Back to Free Will

*Quantum Probability* is the claim that free will is incompatible with the probabilities predicted by Quantum Mechanics.

**Table of Contents**

- Cartesian Interactionism
- John Eccles’ Interactionism
- Altering Quantum Probabilities
- Erwin Schrödinger, from Science and Humanism (1951)

#### Cartesian Interactionism

- If human beings have categorical free will they can control whether upper motor neurons fire.
- Descartes explained how this would work:
- A person, who’s an immaterial self, has direct control over their acts of will. A person’s volition to move a body part causes changes in the pineal gland which, by the spreading of animal spirits, makes muscles contract and body parts move.

- [Pineal pronounced PIN-ee-ul]
- [The pineal gland, located in the middle of the brain, synthesizes melatonin and appears to have no direct effect on upper motor neurons. ]

#### John Eccles’ Interactionism

- In How the Self Controls its Brain, John Eccles proposed a theory like Descartes’ but with an updated mechanism of interaction.
- Whether a neuron fires depends on the amount and kind of neurotransmitter molecules it receives from its presynaptic neurons.
- Eccles proposed that
- Whether presynaptic neurons release their neurotransmitters is governed by Quantum Theory and is therefore a matter of quantum probabilities.
- “Mental events merely alter the probability of a vesicular emission” (pages 72-73)

- That is, human beings can control whether an upper motor neuron fires by altering the quantum probabilities of prior events that encourage or inhibit the neuron’s firing.

#### Altering Quantum Probabilities

- The problem with Eccles’ view is that by “altering the probability” of an event, human beings would violate the postulates of Quantum Mechanics and Quantum Field Theory.
- In the famous Stern-Gerlach Experiment of the 1920s, a stream of silver atoms is shot through a special magnet. Quantum Mechanics predicts a probability of 0.5 that a given silver atom is deflected upward. The statistical results confirm the prediction.
- Were a person, by telekinesis, able to alter the probability from 0.5 to 0.75, say, which could be confirmed by statistical results, they would refute Quantum Mechanics and violate the laws of nature.
- The same is true of a person altering the quantum probabilities of brain events.

#### Erwin Schrödinger, from *Science and Humanism* (1951)

“According to our present view the quantum laws, though they leave the single event undetermined, predict a quite definite statistics of events when the same situation occurs again and again. If these statistics are interfered with by any agent, this agent violates the laws of quantum mechanics just as objectionably as if it interfered —in pre-quantum physics—with a strictly causal mechanical law. … The inference is that … the direct stepping in of free will to fill the gap of indeterminacy … does amount to an interference with the laws of nature, even in their form accepted in quantum theory.”