correctly predict if it sends the signal "you won't press the button", the person will press it.
    – Hypnosifl
    Apr 8, 2022 at 22:09

Just as observation: In a purely deterministic universe — one without free will — no one would be able to do the opposite of what the printer said they would do. if people can do the opposite of a perfect information assessment, then the universe is not deterministic.

The real problem your friends face (or rather, aren't facing) is that the assertion of a deterministic universe is at best a supposition and at worst an article of faith. Even the things that we take for granted as deterministic — such as the laws of macro-physics — have error terms, and error terms literally point at the existence of unknown or unanalyzed effects. Are effects that we do not know and have not analyzed deterministic? Are they even law-like? We don't know, because we do not know what they are and have not analyzed how they work.

I mean, for all we know all these 'deterministic' physical processes are merely the statistical Law of large Numbers result of untold gazillions of perfectly non-deterministic quantum processes. Flip a billion coins and you'll almost always get something vanishingly close to 50% heads, sure; but what of it?

As I have occasionally said elsewhere, strict determinist are almost always working from an anti-theist mindset. To that mindset, theisms use 'free will' as an argument to imply the existence of a soul, and thus of a god. This school of determinism takes the nuclear option: deny the existence of free will to preclude arguments about the existence of a soul. Ultimately they have to deny the experience of their own senses, since each of us has a distinct impression that we are capable of making moral and intellectual choices, an impression that must be ignored to maintain the stance of strict determinism. C'est la vie...


The question seems to presume that if we live in a deterministic universe, an implication of this is that it should be possible to build a machine inside this universe that can predict what we do before we do it, but that doesn't actually follow. A deterministic universe is just one where later states are in principle perfectly predictable from prior states, but there is no requirement that it's actually possible to compute the predictions within the universe itself, using only computing resources available within the universe, in such a way that the computer can predict events before they actually happen (I'll assume for simplicity that the deterministic laws involve only computable functions). After all, to do such a prediction, the computer would need bits in its memory that represent not only all the details of the world around it, but also all the details of its own memory! So it would need to use less than N bits to represent all the N bits of its own memory, otherwise it wouldn't be able to store any additional data about the external world. This isn't necessarily conceptually impossible if the initial state of the world and the computer's memory are such that a large amount of algorithmic compression can be applied to represent them as data, but there's no reason to expect it would be possible in general for any arbitrary initial state.

One interesting variation would be a finite universe that is continually growing in complexity, so that the largest computer that intelligent beings within the universe could construct would be continually growing in memory as well. In that case it could be that if N bits are required to represent the complete state of the universe at some time T1, there will always be some later time T2 where the computer can store far more than N bits. So case it might be that at some sufficiently late time, the inhabitants of the universe could deduce the universe's initial state and then simulate it forward, showing that the simulation perfectly reproduced their real history up to some time T that would always lag behind the present moment of the beings running the simulation. That way the beings in this universe could always verify that everything in their world's past, including their own behavior, had followed deterministically from the initial conditions and deterministic laws, but they would never be able to predict their own future before it happened, so the paradox you suggest would not arise.

Instead of the whole universe's future, could we just predict one person's future?

Another case to consider is a deterministic universe where the laws of nature obey some sort of principle of locality, so that a distant event can only affect you by means of some physical influence that travels at a finite speed from the event to your location. In this case, if you know the initial state of some finite volume of space, and you know all the physical influences (particles, say) that cross inward through the boundary surrounding that volume, then under local determinism you can perfectly predict the behavior of everything inside that volume, provided you have enough computing power outside. In this case, if you can control what influences will be going into the volume, it might even be possible to predict how a system inside the volume will behave in the future, so if it's a person in there you might be able to predict what they do before they do it.

But even if the person's behavior really is deterministic, it might be that there would be no way that you could send a signal into the volume telling the person what you predict they'll do without the signal itself causing them to behave differently than the prediction. Suppose for example that the person has two buttons in front of them, one marked "left" and the other marked "right", and you want to not only predict which button they'll press, but send a signal to them before they press it, with the plan that you'll send a signal coming in from their left if you predict they'll press the "left" button, and send a signal coming in from their right side if you predict they'll press the "right" button. In this case, it might be that when you try to predict their behavior given different possible external influences in your computer simulation, you find that if you simulate a scenario where a signal is sent from their left, you get the deterministic prediction that they will then press the button marked "right"; but if you simulate a scenario where a signal is sent from their right, you get the deterministic prediction that they will then press the button marked "left".

This situation doesn't imply any violation of determinism, since you were explicitly predicting their behavior in response to different possible external influences using a deterministic computer simulation. In fact, you could make this scenario even simpler by replacing the person with a simple machine with sensors on its left and right, which is built to automatically press "right" if its left detector is activated, and to automatically press "left" if its right detector is activated. Of course such a simple machine could not actually understand that the signals were supposed to be predictions about its own behavior, but the principle is the same--a simple system can be set up in such a way as to deterministically do the opposite of whatever we understand a given signal to be predicting about its behavior.

edit: Since writing this I came across a paper that deals with many of the same issues about the impossibility of perfect prediction for an "embedded" system in a deterministic world: "The Paradox of Predictability" by Victor Gijsbers.

  • One might say that predictions within a deterministic system are impossible, because the system itself is calculating (~predicting) its future states as fast as it can. It is physically impossible to calculate future events faster than they actually occur. Apr 9, 2022 at 4:33

I would favor the idea that we have free will because the universe is deterministic.

Let's start with the assumption, widespread in many cultures, that people can sometimes remember the future. This should be considered as dangerous (in a practical sense at least), because the future cannot be changed, and the future often "wouldn't" have happened that way, if you hadn't remembered it. And do you think what you're likely to remember a day or a year in advance is going to be something good, safe, tame and boring, another day of contentment?

We therefore have a situation in which a given universe could have two histories: one in which you see something coming, change your behavior because of it, and cause it to happen - and one where you don't. This means that you, the human observer, are a boundary condition of the entire universe. Its past and future history depend on which value that boundary condition has.

But how is that established? Imagine you had a psychic telegraph machine that can send any string of letters back in time a minute. Whatever message it receives, will be sent, so somehow that message will end up back in the telegraph. But what determines the message that it would print out? It isn't random, because there is no point at which a random event changes anything - there's no chaos; the same series of letters is sent and received. It can't be predicted by any physics. If anything determines the message, it is from outside the universe. But it can be copied and affects the rest of the universe.

Now suppose nonetheless someone writes a telegram and hands it to you, and you put that into the machine. Well, this message naturally matches what the machine produced. You could hold the two messages up and compare them before you put one into the reader slot, and they would always be the same. So the machine's output can actually be copied not just to future events, but to past events. Now when we put these into the false assumption of past causality, we think -- the machine sensed the past message, and responded to it by copying it, but it caused the future message, which was copied by others. But these are the same phenomenon in opposite orientations in time.

That is to say, what we call "qualia" - really feeling something - should be the same process as free will. But it happens backward rather than forward in time, and the interaction of entropy with what solutions are possible affects the perception of it.

I don't mean to suggest a very spooky method for how any of this happens, mind. Let's suppose that in the context of the ever-increasing entropy and information of the universe, a biochemical change in a neuron causes a very obvious effect in its future state, but also a subtle effect on its past state. Evolution of biological organisms might then eventually find a way to recognize and amplify the signs of the future information using a standard neural network. This should be sufficient to cause all the paradoxes mentioned, and would be the deciding event that distinguishes organisms that "really choose" and "really think and feel", i.e. sentient, from those that are not. Sentience should introduce a species to endless new possibilities, the rise of civilization - but there are also the risks to consider, so in a healthy person, all this foretelling of the future would be very tightly locked down, with memories from only a moment in advance. This would still induce a temporal paradox, allowing messages to enter from somewhere beyond the cosmos we know, without necessarily implying an increased rate of memorably bad news.

This is also very old time Heisenberg Interpretation in the most vulgar sense - the conscious observer can only be in one state or the other, so can be directly blamed for collapsing a quantum superposition. We can also think of consciousness as an I/O device for the universe - what is beyond remains a mystery, but might conceivably involve a succession of parallel universes matched up in a perpendicular dimension of time. This would make it easier to see how a conscious choice or sensation would be made relevant to some higher-order process by which the history of the world is revised.

  • I did not find anything here that answers the question.
    – tkruse
    Apr 10, 2022 at 1:00
  • It is like the movie Arrival.
    – Scott Rowe
    Apr 10, 2022 at 15:26

The debate on free will is not over. However, the debate is not about whether free will exists or not. The debate is about the definition of free will, what it means.

Some define free will as something illogical or impossible. Some define it as a real phenomenon. Any definition that leaves the question (about free will's existence) open, subject to debate, is incomplete or invalid.

Determinism is another thing. It is clearly defined as an imaginary state of affairs. Determinism does not describe reality or explain anything. There has been no debate since the advent of quantum mechanics. It is logically impossible to believe that determinism is "true".

Your friends who call themselves "determinists" have clearly misunderstood something about determinism.

As I already mentioned in my comment, your conclusion is correct, but your deduction path how you reach that conclusion is wrong. If we assume a deterministic universe, there could be no-one to want a prediction and no-one capable of making predictions.

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