Clearly, what is immaterial in the human mind can influence the physical world, or our acts of will and understanding would be without effect. If our will is free these physical effects are not wholly predictable.
A merchant in Baghdad sent his servant to the market. The servant returned, trembling and frightened. The servant told the merchant, "I was jostled in the market, turned around, and saw Death."
"Death made a threatening gesture, and I fled in terror. May I please borrow your horse? I can leave Baghdad and ride to Samarra, where Death will not find me."
The master lent his horse to the servant, who rode away, to Samarra.
Later the merchant went to the market, and saw Death in the crowd. "Why did you threaten my servant?" he asked.
Death replied, "I did not threaten your servant. It was merely that I was surprised to see him here in Baghdad, for I have an appointment with him tonight in Samarra."
We have all heard stories such as the parable of Death in Samarra, or the tragedy of Oedipus, who was fated to kill his father and marry his mother and went on to do precisely that, despite his knowledge of the prophecy and his attempts to forestall it. These stories frighten us, and rightly so, by raising the specter of a future that is fixed, and that we each have an inexorable fate that, no matter how hard we run from, we only reach all the sooner.
Advocates of dualism sometimes claim that something very like this must be the case if materialism is true. After all, the argument goes, if we have no supernatural souls exempt from the principle of cause and effect, then our brains must be nothing but machines obeying the laws of physics, and if that is the case, then however complicated they are, their operation can in principle be predicted. Given complete knowledge of the state of the world, plus knowledge of the laws of physics governing how that state evolves, one could predict events arbitrarily far into the future. In this view our future behavior would be just as predictable in principle as the landing spot of a baseball thrown at a certain angle upward with a certain speed. And isn't that a terribly gloomy, disheartening vision? Don't we want to be more than thrown baseballs?
This intuition pump is what Daniel Dennett, in his book Elbow Room, calls the "Malevolent Mindreader", an entity who always knows in advance exactly what you are going to do and uses that knowledge to foil you every time. Playing chess against a Malevolent Mindreader is a doomed proposition, since he knows exactly how the game will end. A variant is the "Nefarious Neurosurgeon", who uses his knowledge not just to predict but actively to control you, typically by surreptitiously implanting electrodes into your brain that cause you to think, believe and decide just as he wishes, while leaving you the illusion of being in control. Are these sinister figures really waiting in the wings for us if materialism is true?
Let us explore this proposition in more detail with a thought experiment. Suppose that it is the year 2096, and the mad scientists at the Materialism Stereotypes Institute, thanks to a large government grant, are about to build the world's first fully functional Prediction Machine. This machine is an extraordinarily sophisticated piece of hardware, possessing a wide variety of sensors that allow it to gather every conceivable piece of data about its environment and a computer brain programmed with all the laws of physics. The purpose of the Prediction Machine is to survey in complete detail the state of a person's brain, then extrapolate that information to infallibly predict that person's future actions, thus proving that we are nothing more than complicated but deterministic machines ourselves. Later versions may be able to predict actions days or years in advance, to make Oedipus or Death-in-Samarra scenarios possible, but PM Mark I is merely a proof of concept and will only predict decisions a few minutes into the future.
Nevertheless, this is enough to prove the point the mad scientists of the MSI are trying to establish. To demonstrate their sinister powers, they recruit test subjects who agree to play several rounds of rock-paper-scissors with the Prediction Machine. If their hypothesis is correct and our actions are predictable, PM Mark I will always win, since it will infallibly anticipate what sign a person will throw and then throw the correct sign that beats that one.
The first test subject is hooked up to the Prediction Machine, which scans his brain and makes its prediction. Then, on a count of three, they each throw their signs simultaneously.
The human throws rock. The Prediction Machine throws paper. The mad scientists grin and exchange high-fives.
But even at the Materialism Stereotypes Institute, they are well aware that repeatability is vital for science. In order to prove that the Prediction Machine's victory was not due to chance, they continue the test.
In the second round, the human throws scissors. The Prediction Machine throws scissors as well.
The mad scientists' grins fade. They put the test on hold, pop open the machine's casing, check all its connections and recalibrate its software to make sure nothing has gone wrong. But it seems to be in perfect working order, so they dismiss the result as a fluke and continue the test.
In the third round, the human throws rock. The Prediction Machine throws scissors.
Seeing their chances at a Nobel Prize slipping away, the mad scientists give their machine the most thorough going-over they possibly can, but quickly discover that their efforts are to no avail. They are absolutely certain that there is nothing wrong with the machine, and yet it cannot win any more often than would be expected by chance. The more trials they run, the clearer this becomes. After all the millions of dollars and decades of research that went into building it, the Prediction Machine is no more accurate than a device that chooses signs at random. Like John Henry defeating the pile-driver, humanity has triumphed over the machine, and, it seems, retained its free will.
Something has gone wrong here, but what? Why doesn't the Prediction Machine work?
To see what the flaw is, consider a similar project: the quest to build a Prediction Machine for the stock market. Taking into account the current prices and past trends of every publicly traded stock, this machine would infallibly predict which stocks would rise and which would fall, allowing anyone who used it to effortlessly make a killing.
Such a plan could never work, and a moment's thought will reveal why: the mere existence of this machine would itself be an influence on the stock market that would have to be taken into account. If you used the knowledge it gave you to buy stocks, this will be a new causal factor on the market which would cause other traders to react differently than they might otherwise have done. In order for its original prediction to be accurate, the machine would have to predict this and incorporate that knowledge into its forecast. But that would change what the original forecast was going to be, thus changing what stocks you would buy in response, thus changing other traders' reactions, thus forcing the machine to alter its original prediction yet again... and so on, in a recursive, endless loop, as the machine tried in vain to construct an accurate model of the world that included itself in that model. To include itself in its own model, it would have to include itself containing that model in its model, which would have to include itself containing that model containing that model in its model, in an infinite iteration. This is impossible, clearly. It would be like trying to store a box inside itself.
Now, there is one way out of this paradox: use the machine to predict the stock market, but do not act on its predictions in any way, not even to tell anyone else what they are. If this were the case, the machine's predictions would not alter the state of the market and thus force it to recalibrate its own prediction. You would be the proverbial observer behind glass, always knowing in advance what the market would do, but utterly unable to act on that knowledge in any way at all, because the mere attempt to do so would render it untrue. In such a scenario, a Malevolent Mindreader could theoretically exist, but would be forever cut off from the rest of the world, unable to interact, an observer only.
But when it comes to the human brain, not even this strict separation can be maintained. When it comes to the stock market, one can learn information about a particular stock without actually affecting it. But imagine if this were not the case. Imagine if the only way to learn a stock's price was to buy a share of it. Then the machine would have to take its own existence into account, in which case infallible prediction truly would be impossible. By terminating the infinite recursion of self-prediction at some arbitrary depth, one could force the machine to make a prediction, but it could never be more than an educated guess, and would never be the dreaded statement of unavoidable destiny enshrined in tragic literature.
Human brains are like the latter, not the former, type of stock market. To exactly describe the state of a person's brain at a given time, one would have to measure the exact electrical potential of each neuron, the exact number of neurotransmitter molecules released from every synapse, and so on. But acquiring this level of detail would require an extremely sophisticated scan of the brain, down to the level of individual molecules.
In some worlds it might be possible to do this without changing the brain's state in any way, but ours is not such a world. In our world the theory of quantum mechanics reigns, which says, among other things, that all events have a component of irreducible chance. QM has been used and misused in many ways when it comes to free will, but it has one uncontroversial implication that is relevant here. That implication is that the mere act of observing something unavoidably changes it. For example, to see something, you have to bounce photons off it. On the scale of macroscopic objects such as baseballs, the influence of this is so small as to not significantly affect the accuracy of our predictions. But on the scale of the very small, such as an atom or a molecule, an impinging photon represents a significant disturbance indeed - and scanning the brain in the level of detail that the Prediction Machine needs requires us to descend to this level. Merely by scanning a person's brain, the Prediction Machine inevitably changes that brain's state, forcing it to take its own influence into consideration when making its prediction; and this leads straight back to the problem of infinite recursion that reared its head when trying to predict the fluctuations of the stock market. This is why the quest of the mad scientists at the Materialism Stereotypes Institute was doomed to failure from the beginning. If they had only listened to us compatibilists, we could have told them that in advance and saved them a lot of work.
What is the point of all this? Advocates of dualist free will claim that theirs is a model where even complete knowledge of the state of the universe at time T would not make it possible to infallibly predict what a person would do at time T+1. But we have just seen that materialism has exactly the same consequence. And this means that our actions are genuinely not determined in the way that the flight of a baseball is.
Some readers may consider this a logical sleight of hand. Even if no one could possibly have known in advance that you would do X, the argument goes, doesn't that knowledge still exist "somewhere", in some hidden dimension of determinism? I urge them to reject this conclusion. The separation of "X was determined to happen" and "It was knowable in advance that X would happen" is illicit, because there is, by definition, no conceivable test that could differentiate between them; no experiment could show that one holds true but not the other. Therefore, despite the apparent difference in wording, these two propositions express exactly the same idea. If one is false, so is the other.
We can now see why there is no need to fear that we live in the world of Oedipus. His is a world of fatalism, a world of Prediction Machines, where certain events will happen regardless of what you choose. But compatibilism is not fatalism. Compatibilism means that certain events happen because of your choices. In a fatalist world you cannot use knowledge of the future to alter the future, but in a compatibilist world you can, because the mere act of providing information creates a new causal factor that alters what would have happened in the absence of that information.
We need not fear these lurking intuition pumps; on closer inspection, they simply evaporate. The Malevolent Mindreader cannot exist. Neither can his comrade the Nefarious Neuroscientist, because precisely controlling one's will through external influence would require perfect knowledge of the state of one's mind to know how it had to be changed. Like many nightmares, these two seem tangible only as long as they keep to the shadows. Daylight reveals that they are without substance.
Next: What does it mean to make a choice? Can human beings be responsible for what they choose in a universe where every event is subject to the law of cause and effect, and could we have chosen differently in any situation? Stay tuned...
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