The double blind Full-Libet experiment

Hypothesis: A Laplacian demon, i.e. a being who can correctly predict all future actions, contradicts our actual experience of following instructions with some failure rate.

Set up: You are in a room with two buttons, A and B. This is the same set-up Soon’s free-will experiment, but the instructions are different.

Instructions: You are told that you will have to push a button every 30 seconds, and that you will have fifty trials. The clock will start when a sheet of paper comes out of a slit in the wall that says A or B. Your instructions are to push the opposite of whatever letter comes out.

The Apparatus: the first set of fifty trials is with a random letter generator. The second set of trials is with letters generated by a Laplacian demon who knows the wave function of the universe and so knows in advance what button will be pushed and so prints out the letter.

The Results: In the first set of trials, which we can confirm with actual experience, the success rate is close to 100%, but, the world being what it is, there is a 2% mistake rate in the responses. In the second set of trials the success rate is necessarily 0%. In the first set of trials, subject report feelings of boredom, mild indifference, continual daydreaming, etc. The feelings expressed in the second trial might be any or all of the following: some say they suddenly developed a pathological desire to subvert the commands of the experiment, others express feelings of being alienated from their bodies, trying to press one button and having their hand fly in the other direction, others insist that they did follow instructions and consider you completely crazy for suggesting otherwise, even though you can point to video evidence of them failing to follow the rules of the experiment, etc.

The Third Trial: Run the trial a third time, this time giving the randomly generated letter to the subject and giving the Laplacian letter to the experimenter. Observe all the trials where the two generate the same number, and interate the experiment until one has fifty trials. Our actual experience tells us that the subject will have a 98% success rate, but our theoretical Laplacian demon tells us that the success rate should be necessarily 0%. Since asserting that the random-number generator and the demon will never have the same response would make the error-rate necessarily disappear and cannot explain our actual experience of failures, the theoretical postulation of a Laplacian demon contradicts our actual experience. Q.E.D.



  1. David said,

    February 16, 2017 at 11:37 am

    Aren’t you just cherry picking data? “Observe all the trials where the two generate the same number.” How is that not just, “pick all the trials where the subject fails”?

    • David said,

      February 16, 2017 at 11:38 am

      …And then ask, “why is the success rate 0% across these 50 examples of the subject failing?”

      • February 16, 2017 at 12:42 pm

        aren’t you just cherry picking data? “Observe all the trials where the two generate the same number”

        Cherry picking requires leaving aside evidence against your hypothesis. But the times when the random number generator and the Laplacian demon disagree do not provide evidence one way or another whether Laplacian demons are consistent with experience or not.

  2. February 16, 2017 at 10:03 pm

    I’m too close to the argument now to be objective. Does anyone else want to weigh in on EU’s critique?

  3. David said,

    February 17, 2017 at 9:29 am

    I agree with EU that a physical determinist could simply say it’s impossible to write down the result (and show it to the subject), because the function is tied to the “input”. It’s like F(X) = not(X) “Now input X such that F(X) = X.”

    I’m not sure where God came into this, though. We were talking about Laplacian Demons :D. The difference between the two sounds like a great future blog post, but I’m sure James has written at length about that already.

    • February 17, 2017 at 12:15 pm

      it’s impossible to write down the result (and show it to the subject), because the function is tied to the “input”. It’s like F(X) = not(X) “Now input X such that F(X) = X.”

      I agree the physical determinist’s equation can’t be satisfied for all values, and that what makes it possible is the presence of a sort of recursion. But in the context of the experiment this means that the letter on a sheet of paper together with a snapshot of the rest of the universe can never be an initial condition, but I see no reason why this would be the case. Even if I granted their claim that there was some reclusive contradiction, it does not arise merely because the letter is given in advance, since the LD could print out the letter in advance just fine if the initial conditions were, say, a test particle flying though empty space toward button A with enough force to push it.

      • February 17, 2017 at 1:18 pm

        Just a note, one point I am thankful to EU for is the idea that a trained dog might be a good test subject too. If this is right, then the recursive loop might not be from intelligence as such but the intrinsic indeterminism of nature, which we find in one way through (what Aristotle called) matter being present in the initial conditions and the working of the laws and in another through intelligence. But space is opened for one with the allowing of the other, since on either account nature has to allow for teleology.

  4. February 17, 2017 at 11:52 am

    Okay, I’ve got some distance and have had some time to think about it. I’ be happy to drop my argument in the face of a better one, but I simply don’t see your counter. This might be some combination of my own vanity and the subtlety of the thing we’re speaking of – I’d be interested in knowing what all of you guys other than EU and myself think of this.

    Here are my responses:

    That is impossible… I know what will actually be written, and I know what the person will do. But I also know that those do not and cannot match

    But “what will actually be written” is, together with a snapshot of the rest of the universe, an initial condition and “what the person will do” is an outcome. Saying these “can never match” means the demon is saying “the laws of nature do not suffice to go from some this initial condition to one of its outcomes” which is to deny Laplacian demons altogether.

    If you answer, as you should, that God cannot write what the person will do, but he can know it, the same applies to the Laplacian demon.

    When God announces what will happen he can be speaking about what he intends to do, while a LD cannot. I’m also very impressed by John of St. Thomas’s arguments that the world is not only notionally present to God but even physically present within him, which makes for a dimension of his speaking of the future that could never be said of an LD. This is in keeping with the Biblical idea that God not only looks at the world but responds and interacts with it. The character of prophesy is also very different from the thought experiment we’re trying to do with an LD: LD’s are all about what we can predict in advance, but Biblical prophesies do not seem to be overly concerned with what can be predicted in advance, as should be shown from the long history of failed attempts to turn the NT into a predictive tool.

    we can easily train a dog in such a way that no one can possibly write down the levers it will choose, since it will trained to choose the opposite ones.

    Such an animal would follow instructions with some errors, and so would be a fine test subject for my experiment. This is exactly what my subject does in trial #1. I say the same for your robot example.

    (ADDED LATER) I’m thankful for this point and developed for reasons given above on the thread.

    As for the third set, if I understood it correctly you are indeed cherry picking — you are simply selecting the trials where the human made a mistake,

    LD’s can’t be mistaken. If they foresee outcome O from initial conditions C, then no mistake can fail to make O come about. But this isn’t my main point, which is simply to repeat what I said to David: cherry picking requires disregarding evidence that goes against your conclusion, but the times when the random number generator and the LD disagree provide no evidence whether LD’s are consistent with our experience of following instructions with some errors.

    • David said,

      February 17, 2017 at 12:19 pm

      “Cherry picking requires disregarding evidence that goes against your conclusion.”
      I think you need to go one layer deeper, and then ask what is my conclusion or hypothesis? The conclusion of the whole thought experiment is that LD’s are inconsistent with our experience, yes. James may not be, but the experimenter within James’s thought experiment is the one cherry picking. The experimenter’s hypothesis is that subjects have a certain % success rate. He’s leaving out all the evidence for the success rate being above 0%.

      That’s all I’ve got.

      • February 17, 2017 at 12:23 pm

        Okay, I can see that, but does the experiment logically demand that one cherry picks anything? There is no reason why the first fifty results wouldn’t be hits. To be honest, the point is not even to get fifty results, but to point to a contradiction that has to occur when the random number generator and the LD (a) give the same result in trial #3 or (b) if they never fail to differ. If (a) then what we expect from experience (following the rule) contradicts with our theory of LD, and if (b) then what we expect from experience (making some errors) contradicts our theory (i.e. since LD’s can’t make mistakes)

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