Common Questions

Frequently asked questions about the relationship between science and the Catholic faith.

Q13: Doesn’t science show that free will is an illusion?

No.  There was once a powerful argument against free will based on the supposed “determinism” of the laws of physics, but this determinism was overthrown by quantum mechanics in the 1920s.

“Physical determinism” is the idea that the laws of physics uniquely determine how any physical system will develop over time.  In other words, given any “initial” state of any system, there would only be one way consistent with the equations of physics for the system to behave at later times.  In the 19th century, this appeared to be correct, because all the physical laws that had been discovered, such as Newton’s laws of mechanics and gravity, had deterministic equations.  It was widely expected that this would also turn out to be true of the complete laws of physics.  Consequently, many people began to doubt the reality of free will.  The point was that if physical determinism is correct, then anything a human being does is uniquely determined by the past state of the physical universe.  This created a serious crisis for Christian belief.

The situation radically changed, however, in the 1920s, when the fundamental principles of quantum mechanics were discovered.  According to those principles, the laws of physics do not uniquely determine what is going to happen in the future, but only the relative probabilities of various things happening.  It is now generally accepted, therefore, that physical determinism is false.  Nevertheless, some people continue to argue against free will, claiming that the non-determinism of quantum mechanics is irrelevant to how the human brain functions.  These arguments are rather loose and hardly compelling, especially given that even so basic a mental phenomenon as consciousness is still not understood in terms of physics and brain function.  (Another commonly heard anti-free-will argument is based on the claim that “conservation laws,” such as conservation of energy and momentum, uniquely determine how physical systems will behave. This claim is erroneous, however. These conservation laws hold also in quantum mechanics, and quantum mechanics is not deterministic.)

Another argument against free will is based on neuroscience.  Famous experiments were done in the 1980s by Benjamin Libet, in which the experimental subjects were asked to make voluntary movements, at times of their choosing, while their brain activity was monitored.  It was found that something called the “readiness potential” started to build up in the brains of experimental subjects a fraction of a second before they were consciously aware of having made their decisions.  Because the readiness potential was interpreted as the brain preparing to initiate the movement, it was argued that the decision to move had already been made before any consciousness of it and therefore could not have been free, as free decisions require conscious deliberation.  Many philosophers have pointed out problems with this argument, and Libet himself did not regard his experiments as disproving free will.  Moreover, more recent experiments and theoretical work1 strongly suggest that the “readiness potentials” in these experiments are not the brain preparing to carry out an already-made decision, but the brain weighing the decision, which if true would fatally undermine the anti-free-will interpretations of the Libet experiment.2

Two further points should be noted.  First, it is obvious that many material factors influence our thoughts and choices, but that does not mean that they completely determine them. Second, if physical factors did completely determine our thoughts, then no one would be responsible for what they believed.


1..  Aaron Schurger, Jacobo D. Sitt, and Stanislas Dehaene,  “An accumulator model of spontaneous neural activity prior to self-initiated movement,” Proceedings of the National Academy of Sciences, October 16, 2012, 109 (42) E2904-E2913.

2.. Explanations of this work can be found in the following articles: “A Famous Argument Against Free Will Has Been Debunked,” Behar Gholipour, The Atlantic, September 10, 2019., “Libet and Free Will Revisited,” by Neuroskeptic, Discover Magazine, September 21, 2019., and at the Information Philosopher website:

Resources for further study

Modern Physics and Ancient Faith,  Stephen M. Barr (University of Notre Dame Press, 2003), Chapter 20.

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