I have recently been thinking a lot about determinism and pretty much decided there was no reason to refute it as there was no evidence of free will that I could find. However, a couple of days ago, this thought experiment came to me and it seems to be a point against determinism so I wanted to see what the SE thought:

Assume that in a few years, someone finally built the determinism computer that took every variable into account and could therefore predict the next action of everything, (including people assuming there is no free will). So I print out the page of Mr Jack Robert and it happens to say he will eat cereal in exactly one hour. In an effort to prove free will, I run over to his house and pay him 100$ to eat salami instead.

By this type of line of though, as long as I knew what the computer predicts, I should be able to do something different than what it asserts I will do every-time and quite literally make it impossible for it to make the right prediction, something impossible if there was only determinism.

Is this a good case against determinism?

  • 1
    "pretty much decided there was no reason to refute it" if absolute determinism exists then you couldn't decide anything because choices aren't real. On a more serious note, you are proposing that the computer does actually compute the exact deterministic outcome of an event. If humans have no free will and the computer computes their actions (as you stipulated) then there isn't anything it could print on a piece of paper that wouldn't come true. If you did something against what the computer says then you have free will which contradicts your earlier premise.
    – Not_Here
    Commented May 1, 2017 at 0:25
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    If you are stipulating that human beings don't have free will and are therefore subject to a deterministic outcome then you can't 'make the choice to go and pay the man to eat salami instead' because you cannot make choices. If you are stipulating that human beings do have free will, then the computer doesn't determine what your actions will be so you aren't refuting determinism.
    – Not_Here
    Commented May 1, 2017 at 0:29
  • I see your argument and it seems a good one, or at least it's an interesting question. But it looks like a sledgehammer to crack a nut. There is a reason why philosophers tend to endorse compatabilism.
    – user20253
    Commented May 1, 2017 at 9:56
  • "there was no reason to refute it as there was no evidence of free will". Free will is not required for determinism to be false, indeterminism is a much simpler concept than free will. And on conventional interpretations quantum mechanics is indeterministic.
    – Conifold
    Commented May 1, 2017 at 17:41

6 Answers 6


There's a few issues. The first one is the easy one. It's mentioned by several answers already. You start by assuming you have freewill and thus prove that you have freewill. Pesky construct that one.

To get around it, we need the computer to be smarter. Much smarter than you're giving it credit for. The version you are looking at is the one where the computer prints out what would have happened had it not printed anything. As you noticed, it is rather easy to do something different, but that doesn't refute determinism because the prediction was for a hypothetical universe where nothing was printed. In this world, something was printed so the prediction is no longer 100% accurate. You might as well have asked it what the lottery numbers were in an alternate universe!

The more interesting case arises when the computer prints out what will happen, accounting for the fact that it printed something. These predictions are far more nuanced and much harder to disprove with such simple constructs. As DeeDuu mentions in their answer, there's no guarantee that you merely wanting to disprove the computer will actually lead you to succeed. In fact, the argument for the computer is that, in fact, you cannot disprove it. Trying to disprove it may simply yield unexpected results (such as breaking your leg going down the stairs trying to stop Mr. Jack Roberts from eating cereal).

The more interesting issue arises when you start playing mind games with the computer. When you start trying to play along when it thinks you'll do the opposite and so forth. You can play that game for a while, but there's some interesting mathematical oddities that arise. You see, the computer needs to predict what will happen, including what will happen within itself while the calculations are taking place. Otherwise, how can it figure out what it might print on the paper, so that it can figure out what you would do with it.

There's a theorem out there known as Godel's incompleteness theorem which starts to apply here. It turns out that the computer you seek to create cannot possibly exist within the universe. There's a strange pattern which occurs when a perfectly truthful machine tries to predict everything, including itself. It turns out that such a machine simply cannot be made from a mathematical perspective.

What you need is an Oracle: a device which can tell this sort of truth without being part of the universe at all. However, once you start adding oracles, it becomes impossible to prove their correctness -- the correctness of an oracle is always assumed, rather than proven.

Thus, the question becomes not whether the universe is deterministic, but whether such an oracle can be created outside of the universe.

  • This is not the right answer. Free will is a red herring; all you need to have this "problem" is for the act of predicting to cause the prediction to be wrong. You don't need a sophisticated human with free will, just a light bulb with a switch. The problem is simply that you by setup have predicting A result in B, and predicting B result in A. It's not a problem for determinism at all; all effects have antecedent causes. It's simply an interesting setup where there's nothing the predictor can do that will match the prediction, because your machine spites it.
    – H Walters
    Commented Apr 17, 2018 at 0:34

Your thought experiment doesn't disprove determinism for two reasons:

  • First, even if your argument is correct, the most it can do is show that no computer (or the like) could predict all future events. This is well compatible with determinism. Determinism only says that given the current state of the world (and the laws of nature), there is only one way for the world to develop. Even if this were the case, it doesn't mean that anyone is able to know, or that any computer is able to predict, how the world will develop.

  • Second, your argument is based on a petito principii: You already presuppose that determinism is false, because you assume that – pace the predictions – you are able to do whatever you want. In particular, you presuppose that you will be able to disprove the prediction. But if determinism is true, and the computer indeed predicts some future event, it follows that you cannot (or will not, at any rate) disprove the prediction. How so? Just one possibilty: Might it not also be determined that in your attempt to make Mr Jack Robert eat salami, you fall down the stairs in excitement and end up in hospital?


Here is a video narrated by professor of philosophy Richard Holton, in which he presents the problem you have come up with: https://www.youtube.com/watch?v=iSfXdNIolQA

He simplifies the scenario into the problem of predicting whether a light bulb will be on or off at a particular time in the future. His conclusion is that determinism does not entail foreknowledge.

Respect for coming up with this problem!


What you are describing is likely an impossible situation. Like most situations when you start with an impossibility you can prove anything in the "therefore". ( Just like those sneaky math equations that prove 1 = 2 by disguising a hidden divide by zero )

What can be computed is limited. The classic example of this is "the halting problem". Also if such a computer program existed it starts getting super fun when we get the computer to make predictions about the computer itself which hits on Gödel's incompleteness theorems.

But that's all a problem with computation, not determinism. The universe could be acting in a pre determined way even though we can't determine what way that is or why.

Also, to take another angle, Thomas Hobbes advanced the idea that free will and determinism can be compatible.


Did this computer account for your intervention of Mr. Robert eating cereal in exactly one hour? Because this computer can only accurately predict future events if and only if it accounts for all things, then you would have only succeeded in taking advantage of this computer's lack of additional information.

  • Hello, if determinism is correct, then it should have also predicted my intervention as that is also nothing more than the effect of some distant cause which it took into account. Anyway, the point is: this computer knows everything. But no matter how i play it, as long as i can print out its future predictions, i can make them false. Such should be impossible if i had no ability to think with a cauntiousness independent of causality.
    – Anthony B
    Commented Apr 30, 2017 at 23:29
  • If the computer knows everything, then it would know that you would attempt to manipulate it. Otherwise, it doesn't know everything. Since this is a Q&A site, I don't think it's appropriate for me to continue. You are free to continue in the chat, of course. Commented May 1, 2017 at 6:46

This is not a good counter to determinism because the computer is obviously flawed if it predicts wrong. And also, you only paid him to eat salami because you wanted to prove we had free will. So by paying him you are showing you dont have free will.

  • Hi, I understand all the rebuttals so perhaps I will slightly modify the scenario. The computer instead predicts that in exactly 3 seconds I will pick up the phone and call my mom. It prints that out and I read it. At this point, it is unimaginable to me that I would feel compelled to do it and I think that if I wanted to I could simply not pick it up. So as long as I wanted to and I knew it's prediction, could I not always always do the opposite?
    – Anthony B
    Commented Jul 11, 2017 at 0:06

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