Saturday, April 3, 2010

Free will and quantum mechanics

I never used to spend much time thinking about the problem of free will. If I can think, "I'm going to move my hand now," and then move my hand - well, that seems like enough free will for me.

Recently, though, I started reading up on the free will debate. Here are two of the arguments philosophers use. (The first argument is courtesy of the Stanford Encyclopedia of Philosophy, the second is cribbed from Norman Swartz.)

Argument #1 – There is No Moral Responsibility if Determinism is True

Premise 1. No one has power over the facts of the past and the laws of nature.
Premise 2. No one has power over the fact that the facts of the past and the laws of nature entail every fact of the future (i.e., determinism is true).
Premise 3. Therefore, no one has power over the facts of the future.
Thus: If determinism is true, it appears that no person has any power to alter how her own future will unfold, and therefore no moral responsibility for it.

Argument #2 - Causal Determinism is a Necessary Condition
for Moral Responsibility
Premise 1: Unless there are extenuating circumstances, persons are (to be) held morally responsible for their actions.
Premise 2: Being unable reasonably to have foreseen the consequences of their actions is one such extenuating circumstance. (Recall that young children who cannot reasonably foresee the consequences of their actions are not to be held morally responsible for the consequences.)
Premise 3: In order to be able to anticipate or foresee the likely (or even the remotely likely) consequences of one's actions, the world must not be random, i.e. the world must be fairly regular (or causally determined).

Thus: Moral responsibility requires that there be causal determinism.

So the first argument (known as the consequence argument) seems to show that if the universe is deterministic, then there is no free will (because everything in the future is determined by events that happened long ago), and hence no moral responsibility. But the second argument (I don't know if it has a name) says that if there is no determinism, then there can't be any moral responsibility, either. Taken together, the two seem to imply that there can be no moral responsibility. I don't think this conclusion is correct, and neither does Swartz. You can read his notes to see how he resolves the problem; here, I want to consider a different aspect.

The current best models of how the universe works are based on quantum mechanics, and quantum mechanics is not deterministic. Or perhaps I should say quantum mechanics is partially indeterministic. That is, the quantum state of a closed system determines the quantum state of the system at a later time,  but the quantum state of the system doesn't determine all of the interesting aspects of the system. In fact, for any quantum system, there will always be questions I can ask about the system that don't have a definite answer - even if the quantum state of the system is exactly known. However, quantum mechanics can give us the probabilities of the various possible answers to any question, and so the future is partially determined: determined up to the range of possible values allowed by quantum mechanics.

How does quantum mechanics relate to the two arguments outlined above? The consequence argument is right out: Premise 2 is simply false if quantum mechanics gives a true picture of the world. What about Argument #2? According to Premise 3 of this argument, the world must be "fairly regular," which, according to quantum mechanics, it is. But the indeterminacy of quantum mechanics provides some elbow room (in Daniel Dennett's phrase), so that the future is not completely determined by the past. Quantum mechanics thus seems to walk a fine line between a clockwork universe in which every action of every person is, in principle, completely predictable, and a chaotically random universe in which nothing is predictable and so there can be no moral responsibility.

Quantum mechanics may not be the explanation of free will, but it does seem to allow for free will.

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