In a remote corner of the universe, on a small blue planet gravitating around a humdrum sun in the outer districts of the Milky Way, organisms arose from the primordial mud and ooze in an epic struggle for survival that spanned aeons.

Despite all evidence to the contrary, these bipedal creatures thought of themselves as extraordinarily privileged, occupying a unique place in a cosmos of a trillion trillion stars. Conceited as they were, they believed that they, and only they, could escape the iron law

of cause and effect that governs everything. They could do this by virtue of something they called free will, which allowed them to do things without any material reason.

Can you truly act freely? The question of free will is no mere philosophical banter; it engages people in a way that few other metaphysical questions do. It is the bedrock of society’s notions of responsibility, praise and blame. Ultimately it is about the degree of control you exert over your life.

Let’s say you are living with a loving and lovely spouse. A chance meeting with a stranger turns this life utterly upside down. You begin talking for hours on the phone, you share your innermost secrets, you start an affaire de coeur. You realize perfectly well that this is all wrong from an ethical point of view; it will wreak havoc with many lives, with no guarantee of a happy and productive future. Yet something in you yearns for change.

Such gut-churning choices confront you with the question of how much say you really have in the matter. You feel that you could, in principle, break off the affair. Despite many attempts, you somehow never manage to do so.

In my thoughts on these matters of free will, I neglect millennia of learned philosophical debates and focus on what physics, neurobiology and psychology have to say, for they have provided partial answers to this ancient conundrum.

Shades of Freedom

I recently served on a jury in United States District Court in Los Angeles. The defendant was a heavily tattooed member of a street gang that smuggled and sold drugs. He was charged with murdering a fellow gang member with two shots to the head.

As the background to the crime was laid out by law enforcement, relatives, and present and past gang members—some of them testifying while handcuffed, shackled and dressed in bright orange prison jumpsuits—I thought about the individual and societal forces that had shaped the defendant. Did he ever have a choice? Did his violent upbringing make it inevitable that he would kill? Fortunately, the jury was not called on to answer these irresolvable questions or to determine his punishment. We only had to decide, beyond a reasonable doubt, whether he was guilty as charged, whether he had shot a particular person at a particular place and time. And this we did.

According to what some call the strong definition of free will, articulated by Ren Descartes in the 17th century, you are free if, under identical circumstances, you could have acted otherwise. Identical circumstances refer to not only the same external conditions but also the same brain states. The soul freely chooses this way or that, making the brain act out its wishes, like a driver who takes a car down this road or that one. This view is the one most regular folks believe in.

Contrast this strong notion of freedom with a more pragmatic conception called compatibilism, the dominant view in biological, psychological, legal and medical circles. You are free if you can follow your own desires and preferences. A long-term smoker who wants to quit but who lights up again and again is not free. His desire is thwarted by his addiction. Under this definition, few of us are completely free.

It is the rare individual—Mahatma Gandhi comes to mind—who can steel himself to withhold sustenance for weeks on end for a higher ethical purpose. Another extreme case of iron self-control is the self-immolation of Buddhist monk Thich Quang Duc in 1963 to protest the repressive regime in South Vietnam. What is so singular about this event, captured in haunting photographs, is the calm and deliberate nature of his heroic act. While burning to death, Duc remained in the meditative lotus position, without moving a muscle or uttering a sound, as the flames consumed him. For the rest of us, who struggle to avoid going for dessert, freedom is always a question of degree rather than an absolute good that we do or do not possess.

Criminal law recognizes instances of diminished responsibility. The husband who beats his wife’s lover to death in a blind rage when he catches them in flagrante delicto is considered less guilty than if he had sought revenge weeks later in a cold, premeditated manner. Norwegian Anders Breivik, who shot more than 60 people in a cold-blooded and calculated manner in July 2011, is a paranoid schizophrenic who was found to be criminally insane and will probably be confined to a psychiatric institution. Contemporary society and the judicial system are built on such a pragmatic, psychological notion of freedom.

But I want to dig deeper. I want to unearth the underlying causes of actions that are traditionally thought of as “free.”

A Clockwork Universe

In 1687 Isaac Newton published his Principia, which enunciated the law of universal gravitation and the three laws of motion. Newton’s second law links the force brought on a system—a billiard ball rolling on a green felt table—to its acceleration. This law has profound consequences, for it implies that the positions and velocities of all the components making up an entity at any particular moment, together with the forces between them, unalterably determine that entity’s fate—that is, its future location and speed.

This is the essence of determinism. The mass, location and velocities of the planets as they travel in their orbits around the sun determine where they will be in a thousand, a million or a billion years from today, provided only that all the forces acting on them are properly accounted for. The universe, once set in motion, runs its course inexorably, like a clockwork.

A full-blown setback for the notion that the future can be accurately forecast was revealed in the form of deterministic chaos. The late meteorologist Edward Lorenz came across it while solving three simple mathematical equations characterizing the motion of the atmosphere. The solution predicted by his computer program varied widely when he entered starting values that differed by only tiny amounts. This is the hallmark of chaos: infinitesimally small perturbations in the equations’ starting points lead to radically different outcomes. In 1972 Lorenz coined the term “butterfly effect” to denote this extreme sensitivity to initial conditions: the beating of a butterfly’s wings creates barely perceptible ripples in the atmosphere that ultimately alter the path of a tornado elsewhere.

Remarkably, such a butterfly effect was found in celestial mechanics, the epitome of the clockwork universe. Planets majestically ride gravity’s geodesics, propelled by the initial rotation of the cloud that formed the solar system. It came as a mighty surprise, therefore, when computer modeling in the 1990s demonstrated that Pluto has a chaotic orbit, with a divergence time of millions of years. Astronomers cannot be certain whether Pluto will be on this side of the sun (relative to Earth’s position) or the other side 10 million years from now! If this uncertainty holds for a planet with a comparatively simple internal makeup, moving in the vacuum of space under a sole force, gravitation, what does it portend for the predictability of a person, a tiny insect or an itsy-bitsy nerve cell, all of which are swayed by countless factors?

Chaos does not invalidate the natural law of cause and effect, however. It continues to reign supreme. Planetary physicists aren’t quite sure where Pluto will be aeons from now, but they are confident that its orbit will always be completely in thrall to gravity. What breaks down in chaos is not the chain of action and reaction, but predictability. The universe is still a gigantic clockwork, even though we can’t be sure where the minute and hour hands will point a week hence.

Origins of Uncertainty

The deathblow to the Newtonian dream—or nightmare, in my opinion—was the celebrated quantum-mechanical uncertainty principle, formulated by Werner Heisenberg in 1927. In its most common interpretation, it avers that any particle, say, a photon of light or an electron, cannot have both a definite position and a definite momentum at the same time. If you know its speed accurately, its position is correspondingly ill defined, and vice versa. Heisenberg’s uncertainty principle is a radical departure from classical physics. It replaces dogmatic certainty with ambiguity.

Consider an experiment that ends with a 90 percent chance of an electron being here and a 10 percent chance of it being over there. If the experiment were repeated 1,000 times, on about 900 trials, give or take a few, the electron would be here; otherwise, it would be over there. Yet this statistical outcome does not ordain where the electron will be on the next trial. Albert Einstein could never reconcile himself to this random aspect of nature. It is in this context that he famously pronounced, “Der Alte wrfelt nicht” (the Old Man, that is, God, does not play dice).

The universe has an irreducible, random character. If it is a clockwork, its cogs, springs and levers are not Swiss-made; they do not follow a predetermined path. Physical determinism has been replaced by the determinism of probabilities. Nothing is certain anymore.

But wait—I hear a serious objection. There is no question that the macroscopic world of human experience is built on the microscopic, quantum world. Yet that does not imply that everyday objects such as cars inherit all the weird properties of quantum mechanics. When I park my red Mini convertible, it has zero velocity relative to the pavement. Because it is enormously heavy compared with an electron, the fuzziness associated with its position is, to all intents and purposes, zero.

Cars have comparatively simple internal structures. The brains of bees, beagles and boys, by comparison, are vastly more heterogeneous, and the components out of which they are constructed have a noisy character. Randomness is apparent everywhere in their nervous system, from sensory neurons picking up sights and smells to motor neurons controlling the body’s muscles. We cannot rule out the possibility that quantum indeterminacy likewise leads to behavioral indeterminacy.

Such randomness may play a functional role. If a housefly pursued by a predator makes a sudden, abrupt turn midflight, it is more likely to see the light of another day than its more predictable companion. Thus, evolution might favor circuits that exploit quantum randomness for certain acts or decisions. Both quantum mechanics and deterministic chaos lead to unpredictable outcomes.

Afterthought to Action

Let me return to solid ground and tell you about a classical experiment that convinced many people that free will must be an illusion. This experiment was conceived and carried out in the early 1980s by Benjamin Libet, a neuropsychologist at the University of California, San Francisco.

The brain and the sea have one thing in common—both are ceaselessly in commotion. One way to visualize this is to record the tiny fluctuations in the electrical potential on the outside of the scalp, a few millionths of a volt in size, using an electroencephalograph (EEG). Like the recording of a seismometer, the EEG trace moves feverishly up and down, registering unseen tremors in the cerebral cortex underneath. Whenever the person being tested is about to move a limb, an electrical potential builds up. Called the readiness potential, it precedes the actual onset of movement by one second or longer.

Intuitively, the sequence of events that leads to a voluntary act must be as follows: You decide to raise your hand; your brain communicates that intention to the neurons responsible for planning and executing hand movements; and those neurons relay the appropriate commands to the motor neurons that contract the arm muscles. But Libet was not convinced. Wasn’t it more likely that the mind and the brain acted simultaneously or even that the brain acted before the mind did?

Libet set out to determine the timing of a mental event, a person’s deliberate decision, and to compare that with the timing of a physical event, the onset of the readiness potential after that decision. He projected onto a screen a point of bright light that went around and around, like the tip of the minute hand on a clock. With EEG electrodes on his or her head, each volunteer had to spontaneously, but deliberately, flex a wrist. They did this while noting the position of the light when they became aware of the urge to act.

The results told an unambiguous story, which was bolstered by later experiments. The beginning of the readiness potential precedes the conscious decision to move by at least half a second and often by much longer. The brain acts before the mind decides! This discovery was a complete reversal of the deeply held intuition of mental causation.

The Conscious Experience of Will

Why don’t you repeat this experiment right now: go ahead and flex your wrist. You experience three allied yet distinct feelings associated with the plan to move (intention), your willing of the movement (a feeling called agency or authorship), and the actual movement. If a friend were to take your hand and bend it, you would experience the movement but neither intention nor agency; that is, you would not feel responsible for the wrist movement. This is a neglected idea in the debate about free will—that the mind-body nexus creates a specific, conscious experience of “I willed this” or “I am the author of this action.”

Daniel Wegner, a psychologist at Harvard University, is one of the trailblazers of the modern study of volition. In one experiment, Wegner asked a volunteer to wear gloves and stand in front of a mirror, her arms hanging by her sides. Directly behind her stood a lab member, dressed identically. He extended his arms under her armpits, so that when the woman looked into the mirror, his two gloved hands appeared to be her own. Both participants wore headphones through which Wegner issued instructions, such as “clap your hands” or “snap your left fingers.” The volunteer was supposed to report on the extent to which the actions of the lab member’s hands were her own. When she heard Wegner’s directions prior to the man’s hands carrying them out, she reported an enhanced feeling of having willed the action herself, compared with when Wegner’s instructions came after the man had already moved his hands.

The reality of these feelings of intention has been underscored by neurosurgeons, who must occasionally probe brain tissue with brief pulses of electric current. In the course of such explorations, Itzhak Fried, a surgeon at U.C.L.A., stimulated the presupplementary motor area, which is part of the vast expanse of cerebral cortex lying in front of the primary motor cortex. He found that such stimulation can trigger the urge to move a limb. Michel Desmurget of INSERM and Angela Sirigu of the Institute of Cognitive Science in France discovered something similar when stimulating the posterior parietal cortex, an area responsible for transforming visual information into motor commands. Patients commented, “It felt like I wanted to move my foot. Not sure how to explain,” or “I had a desire to roll my tongue in my mouth.” Their feelings arose from within, without any prompting by the examiner.

Free the Mind

I have taken two lessons from these insights. First, I have adopted a more pragmatic conception of free will. I strive to live as free of constraints as possible. The only exception should be restrictions that I deliberately and consciously impose on myself, chief among them restraints motivated by ethical concerns: do not hurt others and try to leave the planet a better place than you found it. Other considerations include family life, health, financial stability and mindfulness. Second, I try to understand my unconscious motivations, desires and fears better. I reflect deeper about my own actions and emotions than my younger self did.

I am breaking no new ground here—these are lessons wise men from all cultures have taught for millennia. The ancient Greeks had “gnothi seauton” (“know thyself”) inscribed above the entrance to the Temple of Apollo at Delphi. The Jesuits have a nearly 500-year-old spiritual tradition that emphasizes a twice-daily examination of conscience. This constant internal interrogation sharpens your sensitivity to your actions, desires and motivations. This will enable you not only to understand yourself better but also to live a life more in harmony with your character and your long-term goals.

This article was published in print as "Finding Free Will."

Adapted from Consciousness: Confessions of a Romantic Reductionist, by Christof Koch, Massachusetts Institute of Technology, 2012. All rights reserved.