In chapter 5 of his book Probability Theory: The Logic of Science, E. T. Jaynes discusses ESP:
I. J. Good (1950) has shown how we can use probability theory backwards to measure our own strengths of belief about propositions. For example, how strongly do you believe in extrasensory perception?
What probability would you assign to the hypothesis that Mr Smith has perfect extrasensory perception? More specifically, that he can guess right every time which number you have written down. To say zero is too dogmatic. According to our theory, this means that we are never going to allow the robot’s mind to be changed by any amount of evidence, and we don’t really want that. But where is our strength of belief in a proposition like this?
Our brains work pretty much the way this robot works, but we have an intuitive feeling for plausibility only when it’s not too far from 0 db. We get fairly definite feelings that something is more than likely to be so or less than likely to be so. So the trick is to imagine an experiment. How much evidence would it take to bring your state of belief up to the place where you felt very perplexed and unsure about it? Not to the place where you believed it – that would overshoot the mark, and again we’d lose our resolving power. How much evidence would it take to bring you just up to the point where you were beginning to consider the possibility seriously?
So, we consider Mr Smith, who says he has extrasensory perception (ESP), and we will write down some numbers from one to ten on a piece of paper and ask him to guess which numbers we’ve written down. We’ll take the usual precautions to make sure against other ways of finding out. If he guesses the first number correctly, of course we will all say ‘you’re a very lucky person, but I don’t believe you have ESP’. And if he guesses two numbers correctly, we’ll still say ‘you’re a very lucky person, but I still don’t believe you have ESP’. By the time he’s guessed four numbers correctly – well, I still wouldn’t believe it. So my state of belief is certainly lower than −40 db.
How many numbers would he have to guess correctly before you would really seriously consider the hypothesis that he has extrasensory perception? In my own case, I think somewhere around ten. My personal state of belief is, therefore, about −100 db. You could talk me into a ±10 db change, and perhaps as much as ±30 db, but not much more than that.
The idea is that after Mr. Smith guesses 7 to 13 numbers correctly (when by chance he should have a probability of 10% of guessing each one correctly), Jaynes will begin to think it reasonably likely that he has ESP. He notes that this is his subjective opinion, saying, “In my own case,” and “My personal state of belief.”
However, Jaynes follows this up by stating that if this happened in real life, he would not be convinced:
After further thought, we see that, although this result is correct, it is far from the whole story. In fact, if he guessed 1000 numbers correctly, I still would not believe that he has ESP, for an extension of the same reason that we noted in Chapter 4 when we first encountered the phenomenon of resurrection of dead hypotheses. An hypothesis A that starts out down at −100 db can hardly ever come to be believed, whatever the data, because there are almost sure to be alternative hypotheses (B1, B2,…) above it, perhaps down at −60 db. Then, when we obtain astonishing data that might have resurrected A, the alternatives will be resurrected instead.
In other words, Jaynes is saying, “This happened by chance,” and “Mr. Smith has ESP” are not the only possibilities. For example, it is possible that Mr. Smith has invented a remote MRI device, which he has trained to distinguish people’s thoughts about numbers, and he is receiving data on the numbers picked by means of an earbud. If the prior probability of this is higher than the prior probability that Mr. Smith has ESP, then Jaynes will begin to think this is a reasonable hypothesis, rather than coming to accept ESP.
This does not imply that Jaynes is infinitely confident that Mr. Smith does not have ESP, and in fact it does not invalidate his original estimate:
Now let us return to that original device of I. J. Good, which started this train of thought. After all this analysis, why do we still hold that naive first answer of −100 db for my prior probability for ESP, as recorded above, to be correct? Because Jack Good’s imaginary device can be applied to whatever state of knowledge we choose to imagine; it need not be the real one. If I knew that true ESP and pure chance were the only possibilities, then the device would apply and my assignment of −100 db would hold. But, knowing that there are other possibilities in the real world does not change my state of belief about ESP; so the figure of −100 db still holds.
He would begin to be convinced after about 10 numbers if he knew for a fact that chance and ESP were the only possibilities, and thus this is a good representation of how certain he is subjectively.
The fact of other possibilities also does not mean that it is impossible for Jaynes to be convinced, even in the real world, that some individual has ESP. But it does mean that this can happen only with great difficulty: essentially, he must be convinced that the other possibilities are even less likely than ESP. As Jaynes says,
Indeed, the very evidence which the ESP’ers throw at us to convince us, has the opposite effect on our state of belief; issuing reports of sensational data defeats its own purpose. For if the prior probability for deception is greater than that of ESP, then the more improbable the alleged data are on the null hypothesis of no deception and no ESP, the more strongly we are led to believe, not in ESP, but in deception. For this reason, the advocates of ESP (or any other marvel) will never succeed in persuading scientists that their phenomenon is real, until they learn how to eliminate the possibility of deception in the mind of the reader. As (5.15) shows, the reader’s total prior probability for deception by all mechanisms must be pushed down below that of ESP.
This is related to the grain of truth in Hume’s account of miracles. Hume’s basic point, that an account of a miracle could never be credible, is mistaken. But he is correct to say that the account would not be credible unless “these witnesses are mistaken or lying” has a lower prior probability than the prior probability of the miracle actually happening. His mistake is to suppose that this cannot happen in principle.
Something like this also happens with ordinary things that we are extremely sure about. For example, take your belief that the American War of Independence happened before the Civil War. You can imagine coming upon evidence that the Civil War happened first. Thus for example suppose you found a book by a historian arguing for this thesis. This would be evidence that the Civil War came first. But it would be very unpersuasive, and would change your mind little if at all, because the prior probability of “this is a work of fiction,” or indeed of “this a silly book arguing a silly thesis for personal reasons” is higher.
We could call this a “settled issue,” at least from your point of view (and in this case from the point of view of pretty much everyone). Not only do you believe that the War of Independence came first; it would be very difficult to persuade you otherwise, even if there were real evidence against your position, and this is not because you are being unreasonable. In fact, it would be unreasonable to be moved significantly by the evidence of that book arguing the priority of the Civil War.
Is it possible in principle to persuade you to change your mind? Yes. In principle this could happen bit by bit, by an accumulation of small pieces of evidence. You might read that book, and then learn that the author is a famous historian, and that he is completely serious (presumably he became famous before writing the book; otherwise he would instead be infamous.) And then you might find other items in favor of this theory, and find refutations of the apparently more likely explanations.
But in practice such a process is extremely unlikely. The most likely way you could change your mind about this would be by way of one large change. For example, you might wake up in a hospital tomorrow and be told that you had been suffering from a rare form of amnesia which does not remove a person’s past memories, but changes them into something different. You ask about the Civil War, and are told that everyone agrees that it happened before the War of Independence. People can easily give you dozens of books on the topic; you search online on the matter, and everything on the internet takes for granted that the Civil War came first. Likewise, everyone you talk to simply takes this for granted.
The reason that the “one big change” process is more likely than the “accumulation of small evidences” process is this: if we want to know what should persuade you that the Civil War came first, we are basically asking what the world would have to be like in order for it to be actually true that the Civil War came first. In such a world, your current belief is false. And in such a world it is simply much more likely that you have made one big mistake which resulted in your false belief about the Civil War, than that you have made lots of little mistakes which led to it.