Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe

by Brian Greene

Even the staunch reductionist realizes that as fatuous as it would be to explain a baseball’s trajectory in terms of molecular motion, it would only be more so to invoke such a microscopic perspective in explaining what a batter was feeling as the pitcher went through his windup, the crowd roared, and the fastball approached. Instead, higher-level stories told in the language of human reflection provide far greater insight. Nevertheless—and this is key—these better-suited human-level stories must be compatible with the reductionist account. We are physical creatures subject to physical law. And so there’s little to be gained by physicists clamoring that theirs is the most fundamental explanatory framework or from humanists scoffing at the hubris of unbridled reductionism. A refined understanding is gleaned by integrating each discipline’s story into a finely textured narrative.8

Although we won’t answer the question of life’s origin (still a mystery), we will see that all life on earth can be traced to a common single-celled ancestral species, sharply delineating what a science of life’s origin will ultimately need to explain. This will lead us to examine life from the broadly applicable thermodynamic perspective developed in previous chapters, making clear that living things share a deep kinship not just with one another but with stars and steam engines too: life is one more means the universe employs to release the entropy potential locked within matter.

Grind up anything previously alive, pry apart its complex molecular machinery, and you’ll find an abundance of the same six types of atoms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, a collection of elements students sometimes remember with the acronym SPONCH (not to be confused with the Mexican marshmallow cookie of the same name).

building up complex atoms is an intricate process that requires time. It requires a highly specific series of steps in which prescribed numbers of protons and neutrons are melded together into various lumps, which then need to fortuitously encounter particular complementary lumps, fuse with them too, and so on. Like a gourmet’s recipe, the order in which the ingredients are combined is essential. And what makes the process particularly tricky is that some intermediate lumps are unstable, meaning that after they form they tend to disintegrate quickly, disrupting the culinary preparations and slowing atomic synthesis. This hindrance is a big deal because the steadily falling temperature and density as the early universe rapidly expands implies that the window of opportunity for fusion quickly closes. By roughly ten minutes after creation, the temperature and density drop below the threshold required for nuclear processes.10

The distribution of electrical charge across a water molecule might seem like an esoteric detail. But it’s not. It proves essential to the emergence of life. Because of water’s skewed charge distribution, it can dissolve nearly everything. The negatively charged oxygen vertex grabs hold of anything with even a slight positive charge; the positively charged hydrogen tips grab hold of anything with even a slight negative charge. In tandem, the two ends of a water molecule act like charged claws that pull apart most anything that’s submerged for a sufficient time. Table salt is the most familiar example. Composed of an atom of sodium bonded to an atom of chlorine, a molecule of table salt has a slight positive charge near the sodium (which donates an electron to the chlorine) and a slight negative charge near the chlorine (which accepts an electron from the sodium). Drop salt into water, and the oxygen side of H2O (negatively charged) grabs hold of the sodium (positively charged), while the hydrogen side of H2O (positively charged) grabs hold of the chlorine (negatively charged), ripping salt molecules apart and dissolving them into solution. And what’s true for salt is true for a great many other substances too. The details vary, but water’s asymmetric charge arrangement makes it an uncanny solvent. Wash your hands, even without soap, and water’s electrical polarity will be hard at work, dissolving foreign matter and carrying it away.

Nobel laureate Albert Szent-Györgyi summarized it eloquently: “Water is life’s matter and matrix, mother and medium. There is no life without water. Life could leave the ocean when it learned to grow a skin, a bag in which to take the water with it. We are still living in water, having the water now inside.”21 As poetry, this is a graceful ode to water and life. As science, there is as yet no argument to establish the statement’s universal validity, but we know of no form of life that challenges the necessity of water.

While it might seem natural to begin with the genesis of life, that topic, still unsettled, is better approached after exploring the quintessential molecular qualities of life itself. And for someone like me, having spent the past thirty years pursuing a unified theory of nature’s fundamental forces, such an exploration reveals a stunning biological unity. We don’t know the exact number of distinct species on earth, microbes to manatees, but studies have provided estimates ranging from a low in the millions to a high in the trillions. Whatever the exact number, it’s huge. The wealth of different species, however, belies the singular nature of life’s inner workings.

Regardless of their source, cells share so many features that the untrained eye examining individual specimens would be hard-pressed to distinguish mouse from mastiff, turtle from tarantula, housefly from human. That’s remarkable. Surely our cells must show an obvious and significant distinguishing imprint. Yet they don’t. The reason, established during the past few decades, is that all complex multicellular life descended from the same single-celled ancestral species. Cells are similar because their lineages radiate from the same starting point.22

Tracing the lineage of the sea mollusk all the way back might have revealed one starting point, while doing the same for wombats or orchids might have revealed others. But the evidence strongly suggests that in seeking life’s origin, the lineages converge to a common ancestor.

The motion of the rabbit arises from the internal processing and transmission of a complex set of instructions that flows through its physical structure: biological software driving biological hardware. Such processes are wholly absent for a rock.

Proteins are built from combinations of twenty smaller subunits, amino acids, similar to the way English words arise from various combinations of twenty-six letters. And much as sensible words require letters to be arranged in specific orders, usable proteins require amino acids to be linked in specific sequences. If such assembly were left to blind chance, the likelihood that the requisite amino acids would happen to bump into one another in just the right way to build a particular protein would be next to nothing. The sheer number of ways that twenty distinct amino acids can be linked in a long chain makes this evident: for a chain with one hundred and fifty amino acids (a small protein), there are about 10195 different arrangements, far larger than the number of particles in the observable universe. Much as the proverbial team of monkeys typing random letters for decades will fail to spell out more than “To be or not to be,” random chance will fail to create the specific proteins required for life.

For humans, the DNA sequence runs about three billion letters long, with your sequence differing from that of Albert Einstein or Marie Curie or William Shakespeare or anyone else by less than about a quarter of a percent, roughly one letter out of every string of five hundred.23 But while basking in the glow of possessing a genome so similar to that of any of history’s most revered luminaries (or infamous villains), note that your DNA sequence also has a 99 percent overlap with any given chimpanzee’s.24 Minor genetic differences can have major impact.

All life codes the instructions for building proteins in the same way.25

I’ve laid out the details for two reasons. First, seeing the code makes the concept of cellular software explicit. Given a segment of DNA, we can read off the instructions which direct the cell’s inner workings, a sophisticated coordination wholly absent in inanimate matter. Second, seeing the code demonstrates what biologists mean when they call it universal. Every molecule of DNA, whether from seaweed or Sophocles, encodes the information needed to build proteins in the same way. That is the unity of life’s information.

All life meets the challenge of energy extraction and distribution in the same way.28

The chemical burning central to life’s processing of energy is called a redox reaction. Not the most inviting name, but the archetypal example—a burning log—clarifies the nomenclature. As a log burns, carbon and hydrogen in the wood relinquish electrons to oxygen in the air (remember, oxygen yearns for electrons), bonding them into molecules of water and carbon dioxide, and releasing energy in the process (the very reason fire is hot). When oxygen grabs electrons, we say that it has been reduced (you can think of this as a reduction in oxygen’s yearning for electrons). When carbon or hydrogen relinquishes electrons to oxygen, we say that it has been oxidized. Together, we have a reduction-oxidation reaction, or redox for short. Scientists now use the term “redox” more broadly, referring to a collection of reactions in which electrons are passed between chemical constituents, regardless of whether oxygen is involved.

In living cells—let’s focus on animals to be definite—similar redox reactions take place but, importantly, the electrons stripped from atoms that you ingested at breakfast are not transferred directly to oxygen. If they were, the energy released would create something akin to a cellular fire, an outcome life has learned the benefit of avoiding. Instead, electrons donated by food pass through a series of intermediate redox reactions, rest stops on a trek that ultimately ends with oxygen but that allows smaller amounts of energy to be released at each step. Like a ball in the bleachers cascading down a stadium’s steps, electrons jump from one molecular receptor to another, with each receptor more electron crazed than the previous, ensuring that each jump results in the release of energy. Oxygen, the most electron-crazed receptor of all, waits for the electron at the bottom of the stairs, and when it finally arrives, the oxygen hugs the electron tight, squeezing out the marginal energy it can still provide, thus concluding the energy extraction process. The process for plants is largely the same. The main difference is the source of the electrons. For animals, they come from food. For plants, they come from water. Sunlight striking chlorophyll in the green leaves of plants strips electrons from water molecules, pumps up their energy, and sets them off on a similar energy-extracting redox cascade. And so the energy supporting all the actions of all living things can be traced ...

of all the innumerable processes, life on planet earth leverages one and only one energy mechanism: a specific sequence of electromagnetic chemical reactions in which electrons engage in a downward-directed sequence of jumps, starting with food or water and ending with the clutching embrace of oxygen. How and why did this energy extraction process become life’s go-to mechanism? No one knows. But the universality, like that of the genetic code, speaks again, and strongly so, to the unity of life. Why do all living things power themselves in the same way? The immediate answer is that all life must have descended from a common ancestor, a single-celled species that researchers believe likely existed around four billion years ago.

In an ordinary battery, chemical reactions force electrons to accumulate on one side of the battery (the anode), where the mutual repulsion of these like-charged particles means they’re primed to flee at the first opportunity. When you complete an electrical circuit by pushing an “on” button or flipping a switch, you free the pent-up electrons, allowing them to flow out of the anode, pass through a device—bulb, laptop, phone—and finally return to the battery’s other side (the cathode). Commonplace though batteries are, they are utterly ingenious. They store energy in a crowded collection of electrons standing at the ready to relinquish that energy on a moment’s notice to power devices of our choosing.

When cellular redox reactions pack protons closely together, they too stand at the ready waiting for the chance to rush away from their enforced companions. Cellular redox reactions thus charge up biological proton-based batteries. In fact, because the protons are all clustered on one side of an extremely thin membrane (just a few dozen atoms wide), the electric field (the membrane voltage divided by the membrane thickness) can be enormous, upwards of tens of millions of volts per meter. A cellular bio battery is no slouch.

In centuries past, such wind-powered turning motion was used to crush wheat or other grains into flour. The cellular windmills undertake an analogous grinding project but instead of pulverizing structure the process builds it. As they turn, the molecular turbines repeatedly cram together two particular input molecules (ADP, adenosine diphosphate plus a phosphate group), synthesizing one particular output molecule (ATP, adenosine triphosphate). Forced together by the turbine, the constituents of each resulting ATP molecule are in a tense arrangement: mutually repelling charged constituents are clasped together by chemical bonds, and so, much like a compressed spring, they strain to be released. That’s extraordinarily useful. Molecules of ATP can travel throughout a cell, releasing that stored energy when needed by snapping the chemical bonds and allowing the constituent particles to relax into a lower energy, more comfortable state. It is that very energy, released by the dissociation of ATP molecules, that powers cellular functions.

Your body contains tens of trillions of cells, which means that every second you consume on the order of one hundred million trillion (1020) ATP molecules. Each time an ATP is used, it splits up into the raw materials (ADP and a phosphate), which the proton battery-powered turbines then cram back together into freshly minted, fully rejuvenated ATP molecules. These ATP molecules then hit the road again, delivering energy throughout the cell. To meet your body’s energy demands, your cellular turbines are thus astoundingly productive. Even if you’re an extremely fast reader, as you scan through this very sentence your body is synthesizing some five hundred million trillion molecules of ATP. And just now, another three hundred million trillion more.

Much as Einstein sought a unified theory of nature’s forces, and much as physicists today dream of an even grander synthesis embracing all matter and perhaps space and time too, there is something thoroughly seductive in identifying a common core within a vast range of seemingly distinct phenomena. That the deep inner workings of all life—from my two dogs resting quietly on the carpet, to the chaotic swirl of insects attracted by the lamp near my window, to the chorus of frogs rising up from the nearby pond, to the coyotes I now hear howling in the distance—rely on the same molecular processes, well, it is spectacular. So set aside the details, take a break before concluding the chapter, and allow that wondrous realization to sink in fully.

Simple and intuitive, Darwinian evolution almost seems self-evident. Yet however compelling its explanatory framework, were Darwinian evolution not supported by data it would have failed to achieve scientific consensus. Logic is not enough. Confidence in Darwinian evolution rests on the overwhelming support it has received from scientists who have traced gradual changes in the structure of organisms and delineated the adaptive advantages many of the changes conferred. If such transformations were absent, or if they occurred without any evident pattern, or if they bore no relation to the bearer’s capacity to survive or reproduce, schoolkids would not be learning Darwinian evolution.

molecular Darwinism. It shows how groups of jostling particles guided solely by the laws of physics can become ever more adept at reproduction—something we ordinarily associate with life. When we seek life’s origin, this suggests that molecular Darwinism may have been an essential mechanism during the era leading up to the emergence of the first life.

RNA is thus both software and hardware. It can direct as well as catalyze chemical reactions. And among such reactions are some that promote the replication of RNA itself. While the molecular machinery that makes copies of DNA uses an elaborate collection of chemical cogs and wheels, RNA itself can promote the synthesis of the base pairs necessary for its own replication. Consider the implication. Molecules of RNA, blending software and hardware, have the potential to sidestep the chicken and egg conundrum: How do you assemble molecular hardware without first having the molecular software, the instructions to carry out the assembling? How do you synthesize molecular software without first having the molecular hardware, the infrastructure to carry out the synthesizing? Embodying both functions, RNA melds chicken and egg, and thus has the capacity to propel an era of molecular Darwinism forward. Such is the RNA World proposal. It imagines that before there was life there was a world suffused with RNA molecules, which through molecular Darwinism evolved over an almost unfathomable number of generations into the chemical structures that constituted the first cells.

Imagine, then, that as RNA molecules continued to replicate, a chance mutation facilitated something novel: the mutant RNA coaxed some of the amino acids in the environmental stew to hook up into chains yielding the first rudimentary proteins (a crude version of the kinds of processes that now take place in ribosomes). If, by chance, some of these basic proteins happened to increase the efficiency of RNA replication—after all, catalyzing reactions is, in part, what proteins do—they would be richly rewarded: the proteins would usher the mutant form of RNA to dominance, and the newly plentiful supply of mutant RNA would help synthesize more of the proteins. In tandem, they would constitute a self-reinforcing chemical loop that would propel the chance molecular aberrations to become the norm. Over time, the continued molecular machinations might hit upon another chemical novelty, a double-railed ladder—a rudimentary form of DNA—that proved to be a more stable and more efficient structure for molecular replication, and thus gradually usurped the replication processes and relegated RNA to a supporting role. The chance formation of molecular bags—cell walls—would increase fitness further by concentrating chemicals in sequestered regions and offering protection from environmental disruption. Spreading throughout the chemical population, the structures necessary for the first rudimentary cells would assemble.32 Life would be born.

To date, laboratory attempts to recreate these processes are intriguing but inconclusive. We have yet to create life from scratch. I have little doubt that one day, perhaps not far off, we will. In the meantime, an overarching scientific narrative for life’s origin is emerging. Once molecules acquire the capacity to replicate, chance errors and mutations will feed molecular Darwinism, driving chemical concoctions along the all-important vector of increased fitness. Playing out over hundreds of millions of years, the process has the capacity to build the chemical architecture of life.

Of course, the molecules don’t know anything. Their behavior is governed by the blind, mindless, unschooled laws of physics. But the question remains: How do they consistently and reliably carry out a stunningly intricate series of complex chemical processes? It’s a question that harks back to my paraphrasing of Schrödinger’s primary query in What Is Life?: The jostling and careening of molecules within a rock are governed by the laws of physics. The jostling and careening of molecules within a rabbit are also governed by the laws of physics. How do they differ? We have now seen that the rabbit’s particles are guided by an additional influence—the rabbit’s internal archive of information, its cellular software. Importantly, critically, vitally: This information does not supersede the laws of physics. Nothing does. Instead, much as a water slide doesn’t supersede the laws of gravity but through its shape guides riders along a specific trajectory they would otherwise not follow, the rabbit’s cellular software is carried by chemical arrangements that through their shape, structure, and constituents guide various molecules along trajectories that they, too, would otherwise not follow.

information in a cell is not abstract. It is not a free-floating set of instructions that molecules need to study, memorize, and execute. Instead, the information is encoded in the molecular arrangements themselves, arrangements that coax other molecules to bump or join or interact in a manner that carries out cellular processes like growth, repair, or reproduction. Even though the molecules inhabiting a cell lack intent or purpose, and even though they are thoroughly oblivious, their physical structure allows them to accomplish highly specialized tasks.

When you use the laws of physics to describe the bumping and jostling of the rock’s molecules, you’re done. But when you use the very same laws of physics to describe the bumping and jostling of rabbit molecules, you are not done. Not by a long shot. Overlaid on the reductionist story is a whole additional story that tells of the rabbit’s unique internal molecular arrangements that choreograph an exquisite spectrum of organized molecular motions. And it is these molecular motions which carry out higher-level processes within the rabbit’s cells.

I like to share the credit more equitably, lauding gravity for causing matter to clump and securing stable stellar environments, but also extolling nuclear fusion for the relentless production of a steady stream of high-quality photons over millions and billions of years. The nuclear force, in tandem with gravity, is a fount of life-giving low-entropy fuel.

In his 1943 lectures, Schrödinger emphasized that the torrent of scientific developments had been so intense that “it has become next to impossible for a single mind fully to command more than a small specialized portion.”39 Consequently, he encouraged thinkers to extend the reach of their expertise by exploring realms outside their traditional intellectual stomping ground. With What Is Life? he unabashedly brought the training, intuition, and sensibility of a physicist to bear on the puzzles of biology.

England calls the process dissipative adaptation. Potentially, it provides a universal mechanism for coaxing certain molecular systems to get up and dance the entropic two-step. And as that’s what living things do for a living—they take in high-quality energy, use it, and then return low-quality energy in the form of heat and other wastes—perhaps dissipative adaptation was essential to the origin of life.42 England notes that replication itself is a potent tool of dissipative adaptation: if a small collection of particles has become adept at absorbing, using, and dispensing energy, then two such collections are better still, as are four or eight, and so on. Molecules that can replicate might then be an expected output of dissipative adaptation. And once replicating molecules appear on the scene, molecular Darwinism can kick in, and the drive to life begins.

Somewhere between the first prokaryotic cells four billion years ago and the human brain’s ninety billion neurons entangled in a network of one hundred trillion synaptic connections, the ability emerged to think and feel, to love and hate, to fear and yearn, to sacrifice and revere, to imagine and create—newfound capacities that would ignite spectacular achievement as well as untold destruction. “Everything begins with consciousness and nothing is worth anything except through it,”1 is how Albert Camus put it.

It is an ironic stance. Descartes’s “Cogito, ergo sum” summarizes our contact with reality. All else could be an illusion, but thinking is the one thing even the die-hard skeptic can be sure of. And notwithstanding Ambrose Bierce’s “I think that I think, therefore I think that I am,”2 if you are thinking, the case for existing is strong. For science to pay no mind to consciousness would be to turn from the very thing, the only thing, we each can count on. Indeed, for thousands of years many have denied the finality of death by hanging existential hope on consciousness. The body dies. That’s apparent, obvious, undeniable. But our seemingly persistent inner voice, as well as the abundant thoughts, sensations, and emotions filling each of our subjective worlds, speaks to an ethereal presence that, some have imagined, stands outside the base facts of physical existence. Atman, anima, immortal soul—it has been given many names, but all connote the belief that the conscious self taps into something that outlasts the physical form, something that transcends traditional mechanistic science. Not only is mind our tether to reality, perhaps it is our tether to eternity.

We have yet to articulate a robust scientific explanation of conscious experience. We lack a conclusive account of how consciousness manifests a private world of sights and sounds and sensations. We cannot yet respond, or at least not with full force, to assertions that consciousness stands outside conventional science. The gap is unlikely to be filled anytime soon. Most everyone who has thought about thinking realizes that cracking consciousness, explaining our inner worlds in purely scientific terms, poses one of our most formidable challenges.

The art of science, of which Newton was the master, lies in making judicious simplifications that render problems tractable while retaining enough of their essence to ensure that the conclusions drawn are relevant. The challenge is that simplifications effective for one class of problems can be less so for others. Model the planets as solid balls and you can work out their trajectories with ease and precision. Model your head as a solid ball and the insights into the nature of mind will be less enlightening. But to jettison unproductive approximations and lay bare the inner workings of a system containing as many particles as the brain—a laudable goal—would require mastering a level of complexity fantastically beyond the reach of today’s most sophisticated mathematical and computational methods. What’s changed in recent years is newfound access to observable and measurable features of brain activity that, at the very least, access processes that reliably accompany conscious experience. When researchers can use functional magnetic resonance imaging to meticulously track blood flow supporting neural activity, or insert deep brain probes to detect electrical impulses firing along individual neurons, or use electroencephalograms to monitor electromagnetic waves rippling across the brain, and when the data reveal clear patterns that mirror both observed behavior and reports of inner experience, the case for approaching consciousness as a physical phenomenon strengthens substant...

Some years ago, during a good-natured but heated exchange on the role of mathematics in describing the universe, I emphatically told a late-night television host that he was nothing but a bag of particles governed by the laws of physics. Not as a joke, although without missing a beat he turned it into one. (“Hey, that’s a great pickup line.”) And not as a jibe, for in this regard, whatever holds true for him applies equally to me. Instead, the remark sprang from my deep-seated reductionist commitment, which holds the view that by fully grasping the behavior of the universe’s fundamental ingredients we tell a rigorous and self-contained story of reality.

a twenty-five-hundred-year-old sentiment of Democritus, “Sweet is sweet, bitter is bitter, hot is hot, cold is cold, color is color; but in truth there are only atoms and the void.”3

To embrace thoughts, emotions, and memories is to embrace the core of human experience. It is also a story that requires a perspective qualitatively different from any we have taken so far. Entropy, evolution, and life can all be studied “out there.” We can fully tell their stories as third-person accounts. We are witnesses to these stories and, if we are sufficiently diligent, our account can be exhaustive. These stories are inscribed in open books. A story that encompasses consciousness is different. A story that penetrates into the inner sensations of sight or sound, of elation or grief, of comfort or pain, of ease or anxiety, is a story that relies on a first-person account. It is a story informed by an inner voice of awareness speaking from a personal script each one of us seemingly authors. Not only do I experience a subjective world, but I have a palpable sense that from within that world I control my actions. No doubt, when it comes to your actions you have a similar sense. Laws of physics be damned; I think, therefore I control. Understanding the universe at the level of consciousness requires a story that can grapple with an utterly personal and seemingly autonomous subjective reality.

I’m all for intellectual revolutions. There is nothing more exciting than a discovery that turns the accepted worldview on its head. And in what follows, we will discuss upheavals that some consciousness researchers envision to be heading our way. But for reasons that will become clear, I suspect that consciousness is less mysterious than it feels. Resonating with my late-night TV exclamation and, more importantly, with a segment of researchers who’ve devoted their professional lives to these questions, I anticipate that we will one day explain consciousness with nothing more than a conventional understanding of the particles constituting matter and the physical laws that govern them. That would yield its own variety of revolution, establishing a virtually unlimited hegemony for physical law, reaching arbitrarily far into the outer world of objective reality and arbitrarily deep into the inner world of subjective experience.

That the brain is awash with influential processes escaping introspection is a premise with a long history, one that has been expressed in myriad forms. Vedic texts written three thousand years ago invoke a notion of the unconscious, and references continue across the centuries as penetrating thinkers have surmised flavors of mental qualities unavailable to the palate of conscious awareness: Saint Augustine (“The mind is not large enough to contain itself: but where can that part of it be which it does not contain?”5), Thomas Aquinas (“The mind does not see itself through its essence”6), William Shakespeare (“Go to your bosom, / Knock there, and ask your heart what it doth know”7), Gottfried Leibniz (“Music is the hidden arithmetical exercise of a mind unconscious that it is calculating”8). Intriguing too are processes that seem to reside below the radar and yet generate echoes accessible to conscious processing. Stories abound, for example, of the unconscious mind solving problems and delivering the solutions unbidden. One of the most colorful comes from German pharmacologist Otto Loewi, who during the night before Easter Sunday 1921 briefly awoke and scribbled down an idea that had just come to him in a dream. In the morning, Loewi had an overwhelming sense that the nocturnal note contained a vital insight, but however hard he tried he was unable to decipher it. The next night he had the same dream, but this time he immediately went to the lab and followed the dream’s di...

your brain surreptitiously coordinates a regulatory, a functional, and a data-mining marvel. Wondrous though these brain activities are, they do not constitute a conceptual mystery. The brain rapidly sends and receives signals along nerve fibers, allowing it to control biological processes and generate behavioral responses. To delineate the precise neural pathways and physiological details underlying such functions and behaviors, scientists face the daunting task of mapping out vast territories dense with complex biological circuitry at a level of precision well beyond what has so far been achieved. Still, everything we’re learning suggests that however challenging, however vast the reserves of creativity and diligence required, there is every reason to believe that the familiar strategies of science will prevail. And were it not for one pesky quality of mind, that would be that. But when we look beyond the mind’s tasks and consider instead the mind’s sensations—the inner experience we identify as the essence of being human—some researchers have reached a different and far less optimistic prognosis for the capacity of traditional science to provide insight. This takes us to what some call the “hard problem” of consciousness.

Particles can have mass, electric charge, and a handful of other similar features (nuclear charges, which are more exotic versions of electric charge), but all these qualities seem completely disconnected from anything remotely like subjective experience. How then does a whirl of particles inside a head—which is all that a brain is—create impressions, sensations, and feelings? Philosopher Thomas Nagel gave an iconic and particularly evocative account of the explanatory gap.18 What’s it like, he asked, to be a bat? Picture it: Aloft on a bed of air as you soar across a dark landscape, you cry out with an incessant patter of clicks, generating echoes from trees, rocks, and insects, which allow you to map the environment. From the reflected sound you realize a mosquito is up ahead and darting to the right, so you swoop in and enjoy a tiny morsel. Since our mode of engagement with the world is profoundly different, there is just so far our imagination can take us into the bat’s inner world. Even if we had a complete accounting of all the underlying fundamental physics, chemistry, and biology that make a bat a bat, our description would still seem unable to get at the bat’s subjective “first-person” experience. However detailed our material understanding, the inner world of the bat seems beyond reach.

In 1994 David Chalmers, a young Australian philosopher, hair flowing past his shoulders, took the stage at the annual consciousness conference in Tucson and described this deficit as the “hard problem” of consciousness. Not that the “easy” problem—understanding the mechanics of brain processes and their role in imprinting memories, responding to stimuli, and molding behavior—is easy. It’s just that we can envision what the shape of a solution to those sorts of problems would look like; we can articulate an in-principle approach at the level of particles or more complex structures like cells and nerves, which seems coherent. The challenge to envision such a solution for consciousness motivated Chalmers’s assessment. He argued that not only are we lacking a bridge from mindless particles to mindful experience, if we try to build one using a reductionist blueprint—making use of the particles and laws that constitute the fundamental basis of science as we know it—we will fail.

It is easy to be flippant about the hard problem. In the past, my own response may have seemed so. When asked, I would often say that conscious experience is merely what it feels like when a certain kind of information processing takes place in the brain. But because the core issue is to explain how there can be a “what it feels like” at all, the response too quickly dismisses the hard problem as not being hard and not even being a problem. More charitably, it is a response that sides with a widely held view that thinks too much is made of thought. While some hard-problem aficionados argue that to understand consciousness we will need to introduce concepts from outside conventional science, others—so-called physicalists—anticipate that cleverly construed and creatively applied, traditional scientific methods, solely invoking physical properties of matter, will be up to the task. The physicalist perspective does indeed summarize my own long-held view.

Imagine that in the far future there is a brilliant girl, Mary, who is profoundly color-blind. Since birth, everything in her world has appeared solely in black and white. Her condition baffles the most renowned doctors, and so Mary decides that it will be up to her to figure it out. Driven by the dream of curing her deficit, Mary undertakes years of intensive study, observation, and experiment. And through it all, Mary becomes the greatest neuroscientist the world has ever known, reaching a goal that has long eluded humankind: she fully unravels every last detail about the structure, function, physiology, chemistry, biology, and physics of the brain. She masters absolutely everything there is to know about the brain’s workings, both its global organization and its microphysical processes. She understands all the neural firings and particle cascades that happen when we marvel at a rich blue sky, enjoy a succulent plum, or lose ourselves in Brahms’s Third Symphony. With this achievement Mary is able to identify the cure for her visual impairment, and she undergoes the surgical procedure to correct it. Months later the doctors are ready to remove the bandages, and Mary prepares to take in the world anew. Standing in front of a bouquet of red roses, Mary slowly opens her eyes. Here’s the question: From this first experience of the color red, will Mary learn anything new? By finally having the inner experience of color, will she acquire new understanding? Playing this story ou...

Strategies for explaining consciousness fan out across an impressive terrain of ideas. At the extremes are positions that either dismiss consciousness as an illusion (eliminativism) or declare that consciousness is the only quality of the world that is real (idealism). In between, we encounter a spectrum of proposals. Some operate within the confines of traditional scientific thought, others slip between the cracks of current scientific understanding, and others still augment the qualities we have long held to define reality at its most fundamental level.

Had you overheard discussions in biological circles during the eighteenth and nineteenth centuries, you would be familiar with vitalism. It was a concept addressing what one might have called the “hard problem” of life: Since the world’s fundamental ingredients are inanimate, how can collections of such ingredients possibly be alive?