10.1.2. Reflexive prediction, determinism, and the impossibility of free will

 6th in free-will series

Will you—or won’t you—take an M & M candy in the next minute after it’s placed in front of you? In this thought experiment, we assume that I’m able to predict your action with absolute accuracy based on physics. I inform you that I predict you’ll take the candy, but you’re a rebellious sort, and just to prove you’re no automaton, you decide on defiance. Your proneness to disconfirm predictions deliberately seems to render impossible any prediction of your choice, provided you’re informed of it in advance. But remember I can predict your choice. 

    At least one of the following assumptions concerning the physical possibilities must be false to remove the contradiction:

    1. That it’s physically possible that you be informed of a deterministic prediction when you’re indifferent to the alternatives

  

    2. That it’s physically possible to predict your behavior precisely

  

    3. That it’s physically possible for you to defy the prediction

A.   Because you don't want to defy it

B.   Because you won't be able to defy it despite wanting to

The possibility of conveying a precise prediction (No. 1)

It might be claimed that physics precludes that you could ever be informed of a determinate prediction when you’re indifferent to the alternatives, since you’ll be moved to falsify it. But if perfect prediction is possible, conveying its substance should be easy: no barriers are apparent.

The possibility of predicting precisely (No. 2)

We may try denying the second assumption, that precise prediction of your behavior is possible. It might be said that human organisms aren’t the kind of systems that can be predicted precisely. But again, one must wonder why not. Organisms are but clumps of matter; they can be described as systems in biological terms, but otherwise they are no less subject to purely physical description than other purely physical systems. No specific barrier limits prediction of biological systems when they’re conceived as physical systems.

The possibility of defying the prediction: You won’t want to defy it (No. 3A)

The possibility of defying the prediction must be questioned. Perhaps you won’t choose to be negative, your character and personality notwithstanding. But the truth of the prediction doesn’t affect any of the things that we use to attribute a reactant tendency to infer what you want to do; its truth affords no cause for bringing other motives into play. A true prediction confronts you no differently from a false prediction: whether the prediction happens to be true or false has in itself no bearing on how it’s received.

The possibility of defying the prediction: You can’t defy the prediction despite wanting to (No. 3B)

Having excluded the other possibilities, we should conclude that you will confirm the prediction despite not wanting to confirm it. This result is due to your taking of the candy being a physical act, which must conform to physical prediction, whereas having an attitude of wanting is identified by psychological criteria and does not necessarily conform to any specific physical criteria.

A note on quantum indeterminacy

Although the ground is well traveled by others, I should say a couple of words about quantum mechanics in its bearing on whether absolutely true predictions really are physically possible. First, the relevant macroscopic states are essentially deterministic.  Second, quantum uncertainty doesn’t explain why in the thought experiment you would want determinately according to your character, for which expectation I argue.

Conclusion

The thought experiment serves as an “intuition pump” (Daniel Dennett) concerning the incongruence between mentalistic and physical concepts. Compatibilism is the doctrine that the world can be deterministic despite your ability to exercise “volition.” Compatibilism thus implies that in describing our “wants” we describe a real cause of our behavior.

Mental concepts, such as "wanting," refer only fictitiously. They serve fallibly to enhance foresight about our behavior and that of other people. Fatally for compatibilism, the thought experiment accentuates that fallibility.