In the spectacular space of a single human brain, endless memories flow upward. Whether you’re recalling your first birthday party or pondering some of society’s most complex problems (such as those rendered in this book), your brain functions like a repository of worlds. Psychiatrist Veronica O’Keane details the evolution of memory as it proceeds from the brain—from the cortex, to the hippocampus, and into the cranial lining of human consciousness.

When Copernicus and Galileo discovered that their beloved planet Earth played second-fiddle to the Sun, society was shaken. The following Scientific Revolution of the 16th and 17th centuries moved some of history’s deepest thinkers to pursue similarly unprecedented scientific questions and formulate equally unusual answers in response. One of the questions many of them asked concerned themselves: Was the content of their minds given or produced, or as Veronica O’Keane writes, was it the product of “innate Knowledge” or “learned through the material senses”? Though this dilemma drew disagreement from various philosophers at the time, its answers impacted the world.

One of the first thinkers to stir these ideological waters was René Descartes, a philosopher who cut humanity in half with his concept of “dualism.” According to the philosophy of Descartes, the body and the soul are separate entities within which the physical is simply a bothersome byproduct rather than a component of one’s truest nature. Just as the thinkers before him did to the Earth, Descartes dethroned the physical, disregarding all of its insights as second order concerns. On the opposite end of this thought spectrum, the Sensationalist philosophers regarded humanity’s relationship with the world beyond the mind as integral to life. Contrary to Descartes, these aptly named Sensationalists advocated for the centrality of what the author terms “sensation”—the human capacity to interact with reality through one’s body—the tool that gives humans something to think about, ponder, and most importantly, remember in the first place.

Though the ideas of the Sensationalists arrived much earlier than the 20th century’s embrace of psychology and the 21st century’s profusion of neuroscience, the so-called “Decade of the Brain,” their findings presage the most crucial aspect of the brain’s capacity for memory. The brain’s exposure to physical reality initiates its ability to gather, organize, and remember. Renowned neuroscientists like Oliver Sacks are beginning to examine the truth of this capacity with experimentation. In his article “To See and Not See,” Sacks discusses a man who, after nearly a lifetime of blindness, grows able to see. The world wasn’t immediately familiar to him, though; rather, he had to accustom himself to it as if it was the site of some alien planet, rather than a home he’d inhabited his entire life.

Studies like this prove that to get a foothold in the world, the brain must first ingest it, taking it in, churning it up, and hopefully turning it into something that resembles its environment. Perhaps Descartes thought physical life was secondary to his inner self, but without its influence, his philosophy would be entirely unthinkable.

Like the lining of a quilt, the human brain is made up of separate components called cortices. Just as sewing needles prick and pull the squares of that quilt together, sensory neurons from the physical world arrive at these various cortices to enter into the brain. Consider your coffee, for instance. While the smell of that beverage is facilitated in your olfactory cortex, the flavor is processed through your gustatory cortex—each section corresponds to specific sensory neurons. If you wanted to actually remember that particular coffee, though (or yourself as you sipped it), a different mechanism of the brain is required. After neurons take a roadtrip through the soft dips of the brain’s cortex, they journey toward the right and left hippocampus, wherein they bundle up into what O’Keane calls “memory codes.” These mechanisms and the hippocampus as a whole help to clarify an individual’s view of reality, strengthening her sense of individuality within it, and making the oftentimes disorienting nature of daily life relatively comprehensible.

Take it from the 20th century neurological wonder Henry Molaison—you don’t want to be without your hippocampus. After a fateful bicycle accident, young Molaison began experiencing epileptic seizures. Evidently, his hippocampus received damage during the incident, prompting all of his neurological functions to encounter an inevitable decline. With little else to do, Molaison’s doctors decided to simply take his hippocampus out in a now dated procedure called a “bilateral hippocampectomy.” Though his seizures thankfully subsided, the state of his memory wasn’t as fortunate. Molaison may have acted like a fully grown, properly adjusted man after his operation, but his perception of the world devolved into that of an infant. Everything swarmed in its foreignness—plants, buildings, faces—nothing was ever the same to him; with the loss of his hippocampal glue, Molaison’s understanding of the world and himself dissolved. His experience proves that what takes place within that seemingly small, oddly shaped space of the hippocampus is disproportionately crucial to the brain’s stability as a whole.

The psychologist Donald Hebb is one of the biggest names in the study of human memory, and his 1949 work The Organization of Behaviour explains what Molaison lost with the extraction of his hippocampus. His “Hebbian model” of memory proposes that inside the hippocampus, particular neurons of the brain’s 68 billion join forces, holding hands with one another via outstretched dendrites. This process of “arborizing” leads to what O’Keane calls the creation of a “cell assembly,” and when one of its members is called into action, the rest come tumbling along too, giving birth to one’s experience of recall. Just as you might spend days rehearsing a speech ad nauseam, as these pockets of neurons are activated more often through what neuroscientists call “consolidation,” their kinship becomes even more powerful.

Sadly stripped of his hippocampus, Molaison lost his ability to conceive of reality in a regular way. Interestingly, though, Molaison didn’t simply crumple to the floor as he awoke from his bed, unable to recall his ability to stand and walk. When the neuropsychologist Brenda Milner investigated the cause of what seemed to be a contradiction, she stumbled upon yet another facet of human recollection—one that stretches its neurons even beyond that crucial hippocampus.

The next stop on memory’s trek through the brain occurs right back in the cortex from which those sensory neurons came—if that memory is significant enough, that is. Various studies in the field of neuroscience, such as the 2013 paper from Alison Preston and Howard Eichenbaum, are investigating the mutual interaction between the brain’s hippocampus and its cortex. According to the findings of Preston and Howard among several others, the portion of the brain called the prefrontal cortex houses a person’s most prized mental memorabilia, what researchers term “biographical memory”—the bits of one’s past that engrave themselves onto the brain over time and the reason why Molaison could remember how to walk even despite the partially empty interior of his mind.

The award-winning novelist and author of Ways of Seeing, John Berger presents a fascinating study of the potency of the cortex in preserving bits of long gone memory. After wading through a murky reality from behind the lens of cataracts, Berger underwent surgery to see anew. With characteristic eloquence, Berger called the instance in which he began to witness the world again a “visual renaissance,” in which O’Keane writes, “a white sheet of paper brought him back to his mother’s kitchen.” The long untread corridors of his cortex which established the basis for his sight roared to life. Unlike the naturally blind man of Sacks’s study, Berger already had a foundation upon which he could construct the world, even after a long optical absence. At first, it just felt stirringly similar to the life of his younger self.

Moreover, according to the self-analysis of the 19th century psychologist Hermann Ebbinghaus and his work Über das Gedächtnis, or On Memory, getting a decent night’s rest is pivotal to the facilitation of lasting thoughts. Consider those days on which you went to sleep too late, or awoke unreasonably early. Perhaps the workday was a bit more challenging than usual, or maybe you forgot to feed your beloved pet fish in the morning. The culprit may not be you or even your brain per se, but simply a lack of adequate sleep. Many neuroscientists have proven that something incredibly special occurs when the brain is submerged in its much-needed REM state. Like kids playing a game of telephone, the hippocampus and the cortex partake in a fascinating correspondence as a person slumbers. One research group witnessed this exchange in the brains of mice whose hippocampus funneled information to their cortex for much safer keeping as they napped.

Often, the hippocampus operates like a holding pit, a warehouse for many things you may or may not remember with time. On the other end of memory’s meander through the brain, the cortex provides a more permanent structure in which those thoughts might set up shop. Taking a short siesta might not be a midday splurge then, but a healthful, replenishing remedy for your brain’s ability to remember.

Perhaps you can’t remember the last time you enjoyed the taste of a madeleine, but you’re probably familiar with the writer and artist of human nostalgia Marcel Proust. His gargantuan work À la recherche du temps perdu, or In Search of Lost Time, provides a strikingly poetic account of what is now called the “Proustian effect.” These instances of what O’Keane terms “emotional memories,” occur when a moment bursts into one’s mind so powerfully that the distant past grows nearly palpable. Within the brain’s cortex, there’s yet another mechanism that handles these literally touchy-feely aspects of human memory—the insula. According to O’Keane, this function allows people to comprehend what neuroscientists call “interoceptive sensation,” or all of those things that stir beneath the skin. And, as the insula is prodded by the physical world, it can make a long gone family Christmas, a night spent with a friend, or even one’s love for someone, internally manifest.

As the 19th century psychologist William James wrote, when one feels something, what they experience is “not a primary feeling, directly aroused by the existing object or thought, but a secondary feeling indirectly aroused.” Interestingly, the insula collects bits of information from the autonomic nervous system (ANS), which facilitates one’s bodily functions, to then assimilate those into actual states of mind. A set of researchers in California, for instance, found that the presence of the insula was essential to people’s ability to process feelings, and those of bodily discomfort in particular. Their group investigated people with improperly working insulas, as many encounter with Alzheimer’s disease, and discovered that without this mechanism fully operative, they were less inclined to register what took place within them.

Beyond this, the insula houses what neuroscientist Antonio Damasio discovered were “emotion-specific neural patterns,” with different parts of the mechanism facilitating various sensations. According to the work of Damasio and other neuroscientists, Proust’s sweet and wistful moment with that French cookie probably received its meaning within the space of his left insula. Moreover, the insula is able to dredge up that wondrous moment by way of its interaction with the memory yearbook placed within the prefrontal cortex. O’Keane describes the process succinctly, noting that “biographical memory inputs can stimulate feeling states,” that are often as powerful as the initial encounter.

When pieces of the physical world move you—a taste, a sound, a word—your brain grabs hold of its individual parts and provokes a memorable re-encounter. As you reminisce, you reignite—the brain allows you to relive your most stirring moments as if they’re entirely new.

Time, memory, consciousness—there are few wonders more thought-provoking than these and still fewer that are more unanswerable. But, as these questions are considered in conjunction with one another, their seemingly impenetrable surfaces begin to shine, revealing more insights into the content of humanity than thinkers could have ever imagined. As the philosopher Henri Bergson artfully writes, “To exist is to change, to change is to mature, to mature is to go on creating oneself endlessly,” a process that requires the intertwined nature of time, memory, and consciousness.

The content of time is top of mind for various thinkers—from physicists to theologians, artists to neuroscientists—it seems that everyone has something to add to the story of that ever-spinning clock. The philosopher St. Augustine, for instance, is a bit of a household name, and neuroscientist Liliann Manning and physicist Sean Carroll are just now beginning to prove his take on time. According to all three thinkers, it often seems that the clock is the all-powerful arbiter of time, but such is not the case. Rather, the individual looking at those two hands circling beneath the glass has far more agency than she would think.

After all, time is nothing more than the buildup of the actions she takes throughout her day. As she reflects upon the fact that she woke up that morning, made breakfast, drove to work, journeyed home, and will do the same again tomorrow, the hours drift and bound before her. Interestingly, according to neuroscientists Endel Tulving and Daniel Schacter, the brain registers those two phenomena—one’s ability to think about what happened and ponder what might occur next—all in one place. They’re indistinguishable as far as neuroscience is concerned, and as others have also proven, both the hippocampus and the prefrontal cortex play starring roles. As the hippocampus helps us participate in everyday life, the prefrontal cortex, the brain’s keeper of “autobiographical memory,” allows us to view this reality from afar. Both are key components within the human brain and influence each other enormously.

Even with these leaps into new forms of intellectual inquiry, the question of “consciousness” is misty. But, its indeterminable nature is one of its most enticing traits. As clues of its contents continue to pour in from every field of thought—ones which seek the substance of memory, reality, and the nature of the physical world itself—the partial hints we receive may be captivating enough to sustain our fervent searching.