Book Review - Homo Deus

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Homo Deus is part two of Yuval Noah Harari’s humble quest to explain all of history. In Sapiens, he created a model of how humans rose to become Earth’s apex predator and most powerful ecological force. In Homo Deus, he refines this model and makes predictions about culture in a transhuman future.

Harari’s a bit of an intellectual maverick. His books bring you on a reckless tour through nearly every field of study, taking potshots at common opinion wherever they can. Each chapter escalates the weirdness until you feel peculiar yourself for agreeing with him.s

Though anthropology is its principal subject, Homo Deus dips into fields as diverse as evolutionary psychology, computer science, and literary criticism. Even if you disagree with Harari’s thesis, you will learn something from this book.

In this review, I’ll look at Harari’s main thesis in three sections. A horizontal line divides each section into two parts: a summary of Harari’s ideas, and my own analysis based on relevant research. Though I will cover only a small fraction of the book's content, I hope you walk away with the information you need to decide if Homo Deus is worth your time.

Intersubjective Reality

Homo Deus considers how science and technology will reshape culture. To explore this, Harari needs to establish what culture is, what motivates it, and how it shapes human activity. As many scholars do, Harari argues that culture is not just an ancillary byproduct of society— it's what made it possible for a particular species of Homo to rise above the rest.

Humans are not special. We are algorithms running on biological hardware, just like every other plant or animal. Early humans understood this, and were therefore animists— they saw the snakes, eagles, and elephants around them as being equal to themselves. Even when they killed an animal, they often made a sacrifice as penance to prevent nature from returning the favor.

Contrast this with the story told in the Bible, which was written by scholars of a much later, agrarian culture. God creates humans “in his own image.” He gives them dominion over bees and trees as well as a nice garden to play around in. When a wicked animal tricks Adam and Eve, he expels them for insubordination. Though other major religions show more empathy to animals, they all nevertheless see them as tools for humans. This mythological shift stands as a symbol for the larger historical shift of humans gaining dominance over other animals and the environment. When did this shift occur, and why?

Harari argues that religion itself provides the answer. Language by itself isn't enough, as examples like Clever Hans prove that animals can understand complex communication while still lacking the ability to form societies. What makes humans different is our ability to create intersubjective realities with language. These are systems of thought that exist only in the human mind and are given force by collective belief. All religions, nations, and currencies—among many other things—are intersubjective realities. After all, a dollar bill has no real value, the United States is not a person with thoughts and feelings, and a god gains power only through his worshipers. These are all fictions. If you want to know if a talked-about entity is real, Harari exhorts, ask if it can suffer. When a country loses territory, the country doesn’t suffer—there is no floating national spirit that cries out at the loss of an appendage. But the soldier who was stabbed in the name of that country suffers. The soldier is real; the country isn’t.

Intersubjective realities are useful because they allow humans to cooperate with strangers. Even highly social animals like dolphins and chimpanzees can only cooperate with other members of their small band. When encountering another band, chimps will likely attack or walk away out of fear rather than ally. For this reason, no matter how dexterous or far-sighted a chimpanzee may be, she won’t be able to rally a dozen local bands together and build a glorious temple to the monkey god. Mass cooperation is impossible.

On the other hand, when you meet a stranger on the road or a research group at a conference, you don’t show your teeth and run. You start a friendly discussion in a shared language using fake intersubjective norms like “politeness.” You talk with great excitement about things that don’t exist, like “wealth,” “beauty”, and “human rights.” You may decide to cooperate in pursuit of these things. This ability to cooperate with strangers is why Homo sapiens walks the Earth today and why Homo naledi, Mammuthus primigenius, and Australopithecus aferensis don’t.

What evidence backs up the claim that intersubjective reality is what gave Homo sapiens dominance over all creatures? Three things must be true for this to hold up:

  1. Intersubjective realities are useful.
  2. Modern humans can create and use intersubjective realities.
  3. Humans are unique in possessing this capacity.

Harari has me convinced of #1 and #2, but I remain suspicious of #3. Let’s look at the research.

Studies based on comparative neuroanatomy have mostly been dead ends because of the brain’s opaque structure. Researchers have focused instead on direct behavior observation. As they are extinct, it’s impossible to directly study the behavior of other hominids. Researchers typically conduct comparative experiments on our closest living relatives — chimpanzees, bonobos, and other great apes — to make predictions about what characteristics are derived in Homo sapiens compared to the last common ancestor of all great apes.

Most of my research on this topic comes from Sherwood et al.

Jane Goodall described chimp societies as “fission-fusion,” meaning that bonds between individuals rapidly break apart and form again, like molecules in water. Even though individuals rarely see each other, they still retain the benefits of group living as well as the ability to recognize affiliations and allegiances. This sounds a lot like “working with strangers.”

The biggest blow to Harari is the fact that chimp societies have both traditions and division of labor. In many bands, adolescents serve as territory patrols, adult males hunt, and females focus on gathering with tools. Chimp traditions are “the most widespread and diverse of any nonhuman primate species,” and include practices like grooming, mating, and tool use. They’re retained across generations and environments and vary from band to band. Can a particular grooming practice suffer? It cannot. It is an intersubjective reality.

What other fundamental differences exist between humans and great apes? Harari focuses on complex language, to which Sherwood et al. add a strong theory of mind (based on things like the mirror test), and superior imitation skills (possibly mediated by mirror neurons).

So the line between sapiens and other animals is not as clear-cut as Harari imagines. However, this does not undermine the central claim. Even if chimps and other Homo species had access to simple traditions and societies, modern humans may have marked a point of inflection beyond which these skills generate exponential returns. Intersubjective reality is not solely a human province; animals occasionally penetrate the borders. But they never get far.

This pattern reflects a lot of what scholars have said about Harari's work. He exaggerates and oversimplifies, but rarely does he get things completely wrong. That’s not necessarily a bad thing: a little oversimplification is good when writing for a popular audience. But the fact that it exists is something to keep in mind when reading Sapiens and Homo Deus.

Today's Reality

Intersubjective fictions contribute to our identities. We change them when our situation changes. Once we invented agriculture and animal husbandry, we changed the narrative. We told ourselves that we deserve to be king— after all, we have some divine spark that animals don’t have. This is an idea that all agricultural religions (which include today’s major religions) share. These religions supply the pretense for the new deal — we sacrifice wheat and pigs to you, oh Lord of Wind and Sea, and you will grant us good harvests in return. While the pig was once an equal, now it’s a currency. Some tribes, like the Nayaka of Southern India, make this distinction obvious when they give a soul to wild plants and lions, but not to organisms they’ve domesticated like cattle and tea bushes.

In the West, the idea of human distinctiveness developed into the idea of a soul. Christianity borrowed the psyche from the Greeks because it usefully explains why God cares so much about our everyday actions. Though the modern West pretends to be secular, it retains the notion of a divine soul. In fact, the soul is the foundation of modernity’s chief religion — Humanism. The media may have renamed it something like “the human spirit” and may disagree that it persists after death, but it’s fundamentally the same thing. However they call it, Humanists say that the human soul is unique, incorporeal, inviolable, and the fundamental purpose of existence.

Liberal Humanism, which dominated after the Cold War, goes even further. Liberal Humanists posit that each person has an “inner voice” or “identity” that only they can interpret. Liberal governments value freedom because only the individual can know his or her life purpose.

This notion lies at the core of all Western art, all fiction, and certainly all popular culture. It resounds in advertisements that tell you to “follow your heart” or “do what feels good.” It motivates the novel, the fundamental form of modern storytelling, which features conflict within and between people’s feelings. By contrast, it’s far less important in the medieval chivalric romance that celebrates God-approved valor, and it’s almost absent from the Athenian tragedy that puts plot first and characters second. Modernity didn’t abandon religion, it just replaced the divine command of God with the divine command of your feelings.

This argument sent chills down my back. It’s like watching surgeons vivisect my brain in real-time. Is this why I believe what I believe about the worth of human life, about the equality of all people, about the meaning to be found in human work? You may know on some level that your morality is arbitrary, but Harari makes you feel it. For a moment, you fathom the full length and depth of unexplored idea-space. All that you know in your heart to be good and true is not necessarily so, and likewise for evil. The idea that what your feelings matter at all is a brief, accidental attribute of this moment in history.

Like any religion, Humanism broke into schisms as soon as it gained dominance. Liberal Humanism was one of these sects; the other two were Social Humanism and Evolutionary Humanism.

Social Humanism, rather than emphasizing individual experience, instead focuses on how everything fits into a larger community governed by the rules of politics and economics. If you wish to know what to do, Social Humanism says, do not ask your heart. Ask instead your class, your coworkers, or your local commissariat.

Evolutionary Humanism took an even more divergent route. Inspired by Darwin’s ideas of the survival of the fittest, Evolutionary Humanists strive to do what will best advance the human race. Carving a marvelous statue is better than making a clay fetish, playing a fugue by Bach is better than singing a tribal chant, and you really should choose an embryo with a high IQ rather than one destined for mental impairment.

Though Evolutionary Humanism was popular in the late 19th and early 20th centuries, its famously violent propagation by the Nazis made it taboo after 1945. However, Harari thinks that Evolutionary Humanism is making a comeback. After all, Transhumanism also says that we must advance the evolution of the human race, though in a decidedly less violent manner.

Harari sees World War II and the Cold War as wars of religion. Our current peace is just as arbitrary and fleeting as the Peace of Augsburg. The present wobbles in an unstable equilibrium where, for the moment, only one religion dominates. It cannot do so for much longer.

It’s hard to evaluate Harari’s model because it makes few specific predictions. So what if Humanism is a religion? What evidence would we expect to observe if it weren’t? Furthermore, because every scholar uses a different definition of “Humanism,” it’s hard to compare like to like even at the conceptual level. Was Stalin a Humanist? How about Machiavelli? Most would say Newton was, but given his zeal for theology he would probably hate the title.

Different groups classify themselves in different ways. Take the following scenario: A group that follows the teachings of the prophet Lamor calls itself “Lamoran.” Then, an estranged subgroup splits off and claims that only it has true Lamoran authority. The original group says that these apostate windbags don’t follow Lamor at all. Two centuries later, after everyone who could object is dead, a scholar writes that they were actually all “Ederists” along with a dozen other loosely related groups. Much ink has been spilled debating such labels. What’s the point? How does calling communists “Social Humanists” better map the territory?

Tomorrow's Reality

Harari’s scariest thesis builds on these parts. Modern science may soon end the fragile dominance of Liberal Humanism. Science already undermines many of Humanism’s fundamental assumptions just as Copernicus undermined those of Christianity. Since Humanism is the source of nearly all of our notions about right and wrong, this could significantly alter our moral system.

The first pillar to fall is free will. Humanism uses free will to justify human sacredness by maintaining that humans are the only non-deterministic objects in the universe. In contrast, modern cognitive science understands the human brain as an easily manipulated biological algorithm. Harari cites a study where scientists hooked electrodes to a rat’s brain and used buttons to direct it through a maze. Ethical objections were raised, but then dismissed— after all, there is no coercion. When the professor presses the button to go left, the rat truly desires to go left. A desire is nothing but a pattern of firing neurons. So what if those neurons are firing because of external stimuli? The brain is either deterministic or random— we don’t know yet —but it is not free. The second pillar to fall is the individual. Writers have for centuries remarked that feelings and desires pass as quickly as the wind. We now understand this at the cognitive level. Studies show that the “I” is not a single unit, but the result of conflict between “the remembering self” and “the experiencing self”.

The experiencing self perceives reality in the moment as a series of flashing sights, sounds, feelings, thoughts, and desires. We tell ourselves that these are separate stimuli, but really, they all intermingle. Each stimulus interrupts the one before and likewise passes away instantly. The remembering self takes the memories left by the relentless information stream and pieces together a coherent narrative. It plans, identifies, sorts, segregates, and reminisces. But it cannot act; only the experiencing self can take action. When the two conflict, we procrastinate, do something we later regret, and reinterpret our past in new light.

The finding that our actions result from push-and-pull between these two parts puts the ego in jeopardy. Add to this the biases, heuristics, habits, and mental tics that hamper the remembering self, and we must seriously ask ourselves whether Homo sapiens is best modeled as a person or as a lump of meat. AI will soon send the next pillar – human utility – crashing down. In the past, Humanism thrived in the Manchester era of levée en masse— where practically everybody was useful for something, even if only for operating factory equipment. As algorithms take over an increasingly large proportion of the workplace, we will be forced to question our own merit in the face of objectively superior competitors.

After Humanism dies, Harari predicts that two new religions will compete to fill the power vacuum. He calls them “Techno-Humanism” and “Dataism.” Techno-Humanism is simply Harari’s term for traditional Transhumanism, so I’ll focus on the less-familiar Dataism.

If Humanism is akin to Christianity, Dataism is akin to Buddhism. The world is one massive, interconnected data network, and humans are mere conduits. A country is a data processing unit. So are universities, clubs, and social networks. Capitalism works better than communism because parallel processors are better than central processors at handling rapid change.

Dataism sees right and wrong as being a matter of information flow. To be virtuous is to be connected to the clumsily titled “Internet of All Things.” Soon, algorithms will monitor transpiration in the Amazon and adjust coal burning in Alsace-Lorraine. Machines will analyze employees’ mental states and optimize adrenaline and cortisol levels as deadlines near. AI will take over the world not by sword or silver tongue, but by our own hand.

The most important dataist commandment is “lose your identity.” Assimilate. Consume information, process it, and spit it back out. Share, post, like, and scroll. Quantify the rhythms of mind and body. Give this information to an algorithm. Follow its commands religiously. Information is the highest good. Humans are not special.

Harari argues that the increasing importance of data in research is an early sign of Dataism’s rise. It may be true that data-based research methods are increasingly popular, but that doesn’t mean scientists view their lives in such terms. Scientists have been compartmentalizing for centuries; they won’t stop now.

I know of no social movement with clearly Dataist ideals, and neither does Harari. I wouldn’t be surprised if one existed, but I would be surprised if a small, fringe movement gained enough influence to significantly shift the cultural course of Western history.

So, if Dataism does gain power, it will be the result of a long-term rising trend rather than a swift coup by a specific group. Do we see any such trends today?

One crucial expectation would be for technology to make us less individualistic. Luckily, all signs indicate the opposite: technology strengthens individualism.'

Santos et al. showed in 2017 that “individualism is indeed rising” in nearly every country worldwide, even China. Salehan et al. Found similar results in 2018 and correlated them with an increase in technological development. The 2010s were the most individualistic decade in history, as the internet allowed us to form subcultures unrestricted by geography and define ourselves through cross-cultural means. Many value identity so much that they’re eager to buck traditional gender and sexual norms to express themselves. Though technology may have reduced the rate of “lone wolf”-style independence, it poses no threat to individual identity.

Harari’s version of Dataism would therefore involve a reversal of a long-standing trend, a reversal that seems unlikely.


Here are the major ideas I got from this book:

  • Modern humans dominated all other species because only they can use intersubjective realities to cooperate with strangers.
  • In the West, most morality derives from Liberal Humanism, the dominant intersubjective reality today.
  • Humanism may someday give way to a new ideology. If this happens, everything you think you know about right and wrong will be useless.

Though Harari’s ideas on the future are highly speculative and probably untrue, they’re also hard to ignore. The downfall of Humanism would be the greatest cultural shift in human history in the past 500 years. It would render media ranging from FRIENDS to Shakespeare as distant and bizarre as the Egyptian Book of the Dead.

It would also nullify most values you hold dear. Rationalists like to identify with Galileo before the Catholic Holy Office, defending a new system backed by hard evidence. But imagine instead being a member of the church, and hearing a Dataist Galileo advocate for mass assimilation, or something more bizarre. Would you be strong enough to side with the evidence?

Even if the above ideas don’t interest you, Harari explores so many others that Homo Deus may still be worth skimming. It will stretch your mind in at least one direction.