More than Machines

Physics & Free Will

“What is man, that thou art mindful of him? And the son of man, that thou visitest him?” So asked the psalmist three thousand years ago. The question is still with us and as urgent as ever: What are we? The scriptural answer is that we are made in the image of God, but that answer is not as plausible to many people today as it once was. Reductive theories abound that claim human beings are nothing more than the product of biological and social evolution, of genes and the environment, of instincts and social conditioning, of the wiring of the brain and the chemistry of hormones and neurotransmitters. What gets lost in all this is free will.

Explaining how man is made in the image of God, the second-century theologian St. Irenaeus of Lyons wrote that “man is rational and therefore like God; he is created with free will and is master over his acts.” This freedom is what makes us persons, and it is what allows us to share in the divine life. To share in that life, says St. John the Evangelist, is to abide in love and in truth; and that is only possible for us to do because we are endowed with freedom. If we were entirely controlled by factors of a lower order, by mere material forces, as animals are, we could not be open to realities of a higher order, to goodness, truth, and beauty. According to the Catechism of the Catholic Church, in “his openness to truth and beauty, his sense of moral goodness, his freedom and the voice of his conscience,” man finds evidence “of his spiritual soul,” which is “irreducible to the merely material.”

The belief that human beings have spiritual (or nonphysical) souls and are thereby endowed with free will has been under attack for a long time by philosophers, scientists, and even some theologians. The basis of that attack is a philosophical idea called “physicalism,” which maintains that human beings are completely explicable in material terms: we are, say physicalists, nothing but enormously complicated biochemical machines.

This idea has deep historical roots. The predictable motions of the heavenly bodies led very early to the idea that the astronomical universe is a clocklike mechanism—already a commonplace among medieval thinkers. In the seventeenth century, as it became clear that terrestrial phenomena, like celestial ones, are governed by precise mathematical laws, the idea began to emerge that plants, animals, and even the human body can also be understood as machines. That was the view of both Descartes and Pascal, though they drew the line at the human soul, whose immateriality they continued to affirm. The more radical thinkers of the Enlightenment, including Hobbes, La Mettrie, and Baron D’Holbach, went further. They denied the spiritual soul and its freedom and asserted that man in his entirety is mechanical—hence the title of La Mettrie’s famous work, L’Homme machine.

Such thoroughgoing physicalism was still unusual in the eighteenth century, even among the philosophes, but it has gained enormous traction in our own day. As computer technology has advanced, even ordinary people have become accustomed to the idea, popularized by science fiction, that artificial machines might eventually become as conscious and intelligent as we are. This is the natural corollary of the idea that we ourselves are just “machines made of meat,” in the famous words of Marvin Minsky, a pioneer in artificial-intelligence research.

Many factors have contributed to this development. The defeat of vitalism in the nineteenth century promised that the same reductionist modes of explanation that had been so successful in physics would unlock the secrets of biology as well—as indeed they have, to a remarkable degree. By showing the continuities that exist between ourselves and the lower animals, Darwin’s discoveries weakened belief in human exceptionalism. But a decisive shift had already occurred much earlier, with the discoveries of Isaac Newton, whose laws of gravity and mechanics had two profound features. First, they were universal: they governed everything, from the motions of great planets to the minutest constituents of matter. Second, they were deterministic: they implied that the state of the physical world at one time completely and uniquely determines its state at all future times. The classic statement of physical determinism was given by the great mathematical physicist Pierre-Simon Laplace in 1819:

"[To] an intelligence which could know all the forces by which nature is animated, and the states at some instant of all the objects that compose it, nothing would be uncertain; and the future, as well as the past, would be present to its eyes."

Every discovery in physics in the century that followed bore out this bold assertion. When, for example, the complete laws of electricity and magnetism were written down by James Clerk Maxwell in the 1860s, they were seen to be as rigidly deterministic as Newton’s laws of mechanics and gravity.

The universality and determinism of the laws of physics gave rise to the belief that the entire physical universe is a complete and closed system of cause and effect. This is often called the principle of “causal closure” of the physical world. According to this principle, nonphysical entities (if any exist) can have no effect on anything physical. Whether you have a spiritual soul becomes irrelevant, since it could not affect what you say or do, or what is going on inside your brain. Your brain, as much as the rest of your body, is under the complete control of the laws of physics.

One might say that was nothing new. Since ancient times there had been thinkers who denied the reality of human freedom, claiming that our thoughts and deeds were governed by fate or by the stars. But these ideas were the fruit of mere speculation. What made the Newtonian challenge to free will more pressing was that it came from a powerful scientific synthesis that proved its validity in countless ways over hundreds of years of painstaking research. This doesn’t mean that scientists immediately abandoned belief in free will or in a spiritual soul that animates our bodies. In fact, most of the great scientists up through the nineteenth century, including Newton and Maxwell, rejected the causal closure of the physical world and the physicalist picture of human beings it entails. Still, these ideas slowly gained ground. For many thinkers, they are now axiomatic.

It may seem obvious to most of us that we do in fact exercise free will, but it has become customary for philosophers and scientists to dismiss such intuitions or commonsense beliefs about our own minds as mere “folk psychology.” Some argue that free will is a natural illusion: we cannot fathom our own behavior, they say, because the true physical causes of it lie below the level of consciousness, and so we posit a mysterious power that we call “the will.” The eminent biologist Francis Crick puts it this way: “What you’re aware of is the decision, but you’re not aware of what makes you do the decision. It seems free to you, but it’s the result of things you are not aware of.” The father of sociobiology, Edward O. Wilson, puts it more crisply: “The hidden preparation of mental activity gives the illusion of free will.”

Some might object that if free will were illusory then reward, punishment, and even exhortation would be useless. But this objection is easily met. Even animals that no one regards as free can respond to rewards and punishments-as can, in a sense, computer programs that “learn” by feedback. The more serious objection is that without free will there is no such thing as morality. We do not attach moral blame to animals or machines, or to human beings in the grip of a compulsion, precisely because we recognize that they have no power of free choice and are therefore not responsible for what they do. Nor do we attribute moral obligation to them. Without some concept of free will, most of our common moral language loses its meaning.

Many philosophers who are convinced determinists understand this. They have therefore tried to rescue moral language by showing that some notion of free will, even if not the traditional one, is compatible with physical determinism. The more sophisticated versions of “compatibilism” do this by identifying some manner in which human decision-making differs from that of animals and artificial machines and defining that as free will. To this end, they often focus on the fact that human beings can be self-critical.

One influential version of compatibilism, developed by Harry Frankfurt of Princeton University, analyzes freedom in terms of desires. Like animals we have desires, but unlike animals we also have desires about desires. For example, we may have chaste desires and unchaste desires tugging in contrary directions. But we can also have a general desire not to have unchaste desires, or at least not to act on them. This “higher order” desire could be said to represent our true self. If our lower-order desire wins out over this true desire, we are acting like animals, creatures that are ruled by passion and instinct. To act freely means to be governed by our higher-order desires. According to Frankfurt, what makes us persons—that is, moral agents—is this multitiered structure of our wills. There is obviously a great deal of truth in this analysis. It brings to mind St. Augustine’s theory of the “two wills” and St. Paul’s discussion of the unfree will in the seventh chapter of Romans: “For I do not do what I want, but I do the very thing I hate.”

Other compatibilists, such as Susan Wolf, stress the role of reason in our decision-making. Unlike animals, we can analyze our actions in relation to general principles and abstract standards of right and wrong—clearly a central aspect of what it means to be free.

Compatibilism in its various forms attempts to take seriously the notion of moral agency and moral responsibility, but the moral agency it speaks about is not quite real. Ultimately, the supposed agent has no real choice in anything. If physical determinism is true, as compatibilists believe, then what the “agent” does is, in principle, computable from a complete knowledge of the state of the physical world long before he makes his “decision.” Such a computation could be made entirely without reference to the agent’s desires, whether higher-order or lower-order.

It is helpful here to consider how physical determinism and causal closure work in the case of animals. Consider a cat whose tail has been stepped on. The cat screeches. What caused this? From the point of view of physics, the screech is simply the result of a chain of measurable and calculable physical events: a nerve is compressed; impulses travel up to the brain, where a complicated set of electro-chemical responses is triggered; impulses travel back down certain motor nerves; muscles contract; certain body parts move; and sound waves are produced. This explanation is undoubtedly correct. Nothing important is added if we talk about how the cat felt or what was going on in its mind. A similar analysis can be made of the cat’s desire to eat. Molecules wafting from its prey will enter its nostrils and affect some chemical receptors, signals are sent to the brain, and so on—until, finally, some muscle contractions cause the cat’s body to lunge. The causal-closure principle says that physical events (such as the screech or the lunge) can only have physical causes. The cat might just as well have no feelings or consciousness at all. (Descartes, a consistent physicalist when it came to animals, saw no reason to believe that animals have feelings.)

The fact is that feelings, desires, sensations, and other subjective experiences are not physical phenomena—that is, they are not things that physics explains. Such things do not register in measuring devices, nor do they appear as quantities in the equations of theoretical physics. The paradox, as the Austrian physicist Erwin Schrödinger correctly noted, is that while “all scientific knowledge is based on sense perceptions, the scientific view of natural processes formed in this way lacks all sensual qualities and therefore cannot account for the latter.” For example, theoretical physics explains the difference between the light we perceive as red and the light we perceive as blue (they oscillate at different frequencies), but it has nothing to say about the subjective experience of seeing redness and blueness. Physical science can explain to a blind person what goes on in the brain of a seeing person, but not what it is like to see. Subjective experiences may supervene on physical events in the brain, but they are not themselves physical events. And thus, according to a consistent physicalist, they can have no effects on the brain or the body. It is therefore often asserted by philosophers today that subjective experience and consciousness have “no causal efficacy,” are purely “passive” or “epiphenomenal.”

If one accepts this point of view, what happens to our moral life? Well, to begin with, such things as moral sentiments and feelings of desire, empathy, guilt, shame, or obligation are not actually causes of our behavior. They are just shadows cast on the screen of our consciousness by the unconscious physical processes that are doing all the real work of deciding. Even the “moral reasoning” that goes on in our brains is, in a deterministic world, not essentially different from the computations that go on in a pocket calculator or a chess-playing computer: it is done by material particles unconsciously following the laws of physics. Where is the moral agent in all this? Who is actually doing things? The “who” is really a congeries of particles, not a person.

Notwithstanding the desperately ingenious efforts of compatibilist philosophers, it would seem that physical determinism is fatal to the idea of free will and moral agency, at least as these are usually understood. And most people seem to realize this. Compatibilism is popular among professional philosophers, but it seems that most scientists and ordinary people who end up believing in physical determinism also end up simply concluding that free will is an illusion. They often admit that it is a useful illusion—one we need in order to carry on a decent and productive life. Indeed, it may have been necessary for the survival of the whole species for us to have this illusion; and for that reason, presumably, it was bred into us by natural selection.

The irony in all this is that just as many people were concluding that science compels a belief in physical determinism and a rejection of free will, the science itself was beginning to change. Three hundred years after Newton’s death, physics underwent a profound transformation with the development of quantum mechanics. Quantum mechanics was not just a theory of some particular class of physical phenomena; it was a new conceptual foundation for all of physics. And this new foundation was radically nondeterministic. In 1927, the physicist Max Born demonstrated that the formalism of quantum mechanics required a probabilistic interpretation. In particular, its basic principles imply that, pace Laplace, even complete information about the state of the physical world at one time would not uniquely determine its future state but only the probabilities of various future states. In other words, the scientific basis upon which the doctrine of physical determinism had rested for three hundred years disappeared in the 1920s. Why, then, does determinism still have such a strong grip on contemporary philosophy?

The answer usually given is that “quantum indeterminacy” has no relevance to the way the brain functions. The building blocks of the brain are neurons, and neurons, it is claimed, are sufficiently large that a “classical” (that is, nonquantum) account is perfectly adequate for understanding how they work. But given how rudimentary science’s present understanding of the brain is, such a claim looks like an overconfident, not to say arrogant, extrapolation. It suggests we know more than we do. The great mathematician and physicist Hermann Weyl described our situation well in a lecture delivered at Yale University in 1931:

"We may say that there exists a world, causally closed and determined by precise laws, but...the new insight which modern [quantum] physics affords...opens several ways of reconciling personal freedom with [the laws of physics]. It would be premature, however, to propose a definite and complete solution of the problem.... One of the great differences between the scientist and the impatient philosopher is that the scientist bides his time. We must await the further development of science, perhaps for centuries, perhaps for thousands of years, before we can design a true and detailed picture of the interwoven texture of Matter, Life, and Soul. But the old classical determinism of Hobbes and Laplace need not oppress us longer."

Today, though, many scientists are even less patient than the philosophers. Unable to imagine how free will might be interwoven with matter, they adhere to the crudest forms of classical determinism.

In addition to overthrowing determinism, quantum mechanics furnished an unexpected and very powerful argument against the physicalist view of the human mind. In the traditional understanding of quantum mechanics, the “wavefunctions” that are calculated in it are not simply mathematical representations of “the world as it is,” but rather of that which is known about the world by various particular “observers” who are part of it. And, according to a classic analysis that goes back to the great mathematician John von Neumann in the 1930s, the minds of these observers (or knowers) cannot themselves be entirely described by quantum mechanical wavefunctions—that is, by quantum physics. That’s why the Nobel Prize-winning physicist Eugene Wigner wrote in a famous essay that materialism is “not consistent with present quantum mechanics,” and why Sir Rudolf Peierls, another eminent physicist, said, “The premise that you can describe in terms of physics the whole function of a human being, including its knowledge and its consciousness, is untenable. There is still something missing.” While the argument against physicalism based on quantum mechanics is very controversial, there are many respected physicists who continue to defend its validity.

Quite apart from physics, there are strong philosophical arguments against physicalism. The theory of causal closure claims to be the truth about the world, and yet if it were true, we could have no access to truth. The closure of the physical world would mean the closure of our minds as well. In other words, physical determinism involves a denial of our intellectual freedom just as much as our moral freedom. In 1932, the famous biologist J. B. S. Haldane wrote, “If materialism is true, it seems to me we cannot know that it is true. If my opinions are the result of the chemical processes going on in my brain, they are determined by chemistry, not the laws of logic.”

Interestingly, Haldane eventually reversed his position and came to embrace physicalism. What led him to change his mind was the development of the computer, which demonstrated that a machine, though bound by physical laws, could nonetheless follow the laws of logic. The reason Haldane ended up being misled is that he did not frame the issue in quite the right way to begin with. The real issue is not whether a machine can follow the laws of logic. Certainly it can. If designed and programmed properly, a machine that follows one set of rules (the laws of physics), can by so doing also follow another set of rules (the laws of logic, or arithmetic, or chess). The real question is whether a machine that only follows rules of whatever kind can be open to the truth.

The answer seems to be no. Even if we restrict our attention to the realm of mathematics, openness to truth involves more than mere mechanical rule-following. That seems to be the lesson of one of the great discoveries of the twentieth century, Gödel’s Theorem. Before Kurt Gödel proved his theorem in 1931, a school of thought called “formalism” was very influential in the world of mathematics. According to the formalists, all of mathematical reasoning could be replaced, in principle, by symbolic manipulations that follow prescribed rules. For a mathematical proposition to be “true” meant simply that it could be derived from prescribed axioms by following prescribed rules. Consequently, following mechanical rules could give one access to all of mathematical truth. Gödel stunned mathematicians by showing that no matter how many rules were prescribed, there would remain infinitely many truths of mathematics that could not be reached merely by following them. The power of reason, even in the bloodless realm of mathematics, involves something more than machine-like behavior. It involves understanding of meaning and judgments of truth and falsehood. Hermann Weyl, in the lecture already quoted, made this observation:

"Descartes brings out the decisive point in the problem of free will with particular clarity, when he demonstrates the freedom involved in the theoretical acts of affirmation and negation: When I reason that 2 + 2 = 4, this actual judgment is not forced upon me through blind natural causality (a view which would eliminate thinking as an act for which one can be held answerable), but something purely spiritual enters in."

Weyl went on to explain that the mind, if completely determined by factors that are below it, such as chemical reactions in the brain, is not open to those nonmaterial realities that are above it, such as truth and meaning. This is indeed the decisive point: Our openness to truth not only demonstrates that we are intellectually free, but explains what that freedom is for and why it is important. Truth cannot enter a mind that is not first open to it; it cannot move a mind whose movements are already dictated by “blind natural causality.” The same is true of moral freedom. The soul cannot be open to the morally good if its movements are determined by the blind natural causality of physics, chemistry, and biology. It is our freedom that makes possible our openness to truth, beauty, goodness. Christians believe that it is primarily in the possession of this freedom that we are made in God’s image and likeness. This freedom is not an epiphenomenon of matter or an emergent property of it. Physics cannot explain it or say how it came to be present in the world, but neither does physics contradict it-as it once seemed to. The modern materialism that reduces human beings to the level of machines was a byproduct of the science of an earlier age. That science has been overthrown, and the deepest discoveries of the twentieth century have allowed us to see that neither the processes of matter nor the processes of thought are merely mechanical. Or, as Hermann Weyl put it, “the old classical determinism of Hobbes and Laplace need not oppress us longer.”


Related: "Thinking Again" by Marilynne Robinson

Published in the 2009-11-20 issue: 

Stephen M. Barr is professor of physics at the University of Delaware and author of Modern Physics and Ancient Faith (University of Notre Dame Press).

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