Written on March 18, 2009. Written by .

One of the biggest debates in philosophy is the free will vs. determinism debate. Determinists claim the world is predictable, so we should be able to predict all the actions of a person, at least in principle. Free will proponents claim that we have free will and see this as contrary to determinism. There is a little-known third option called compatibilism that says that our actions are predictable, but we still have free will. Sometimes people define free will so that it is logically inconsistent with determinism, but I think these definitions differ from the commonly understood meaning of the term and I believe compatibilism makes the most sense.

Before I explain why I think this, I need to address an important point from physics. There is a result called Bell’s Theorem, which mathematically proves that there is “no local hidden variable theory for quantum mechanics” (using some input from well-established experimental data). Put simply, this means that there is no obvious way in which the universe could be truly deterministic. The universe seems to have randomness built-in at the lowest level. Even with Bell’s Theorem, it is still possible that the universe is deterministic, but it would have to work in really strange ways; ways much stranger than just being random. Our intuition tells us that the universe is deterministic, but this conclusion is based on limited information. We take the data from our daily lives along with basic explanations from science and apply Occam’s razor to decide that the simplest explanation is that everything is deterministic. But Occam’s razor is all we are going off of. So when it turns out that closer inspection reveals that the simpler explanation is actually that the world has randomness (unpredictability), we have to revise our beliefs. Trust me, I was extremely reluctant to give up on determinism, but Bell’s Theorem definitely throws some strong doubt on it.

On the other hand, the quantum randomness associated with Bell’s Theorem applies mostly at very small scales. There is very little effect on something the size of a neuron (neurons are too big). So regardless of whether the universe is entirely predictable, it is still quite likely that the operations of our brains are predictable with near 100% accuracy. There are already MRI machines than can read people’s minds [Link]. Just today it was announced that an MRI is going to be used as a lie detector in a court of law for the first time in history [Link]. It isn’t far-fetched to expect this technology to develop to the point where any action can be predicted. So does that mean we don’t have free will?

As I alluded to earlier, it depends on your definition of “free will”. To me, free will basically means that I can make whichever choice I want. This is not inconsistent with determinism because having my behavior predicted doesn’t prevent me from doing what I want. I know this sounds a little strange at first and I think it is because it is outside of our usual realm of thought. We more naturally think of a prediction of our behavior as a command that we must obey. But just think of the fact that there are a lot of situations where our behavior is predictable. If you put a person at a fork in the road, you can easily predict which fork they will take if one is covered in lava and the other isn’t. That doesn’t mean they don’t have a choice, they still have free will, it is just predictable free will.

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