Speaker 1: What makes things last and what do very different lasting
Speaker 1: things have in common from the brain's point of view.
Speaker 1: What is the secret behind how music works? And why
Speaker 1: might a space alien not be able to understand the
Speaker 1: concept of music. Why does glass in medieval cathedrals look
Speaker 1: thicker at the bottom and what does this tell us
Speaker 1: about the world's religions. What was the most important weapon
Speaker 1: in ancient history and how did it completely disappear? Today
Speaker 1: we're going to talk about the concept of persistence, from
Speaker 1: sharks to Roman concrete to DNA. So get ready for
Speaker 1: a great brain stretch. Welcome to Inner Cosmos with me
Speaker 1: David Eagelman. I'm a neuroscientist and author at Stanford and
Speaker 1: in these episodes we dive deeply into our three pound
Speaker 1: universe to understand some of the most surprising aspects of
Speaker 1: our world. Today, we're talking about how things persist. I
Speaker 1: want to start today's episode by talking about Greek fire,
Speaker 1: which I'm obsessed with. This story begins with a refugee
Speaker 1: named Kalanikos who arrived in Constantinople in the late six
Speaker 1: hundreds and he handed the emperors a weapon that he'd invented,
Speaker 1: and this completely changed warfare in the Mediterranean. His weapon
Speaker 1: was known as Greek fire. Think of it like a flamethrower,
Speaker 1: a thousand years before the invention of the modern flamethrower.
Speaker 1: Greek fire was a thick liquid. It was petroleum based,
Speaker 1: and you'd hurl this in pots or you'd blast it
Speaker 1: from siphons and it would burn on the water. And
Speaker 1: the key is you could incinerate the enemy's ships this way,
Speaker 1: and no one had ever seen anything like this, So
Speaker 1: the Byzantine army immediately leveraged this as a defense that
Speaker 1: broke the Arab sieges that were attacking Constantinople, and Greek
Speaker 1: fire quickly became the most important weapon of the age.
Speaker 1: The Byzantines were able to turn naval battles completely on
Speaker 1: their head and control the waters. Now here's the thing.
Speaker 1: The recipe for how to make Greek fire was ferociously secretive.
Speaker 1: The recipe was compartmentalized, and it was known only to
Speaker 1: the emperor's circle and Klinikos's genetic line. And this worked
Speaker 1: out well. No one else knew how to make it,
Speaker 1: and over centuries the Byzantine chronicles credit Greek fire for
Speaker 1: saving the capitol, and by extension, the entire empire. But
Speaker 1: the price of extreme secrecy is fragility. They had a
Speaker 1: small bus factor. Now I'm not sure how Cammon this
Speaker 1: term is. We use this in Silicon Valley a lot.
Speaker 1: The idea of a bus factor is how many people
Speaker 1: in your company would have to get hit and killed
Speaker 1: by a bus before no one knows how to do
Speaker 1: something anymore. You want to make sure that you always
Speaker 1: have a bus factor bigger than one like only Bob
Speaker 1: knows the password to our servers, because if Bob gets
Speaker 1: hit by a bus, then your company grinds to a halt.
Speaker 1: The smaller the bus factor, the more likely the company
Speaker 1: is not going to make it. So you always need
Speaker 1: to make sure you have redundancy. So in the case
Speaker 1: of Greek Fire, you've got a very small bus factor
Speaker 1: for secrecy reasons, and all you need are some political
Speaker 1: convulsions or the failure of a single lineage, and suddenly
Speaker 1: no one knows how to make Greek fire anymore. And
Speaker 1: that's what happened after the Fourth Crusades sack of Constantinople.
Speaker 1: After that, the Byzantines all remembered the weapon, but nobody
Speaker 1: remembered how to make it anymore. So the lesson of
Speaker 1: Greek fire is that critical knowledge can easily evaporate. And really,
Speaker 1: when we look across history, we find that's a common story.
Speaker 1: Things or organizations get started or invented, and eventually they
Speaker 1: die out. So this kind of story arc how we
Speaker 1: once knew something and then totally lost it has always
Speaker 1: made me wonder how anything survives for a long time,
Speaker 1: how anything persists because the universe is slipping away moment
Speaker 1: by moment. But everywhere we look we do find systems
Speaker 1: that come into existence that we fuse to let things go.
Speaker 1: Brains hold on to a continuous sense of self, cultures
Speaker 1: hold on to very old traditions, religion's last hundreds of generations.
Speaker 1: The body plans of sharks has continued essentially unchanged for
Speaker 1: hundreds of millions of years. So today's episode is about persistence,
Speaker 1: about things lasting through time. This is something I've always
Speaker 1: been fascinated with. So today we're going to weave together
Speaker 1: several paths to get to the bottom of a question.
Speaker 1: What makes things last from neurons to civilizations, and what
Speaker 1: do very different lasting things have in common? And this
Speaker 1: is part one. Next week, in part two, we're going
Speaker 1: to talk about thinking about our current moment on a
Speaker 1: ten thousand year timescale, we'll talk with Alexander Rose, the
Speaker 1: former executive director of the Long Now Foundation, where we'll
Speaker 1: hear about what organizations last through generations and why. For today,
Speaker 1: we're going to start in the brain with very brief
Speaker 1: windows of persistence, and then we'll move out to see
Speaker 1: some of these same principles across decades and centuries and millennia.
Speaker 1: So let's dive into the inner cosmos. Every sensory system
Speaker 1: faces a fundamental problem, which is that the world arrives
Speaker 1: in small brief pieces. The brain solves this by holding on,
Speaker 1: by making the signals persist longer than they would otherwise.
Speaker 1: For example, when you're hearing my voice, every sound I
Speaker 1: make is only in the outside world between us for
Speaker 1: some tens of milliseconds. But your brain holds onto those sounds,
Speaker 1: it retains them internally, so they're around long enough to
Speaker 1: be combined with whatever follows. So, in other words, all
Speaker 1: that comes through your speakers at any moment is a
Speaker 1: single phoneme, a little unit of speech. So you hear
Speaker 1: one phoneme and then another, and then another and then another.
Speaker 1: And the key is that it wouldn't mean anything to you,
Speaker 1: except they are able to hold them in your brain
Speaker 1: and replay them so that each little moment is heard
Speaker 1: with the others kept in mind, so you hear speech.
Speaker 1: So with these little sounds of speech, these phonemes, the
Speaker 1: brain preserves each of them long enough to be able
Speaker 1: to understand sounds in context. If the sound appeared and
Speaker 1: disappeared as quickly as it does from the world, nothing
Speaker 1: would have any meaning. And I'll give you another example
Speaker 1: which will make this even clearer. Music Music also depends
Speaker 1: on persistence in the brain. You can understand a melody
Speaker 1: only because earlier notes remain present in your brain when
Speaker 1: the later notes arrive. Without that, music would collapse into
Speaker 1: isolated blips with no rhythm and meaning. Why Because a
Speaker 1: single pitch is just an air compression wave. It arrives,
Speaker 1: it vibrates the ear drum, and then it physically disappears
Speaker 1: on its own. It doesn't carry much meaning. But when
Speaker 1: notes arrive in sequence, a melody appears. But the melody
Speaker 1: only exists because of the brain's ability to keep information alive.
Speaker 1: As each note reaches your ears, the previous notes are
Speaker 1: still present in neural activity. They remain in there shooting
Speaker 1: around in the neurons, and they do this long enough
Speaker 1: to overlap with what comes next. This allows your brain
Speaker 1: to create relationships across time. What you hear at any
Speaker 1: moment is shaped by what you heard in the moments before. So,
Speaker 1: just like speech, music depends on this internal persistence. A
Speaker 1: melody requires comparison, just like a word or sentence does.
Speaker 1: When we talk about resolving tension in music, this is
Speaker 1: only possible because the tension has been sustained. Even understanding
Speaker 1: a simple beat depends on your brain's capacity to predict
Speaker 1: the next thump by preserving the timing of at least
Speaker 1: the last two thumps. The stuff of the outside world
Speaker 1: has to be held onto and maintained. So let me
Speaker 1: drive this point home with an example that I love.
Speaker 1: A tune that you recognize has nothing to do with
Speaker 1: the individual notes. Instead, it's all about the relationship between
Speaker 1: the ne notes. And this is why you can transpose
Speaker 1: a tune to any key, so that all the notes
Speaker 1: are different, but their relationship is the same. So here's
Speaker 1: happy Birthday and the key of C. But if I
Speaker 1: transpose this to another key, you'll see that the individual
Speaker 1: notes are different, but you have no trouble recognizing the melody.
Speaker 1: Why can you still recognize the tune? Because it's not
Speaker 1: about the individual notes. A piece of music is about
Speaker 1: the relationship between the notes, and that relationship depends on
Speaker 1: holding in mind what just came before. So here's my
Speaker 1: assertion for today. I suggest that music would not be
Speaker 1: understood by any the alien species that doesn't keep signals
Speaker 1: alive internally in their nervous systems for a little while.
Speaker 1: If some space alien didn't have a way to make
Speaker 1: signals persist, but instead they were just reactive to the moment,
Speaker 1: then they would have an immediate sensory experience for each note,
Speaker 1: but they wouldn't be able to understand the notes as
Speaker 1: having a relationship. Music only makes sense when you think
Speaker 1: about it as the tension between what is being held
Speaker 1: and what arrives next. Same with language, of course, if
Speaker 1: the alien heard each of our phonemes and then forgot
Speaker 1: it before the next phoneme arrived, language couldn't mean anything.
Speaker 1: Everything would just be disconnected. Sounds. Meaning depends on holding
Speaker 1: earlier signals in mind while later ones unfold, so the
Speaker 1: brain is constantly retaining signals and giving them a law
Speaker 1: longer life than they actually have in the outside world,
Speaker 1: and we see this across all the senses. Take vision.
Speaker 1: Imagine you're sitting in a dark room and I flash
Speaker 1: an LED light on and off. It's so brief, let's
Speaker 1: say twenty five milliseconds, that it's already gone before you
Speaker 1: can register it. But here's the wacky part that my
Speaker 1: lab has done a lot of experiments on. The flash
Speaker 1: seems to you to last a little while, like one
Speaker 1: hundred milliseconds. It's not existing in the world anymore, but
Speaker 1: it exists for even longer directly in your conscious experience.
Speaker 1: The neurons that represented in awareness continue firing. That brief
Speaker 1: flash is stretched in time. This phenomenon is called visual persistence,
Speaker 1: and it's a version of the same idea, but this
Speaker 1: time directly in conscious awareness. That perception makes things last
Speaker 1: longer than they exist in the world. And visual persistence
Speaker 1: is the only reason that movies work, because we're able
Speaker 1: to perceive continuity and motion from looking at still frames
Speaker 1: flashed one after the other. In other words, because of persistence,
Speaker 1: successive images overlap in the mind. The brain integrates pictures
Speaker 1: over time, blending what just happened with what is happening now,
Speaker 1: and the result is motion from flashed still frames. You
Speaker 1: can see this visual persistence with a simple demonstration. If
Speaker 1: you flash a light on and off, and on and
Speaker 1: off and on and off really quickly, it will fuse
Speaker 1: into what looks like a steady light. The speed at
Speaker 1: which this happens is called the flicker fusion threshold, and
Speaker 1: it's normally about fifty five hurts, meaning you do this
Speaker 1: on off, on off fifty five times every second. So
Speaker 1: if a light flickers fast enough, the flicker goes away
Speaker 1: and it seems like a solid light now. One of
Speaker 1: the things my lab has studied over the years is
Speaker 1: the way we can leverage that as a diagnostic tool,
Speaker 1: because it turns out that not all brains make things
Speaker 1: persist for the same amount of time, and specifically, people
Speaker 1: who are suffering with schizophrenia have a longer window of persistence.
Speaker 1: A brief flash lingers in their brain for a longer time.
Speaker 1: What this means is that the visual world has more overlap, So,
Speaker 1: for example, a person with schizophrenia has a lower flicker
Speaker 1: fusion threshold. What that means is that I can flash
Speaker 1: a light on and off more slowly and the person
Speaker 1: will see that as a steady light. In other words,
Speaker 1: persistence lasts longer. Now I'm telling you that little detail
Speaker 1: about schizophrenia because this allowed me to build a purely
Speaker 1: visual test for diagnosing schizophrenia, and this is something I'm
Speaker 1: currently running tests on at Stanford. If you're interested, I'm
Speaker 1: linking my papers on this to the show notes. So
Speaker 1: let's zoom out to what we've seen so far about
Speaker 1: how the brain makes very short signals persist. The brain
Speaker 1: constructs what the psychologist William James a century ago called
Speaker 1: this specious present, by which he meant that the present
Speaker 1: moment is not actually an instant, but it's a short
Speaker 1: time window. This little sliding window of now causes the
Speaker 1: present moment to have a certain thickness. Everything sticks around
Speaker 1: for a bit in the brain so that events in
Speaker 1: time can get compared. In other words, your neural circuits
Speaker 1: refuse to let the world disappear too quickly. And we
Speaker 1: see this kind of temporal integration in every sensory system,
Speaker 1: which allows the immediate past to remain active in the present,
Speaker 1: whether the brain is stitching notes into music or still
Speaker 1: frames into motion. It relies on short lived memory to
Speaker 1: transform fragments into meaning. So persistence is something we talk
Speaker 1: about in neuroscience, but today I want to explore the
Speaker 1: notion of persistence from animals to nations to civilizations. And
Speaker 1: in thinking about this, I now think there are five
Speaker 1: main ways that things persist. So let's dive in. The
Speaker 1: First reason something persists is that it's optimized on some
Speaker 1: fitness landscape, meaning that it can't really get better. So generally,
Speaker 1: when we think about evolution, we picture a constant transformation
Speaker 1: of species, with new forms replacing old ones, like the
Speaker 1: cartoon of Homo sapiens evolving from a common ancestor with
Speaker 1: great apes until he's walking upright. But what's interesting is
Speaker 1: that some animals haven't changed in hundreds of millions of years,
Speaker 1: like sharks. Little details have shifted, but the overall body
Speaker 1: plan has persisted. Why Well, because they work, they're streamlined,
Speaker 1: they have very efficient propulsion, and their sensory systems are
Speaker 1: perfectly tunfoer hunting, so the design continues to succeed. So
Speaker 1: this first reason is obvious. Some things persist because they
Speaker 1: are at or near a local optimum, and if they
Speaker 1: were to change, that would probably introduce risk, as in,
Speaker 1: you're more likely to muck up the system rather than
Speaker 1: to improve it. So stability becomes the winning strategy, and
Speaker 1: persistence here is just a signal of success. And of
Speaker 1: course we see this with everything. Technologies persist when they
Speaker 1: meet needs efficiently. We can see these kind of technological
Speaker 1: sharks everywhere. The bicycle rolling down the street still has
Speaker 1: the triangular frame that was designed over a century ago.
Speaker 1: It's a design that balances strength and weight and efficiency
Speaker 1: so well that most attempts to reinvent the bicycle these
Speaker 1: remain curiosities. Or look at steel shipping containers. These stack
Speaker 1: and ports around the world. They have standardized dimensions and
Speaker 1: that locks ships and cranes and trucks and warehouses into
Speaker 1: a single, coordinated system that would only get messed up
Speaker 1: if you tried to change it. Or look at this
Speaker 1: screw with its spiral thread which translates rotation into linear force.
Speaker 1: This solves its little mechanical problems so cleanly that its
Speaker 1: basic form has persisted for centuries. Or take the four
Speaker 1: tined fork. This is centuries. There are variations, with the
Speaker 1: basic form of a fork stays the same because it
Speaker 1: accomplishes spearing and lifting efficiently across lots of food types.
Speaker 1: So these technologies persist because they reached a place of
Speaker 1: alignment with performance and costs and usability. Just like this shark,
Speaker 1: they solve their ecological problems efficiently, and once a form
Speaker 1: settles into a valley where it works well, staying there
Speaker 1: often proves a better decision than constant reinvention. So, like
Speaker 1: I said, I think that's the obvious reason why some
Speaker 1: things persist. But I was thinking about this, and I
Speaker 1: think there are four more interesting angles that we could
Speaker 1: take on this about other reasons why things last. For
Speaker 1: the first one, let's turn to something I think about often.
Speaker 1: Why you can recognize a friend that you haven't seen
Speaker 1: in like ten years. This happened to me at my
Speaker 1: high school reunion, where I saw people whose hair had
Speaker 1: changed entirely, maybe they didn't have hair anymore, Their face
Speaker 1: had changed shape a bit, their body had changed shape,
Speaker 1: and yet I was able to mostly recognize them, so
Speaker 1: something about them remained unmistakable. How do our brains allow
Speaker 1: someone's identity to persist for so long? And, by the way,
Speaker 1: I've wondered the same thing when I look at Google
Speaker 1: or Apple photo albums. In these photo albums, they recognize
Speaker 1: you or your kids through decades of time, even though
Speaker 1: you can see that everyone's faces have changed a lot,
Speaker 1: everyone's gotten older. So what's going on with brains and algorithms? Okay,
Speaker 1: for both of these, it's simply that they're figuring out
Speaker 1: which aspects of a face are worth carrying forward. They're
Speaker 1: sensitive to deep structure rather than the surface detail, so
Speaker 1: things like the spacing of features, how how far apart
Speaker 1: your eyes are, and how that relates to your nose,
Speaker 1: and the details of your mouth and your smile and
Speaker 1: your filterum, that's the vertical groove that connects to the
Speaker 1: bottom of the nose to your upper lip. All that
Speaker 1: geometry remains stable enough to anchor down your identity even
Speaker 1: while everything else changes. This is why airport security cameras
Speaker 1: can recognize the face even if you dye your hair
Speaker 1: or wear colored contacts or whatever. Recognition works because certain
Speaker 1: information persists, and the algorithms can ignore certain kinds of change,
Speaker 1: like shifts in the lighting or changes in the hairstyle.
Speaker 1: They filter all that stuff away. And for your brain,
Speaker 1: in particular, it's training on faces from infancy onward. It
Speaker 1: sees them again and again, and it learns which features
Speaker 1: persist across situations. Over time, it builds an internal model
Speaker 1: that figures out what matters enough to preserve. This question
Speaker 1: of what information to keep is asked by memory. More generally,
Speaker 1: most of your world passes through you and you don't
Speaker 1: remember it. Your memory only retains some experiences and decides
Speaker 1: the rest just doesn't matter. And in fact, culture works
Speaker 1: this way too, It repeats practices that continue to matter
Speaker 1: while letting most others drift into obscurity. What these all
Speaker 1: have in common is that persistence depends on abstraction. What
Speaker 1: survives is something a little removed from the details. So
Speaker 1: the brain's ability to recognize a familiar face after decades
Speaker 1: is one example of a universal strategy, which is preserving
Speaker 1: identity by learning what to ignore. In other words, persistence
Speaker 1: is about knowing what to hold on to. And nowhere
Speaker 1: does this apply more than to the notion of your
Speaker 1: own self. You consider you you, even though you have
Speaker 1: changed enormously over the years. You're not the same person
Speaker 1: that you were five years ago, and certainly all your
Speaker 1: cells have changed. I refer you to Inner Cosmos episode
Speaker 1: eighty two, about the continuity of the Self and the
Speaker 1: ship of theseus for more on this front. But The
Speaker 1: point I want to emphasize now is simply that you
Speaker 1: change through time, but you have certain things in common,
Speaker 1: like your name and the stories you like to tell,
Speaker 1: and where did you go to college and so on,
Speaker 1: and you end up with the illusion of a stable
Speaker 1: self that persists through time. Now here's the thing that
Speaker 1: might come as a surprise. The persistence of your self
Speaker 1: is something that you have to learn. This is what
Speaker 1: we call a cognitive development. So look at very young children.
Speaker 1: A young child can recognize her reflection in a mirror
Speaker 1: pretty early in life. She notices that when she smiles,
Speaker 1: the mirror image smiles. She notices that when she touches
Speaker 1: her face, so does the mirror image. So that's good.
Speaker 1: But this kind of self recognition doesn't yet work across time.
Speaker 1: If you show that same child a video of herself
Speaker 1: from a year earlier, something surprising happens. She might point
Speaker 1: to the screen and say that's a girl, or she
Speaker 1: calls the child by another name. The idea that this
Speaker 1: earlier version of her belongs to herself hasn't yet taken hold.
Speaker 1: So selfhood across time is something we have to figure out.
Speaker 1: What happens over the first few years of life is
Speaker 1: that children gradually assemble a sense of persistence. They come
Speaker 1: to understand that the person who fell down yesterday is
Speaker 1: the same one standing here right now. This depends on
Speaker 1: memory and languag whig and narrative all developing together, so
Speaker 1: the self becomes a story that can be updated while
Speaker 1: retaining a central thread. Now, one consequence of this is
Speaker 1: we all have what's called childhood amnesia, meaning we don't
Speaker 1: remember anything from the first few years of our lives. Now,
Speaker 1: it's not that you didn't have memory during those first
Speaker 1: few years, because you were remembering a ton of stuff
Speaker 1: and learning from it. But what was missing was a
Speaker 1: durable narrative to scaffold everything. You had experiences, but there
Speaker 1: was no persistent identity that you could stitch them into.
Speaker 1: As you grew, you began to tag experiences as belonging
Speaker 1: to you. You learned which details madded enough to carry
Speaker 1: forward and which didn't really matter. So your identity emerged
Speaker 1: as a stable pattern extracted from constant chain, just like
Speaker 1: with face recognition algorithms. And now on to the next
Speaker 1: way that some things persist, and that has to do
Speaker 1: with self repair. So, for example, if you walk along
Speaker 1: an ancient Roman Harbor, you can still find concrete structures
Speaker 1: that have endured wave after wave for two thousand years.
Speaker 1: These blocks were sitting in seawater, exposed to salt and
Speaker 1: stress and erosion, but they only got better with time. Now,
Speaker 1: how in the world does Roman concrete shrug off centuries
Speaker 1: of battering while our modern concrete would spawl and corrode
Speaker 1: under these circumstances, Roman concrete persists because it can self repair.
Speaker 1: This is probably accidental, but it's baked into the chemistry.
Speaker 1: In twenty twenty three, a team at MIT performed all
Speaker 1: kinds of imaging and found that Roman builder hot mixed
Speaker 1: their concrete with quicklime, which left tiny lime clasts scattered
Speaker 1: through the mortar. And these act like self healing capsules.
Speaker 1: So when micro cracks form and water seeps in, calcium
Speaker 1: dissolves and reprecipitates this calcium carbonate, which seals the crack. So,
Speaker 1: whether purposeful or not, the Romans achieved persistence of their structures.
Speaker 1: So even though the concrete lasts, how did this knowledge fade? Well,
Speaker 1: it's partly because history re optimizes, so various other recipes
Speaker 1: for cement took over, most notably Portland cement by the
Speaker 1: eighteen hundreds, mostly because Portland cement cures quickly and it
Speaker 1: pairs well with steel. In other words, we switched goals
Speaker 1: to speed and early strength and skyscrapers and so on,
Speaker 1: and so the recipe changed. But just go stand in
Speaker 1: an aqueduct and run your fingers over the mortar and
Speaker 1: you'll see a material that knows how to persist across
Speaker 1: centuries by self repair. And now I want to move
Speaker 1: on to a fourth way that things persist. And by
Speaker 1: the way, none of these methods are mutually exclusive. To
Speaker 1: understand this, one walk into an old cathedral and look
Speaker 1: closely at the windows. The glass often appears thicker at
Speaker 1: the bottom than at the top. Why well, what was
Speaker 1: explained to me when I was touring a cathedral in
Speaker 1: Europe a while ago, was that the glass behaves like
Speaker 1: a very slow liquid. You give it enough centuries and
Speaker 1: it gradually flows downward under its own weight. What's cool
Speaker 1: about this idea is that it invites you to imagine
Speaker 1: time as a force so powerful that even solid matter
Speaker 1: eventually yields. And by the way, I've heard this explanation
Speaker 1: of cathedral glass a dozen times since then, But that
Speaker 1: explanation is totally wrong. Glass at room temperature behaves as
Speaker 1: a solid, and its viscosity is so high that any
Speaker 1: flow would require time scales far longer than the age
Speaker 1: of the universe. As it turns out, the uneven thickness
Speaker 1: of old windows comes from manufacturing methods. Medieval glassmakers shaped
Speaker 1: pains unevenly, and installers generally placed the thicker edge at
Speaker 1: the bottom for stability. It has nothing to do with
Speaker 1: gravity pulling on it, so the glass hasn't moved. But
Speaker 1: the myth goes on, and the reason the myth persists
Speaker 1: is more interesting than the myth itself. The reason the
Speaker 1: myth persists is that ideas follow rules of survival that
Speaker 1: differ from the rules governing matter. An idea can last
Speaker 1: because it feels explanatory, or it resonates in some way.
Speaker 1: Ideas persist not necessarily because they're true, but because they
Speaker 1: have some other quality that makes them sticky. One might
Speaker 1: hypothesize that the glass myth persists because vast spans of
Speaker 1: time are awe inspiring, and or because the story translates
Speaker 1: the abstract concept of time in something visible, and or
Speaker 1: because it makes the person who says it sounds smart.
Speaker 1: Whatever the reasons that it became sticky. Once an idea
Speaker 1: is embedded in memory and culture, everyone starts to repeat
Speaker 1: it and so it gets carried forward. So what this
Speaker 1: tells us is that with human culture, persistence doesn't require accuracy,
Speaker 1: just good transmission from brain to brain. And we see
Speaker 1: this pattern everywhere, like with urban legends. With an urban legend,
Speaker 1: a story begins somewhere, maybe with a little grain of plausibility,
Speaker 1: and then it travels. There are urban legends about contaminated
Speaker 1: Halloween candy, or vanishing hitchhikers, or a friend of a
Speaker 1: friend who narrowly escaped disaster. And by the way, when
Speaker 1: you travel around the world, you see that urban legends
Speaker 1: shift their details according to local fears and cultural contexts,
Speaker 1: but the core structure stays the same. Why do these
Speaker 1: sorts of stories persist. In part, it's because urban legends
Speaker 1: attach themselves to emotion, usually some combination of surprise and danger,
Speaker 1: and this emotional component buffs up the recall and people
Speaker 1: love retelling it, and the stories are shaped and refined
Speaker 1: for memorability the way that good jokes are. So urban
Speaker 1: legends unmask the same principle of persistence that the myth
Speaker 1: about medieval Glass does, which is that ideas can survive
Speaker 1: just by embedding in shared memory. Once a community starts
Speaker 1: to repeat a story, that repetition becomes a stabilizing force,
Speaker 1: and that lets it endure across time. Now beyond explanatory
Speaker 1: stories about class and urban legends, this is true of
Speaker 1: cultural beliefs of all sorts. Takes something like religion. Now.
Speaker 1: I happen to be of the opinion that religions can
Speaker 1: be very useful for people and communities. But I also
Speaker 1: don't think, with two thousand religions on the globe, that
Speaker 1: one of them is true and all the others are false.
Speaker 1: But for today's purpose, what I think is fascinating is
Speaker 1: the persistence of religions, the way their stories continue to
Speaker 1: be told and retold across tens or hundreds of generations. Now,
Speaker 1: like the glass or the urban legend, some of the
Speaker 1: particular ideas and religions survive because they're well told stories,
Speaker 1: and mostly because they strive to explain some meaningful mystery
Speaker 1: in our lives. And I think there's a social flywheel
Speaker 1: effect such that if your parents told you a story,
Speaker 1: it feels very lovely to pass that on to your
Speaker 1: own children. On top of that, you have ritual repetition,
Speaker 1: you have texts, you have music, you have great architecture.
Speaker 1: And religious systems also endure because they distribute information across
Speaker 1: a huge swath of people and places, and this redundancy
Speaker 1: creates resilience. We'll come back to that in a moment.
Speaker 1: So put all this together, and religions are persistent engines.
Speaker 1: They solve the problem of how to keep an idea
Speaker 1: alive across centuries of change. In this way, cultural ideas
Speaker 1: like religions can often look like a cultural habit, in
Speaker 1: the sense that once they're established, they keep on trucking
Speaker 1: along automatically even when their origins fade. Sticky religions persist
Speaker 1: because they prove effective at capturing attention and moving from
Speaker 1: one mind to another. By the way, the reason I
Speaker 1: say sticky religions is because there are tens of thousands
Speaker 1: of other religions that we don't even see anymore. They
Speaker 1: lasted for a long time, and people gave up their
Speaker 1: life for those deities, but the religions weren't sufficiently well
Speaker 1: constructed to persist to the present. So we see that
Speaker 1: religions and many type of cultural stories last because they
Speaker 1: were optimized for transmit and that lets us revisit Greek fire,
Speaker 1: which I mentioned at the beginning. Greek Fire illustrates the converse.
Speaker 1: I told you that Greek fire was a weapon so
Speaker 1: powerful that as shaped to the fates of empires. But
Speaker 1: the formula vanished precisely because there wasn't enough redundancy for
Speaker 1: carrying it forward. The bus factor was too small. Greek
Speaker 1: fire was super powerful, but power didn't translate into endurance.
Speaker 1: So persistence isn't just about impact, but it depends on transmission.
Speaker 1: So back to the big picture. Some things persist because
Speaker 1: they're optimal, or at least locally optimal, like sharks. Other
Speaker 1: things endure because they're strong and self healing, like Roman concrete,
Speaker 1: and we just saw that. Other things persist because they're
Speaker 1: structured just right to be memorable. They're sticky, and once
Speaker 1: an idea digs in, it can outlast the originators in
Speaker 1: any evidence that got it started in the first place.
Speaker 1: And the idea's mechanism of transmission is moving from one
Speaker 1: brain to another. And now I want to turn to
Speaker 1: the fifth reason why some things last, And for that,
Speaker 1: think about when you were a kid and you played
Speaker 1: rockstis or paper, and one of the rules was that
Speaker 1: paper beats rock. I was thinking about this recently in
Speaker 1: terms of the way that information gets written down. Until recently,
Speaker 1: almost two thousand years ago, people used to carve proclamations
Speaker 1: into stone, and stone has the advantage of feeling really permanent.
Speaker 1: Then paper got introduced almost too millennia ago in China
Speaker 1: and in Europe about a thousand years ago. The problem
Speaker 1: with paper is that it's very fragile by comparison. It tears,
Speaker 1: and it burns and it disintegrates. But what's happened cross
Speaker 1: history is that paper has outlasted stone. Why it's because
Speaker 1: monuments erode. Wherever you find any ancient inscription, you'll see
Speaker 1: that time has softened it and it's nearing disappearance. You
Speaker 1: see that ancient statues lose their faces and their name plaques.
Speaker 1: So as material, stone persists, but the information carved into
Speaker 1: it gradually fades away. Now, the surprising contrast is that
Speaker 1: ideas written on paper have much longer lives. Why it's
Speaker 1: because they're copied and recopied and translated and rewritten. Here's
Speaker 1: the key. The individual fragile pages vanished, but the pattern survives.
Speaker 1: It seems so crazy that paper could beat rock, but
Speaker 1: it does because it can be easily replaced. So what
Speaker 1: does this mean? It means that your ability turns out
Speaker 1: to matter less than replicability. A medium that can be
Speaker 1: reproduced easily travels farther through time than one that demands
Speaker 1: physical permanence. What survives is what can be regenerated when
Speaker 1: it breaks. And once you start looking for this principle,
Speaker 1: you'll see it everywhere in nature. Take genes. The wild
Speaker 1: part about the DNA code is how long it's survived
Speaker 1: and how massively successful it is. This simple little codebook
Speaker 1: has survived across billions of years, and the key is that,
Speaker 1: just like with paper, its longevity doesn't come from physical toughness.
Speaker 1: Individual DNA molecules are unbelievably fragile. They break easily when
Speaker 1: you subject them to heat or other kinds of radiation
Speaker 1: or chemicals they don't like. But the genetic code keeps
Speaker 1: on trucking. Why DNA persists because it is copied. Every
Speaker 1: time a cell divides, its genetic information is rewritten onto
Speaker 1: new base pairs. And whenever you do that, you check
Speaker 1: for errors and try to correct mistakes, and you scan
Speaker 1: for damage and fix it. But the key is that
Speaker 1: the original molecule doesn't need to survive as long as
Speaker 1: the pattern survives. This makes DNA less like stone and
Speaker 1: more like paper. The medium is expendable and the information
Speaker 1: is what matters. Even though the molecule is fragile, the
Speaker 1: information continues. And this is the same principle with memories
Speaker 1: persisting in your brain. You can still remember, for example,
Speaker 1: your third grade teacher's name. Now, why is that weird?
Speaker 1: It's because every cell in your body is getting all
Speaker 1: its pieces and parts replaced every moment of your life,
Speaker 1: such that you're an entirely new person. Some years later,
Speaker 1: none of the the molecules that you had in your
Speaker 1: body in the third grade are there anymore. It's all
Speaker 1: new stuff. But you still have your memories and your
Speaker 1: knowledge and your perception because those are stored in the
Speaker 1: constantly rebuilding structure that persists. Your whole brain is made
Speaker 1: of very fragile stuff, just little cells, but it keeps
Speaker 1: the structure even as the details get replaced again and again.
Speaker 1: So back to religions and cultural stories. Even more generally,
Speaker 1: I argued in a minute ago that cultural stories last
Speaker 1: because they're useful, they're memorable. They propose answers to questions
Speaker 1: that people have, and they give rituals and social coherence.
Speaker 1: But the interesting part here is that you can think
Speaker 1: of something like the Catholic Church living on top of
Speaker 1: generations of humans that are born. They absorb the religious
Speaker 1: dogma from the elders, they pass it to their children,
Speaker 1: and they pass away. The Catholic Church is something like
Speaker 1: information living on top of constantly recycling humans. Culture persists
Speaker 1: because ideas are reenacted and recopied across generation, just like
Speaker 1: the way your memory of your third grade teacher lives
Speaker 1: on top of constantly changing neurons. So across the spectrum
Speaker 1: from brains to civilizations, persistence sometimes depends on renewal. Unlike
Speaker 1: something that lasts because it's optimized or strong, Persistence very
Speaker 1: often emerges from something fragile, like paper or DNA or
Speaker 1: memories or humans, and you keep passing the message along.
Speaker 1: In other words, what you want is not necessarily the
Speaker 1: thing that doesn't get damaged. You want fragile elements organized
Speaker 1: into replicating patterns. So let's wrap up. Today is a
Speaker 1: meditation on how things last. And what we saw is
Speaker 1: that across all scales from neurons to cultures. Persistence follows
Speaker 1: just a few patterns. Systems that rely on a single
Speaker 1: instance those vanish easily, but systems that distribute themselves endure,
Speaker 1: and systems that can be repaired or self repair outlast.
Speaker 1: Systems that demand perfection. Now, coming back to the beginning,
Speaker 1: what I find interesting is that we're always told to
Speaker 1: live in the present, But the fact is that the
Speaker 1: world you inhabit is not just the knife edge of
Speaker 1: the present. It's a dense accumulation of held moments. Every
Speaker 1: habit you have, every story you tell, every building that
Speaker 1: you live in, the culture that you are embedded in.
Speaker 1: These are all solutions to the same problem, how to
Speaker 1: keep things alive for longer. Even podcasts like this one
Speaker 1: participate in that process. Sounds held in your brain for
Speaker 1: seconds become ideas held for years. A story that you
Speaker 1: hear becomes part of your internal landscape, maybe for the
Speaker 1: rest of your life. The cosmos outside continues to change
Speaker 1: every second in its vast motion, but the job of
Speaker 1: your inner cosmos is to stitch moments together to make
Speaker 1: some things in the world persist. And now we're well
Speaker 1: positioned for next week's episode, where I'm going to talk
Speaker 1: with Alexander Rose, the former director of the Long Now Foundation,
Speaker 1: and we're going to dive into his research on organizations
Speaker 1: that outlast their founders might say over a thousand years.
Speaker 1: What makes them last? Join me next week to find out.
Speaker 1: Go to eagleman dot com slash pod cast for more
Speaker 1: information and to find further reading. Join the weekly discussions
Speaker 1: on my substack and check out and subscribe to Inner
Speaker 1: Cosmos on YouTube for videos of each episode and to
Speaker 1: leave comments. Until next time. I'm David Eagleman, and this
Speaker 1: is Inner Cosmos