1 00:00:05,120 --> 00:00:09,320 Speaker 1: What makes things last and what do very different lasting 2 00:00:09,480 --> 00:00:13,160 Speaker 1: things have in common from the brain's point of view. 3 00:00:13,200 --> 00:00:17,279 Speaker 1: What is the secret behind how music works? And why 4 00:00:17,360 --> 00:00:20,880 Speaker 1: might a space alien not be able to understand the 5 00:00:21,040 --> 00:00:26,599 Speaker 1: concept of music. Why does glass in medieval cathedrals look 6 00:00:26,720 --> 00:00:29,280 Speaker 1: thicker at the bottom and what does this tell us 7 00:00:29,320 --> 00:00:33,280 Speaker 1: about the world's religions. What was the most important weapon 8 00:00:33,440 --> 00:00:37,960 Speaker 1: in ancient history and how did it completely disappear? Today 9 00:00:38,000 --> 00:00:41,560 Speaker 1: we're going to talk about the concept of persistence, from 10 00:00:41,880 --> 00:00:45,559 Speaker 1: sharks to Roman concrete to DNA. So get ready for 11 00:00:45,680 --> 00:00:52,559 Speaker 1: a great brain stretch. Welcome to Inner Cosmos with me 12 00:00:52,720 --> 00:00:56,200 Speaker 1: David Eagelman. I'm a neuroscientist and author at Stanford and 13 00:00:56,280 --> 00:00:59,560 Speaker 1: in these episodes we dive deeply into our three pound 14 00:00:59,680 --> 00:01:03,680 Speaker 1: universe to understand some of the most surprising aspects of 15 00:01:03,720 --> 00:01:20,039 Speaker 1: our world. Today, we're talking about how things persist. I 16 00:01:20,080 --> 00:01:24,280 Speaker 1: want to start today's episode by talking about Greek fire, 17 00:01:24,480 --> 00:01:28,560 Speaker 1: which I'm obsessed with. This story begins with a refugee 18 00:01:28,760 --> 00:01:33,720 Speaker 1: named Kalanikos who arrived in Constantinople in the late six 19 00:01:33,840 --> 00:01:38,520 Speaker 1: hundreds and he handed the emperors a weapon that he'd invented, 20 00:01:38,560 --> 00:01:43,600 Speaker 1: and this completely changed warfare in the Mediterranean. His weapon 21 00:01:43,680 --> 00:01:48,320 Speaker 1: was known as Greek fire. Think of it like a flamethrower, 22 00:01:48,760 --> 00:01:52,600 Speaker 1: a thousand years before the invention of the modern flamethrower. 23 00:01:52,960 --> 00:01:56,480 Speaker 1: Greek fire was a thick liquid. It was petroleum based, 24 00:01:56,800 --> 00:01:59,920 Speaker 1: and you'd hurl this in pots or you'd blast it 25 00:02:00,040 --> 00:02:03,480 Speaker 1: from siphons and it would burn on the water. And 26 00:02:03,520 --> 00:02:07,240 Speaker 1: the key is you could incinerate the enemy's ships this way, 27 00:02:07,520 --> 00:02:10,200 Speaker 1: and no one had ever seen anything like this, So 28 00:02:10,400 --> 00:02:14,919 Speaker 1: the Byzantine army immediately leveraged this as a defense that 29 00:02:15,040 --> 00:02:19,680 Speaker 1: broke the Arab sieges that were attacking Constantinople, and Greek 30 00:02:19,720 --> 00:02:23,280 Speaker 1: fire quickly became the most important weapon of the age. 31 00:02:23,720 --> 00:02:28,160 Speaker 1: The Byzantines were able to turn naval battles completely on 32 00:02:28,200 --> 00:02:31,840 Speaker 1: their head and control the waters. Now here's the thing. 33 00:02:32,639 --> 00:02:38,119 Speaker 1: The recipe for how to make Greek fire was ferociously secretive. 34 00:02:38,800 --> 00:02:42,960 Speaker 1: The recipe was compartmentalized, and it was known only to 35 00:02:43,080 --> 00:02:47,880 Speaker 1: the emperor's circle and Klinikos's genetic line. And this worked 36 00:02:47,880 --> 00:02:50,360 Speaker 1: out well. No one else knew how to make it, 37 00:02:50,840 --> 00:02:55,959 Speaker 1: and over centuries the Byzantine chronicles credit Greek fire for 38 00:02:56,080 --> 00:03:00,400 Speaker 1: saving the capitol, and by extension, the entire empire. But 39 00:03:01,240 --> 00:03:06,919 Speaker 1: the price of extreme secrecy is fragility. They had a 40 00:03:07,080 --> 00:03:10,480 Speaker 1: small bus factor. Now I'm not sure how Cammon this 41 00:03:10,600 --> 00:03:13,000 Speaker 1: term is. We use this in Silicon Valley a lot. 42 00:03:13,320 --> 00:03:17,280 Speaker 1: The idea of a bus factor is how many people 43 00:03:17,280 --> 00:03:20,280 Speaker 1: in your company would have to get hit and killed 44 00:03:20,320 --> 00:03:23,679 Speaker 1: by a bus before no one knows how to do 45 00:03:23,760 --> 00:03:26,760 Speaker 1: something anymore. You want to make sure that you always 46 00:03:26,800 --> 00:03:31,440 Speaker 1: have a bus factor bigger than one like only Bob 47 00:03:31,639 --> 00:03:35,520 Speaker 1: knows the password to our servers, because if Bob gets 48 00:03:35,560 --> 00:03:39,080 Speaker 1: hit by a bus, then your company grinds to a halt. 49 00:03:39,640 --> 00:03:43,440 Speaker 1: The smaller the bus factor, the more likely the company 50 00:03:43,480 --> 00:03:45,840 Speaker 1: is not going to make it. So you always need 51 00:03:45,880 --> 00:03:50,480 Speaker 1: to make sure you have redundancy. So in the case 52 00:03:50,600 --> 00:03:54,280 Speaker 1: of Greek Fire, you've got a very small bus factor 53 00:03:54,360 --> 00:03:58,640 Speaker 1: for secrecy reasons, and all you need are some political 54 00:03:58,720 --> 00:04:03,160 Speaker 1: convulsions or the failure of a single lineage, and suddenly 55 00:04:03,520 --> 00:04:07,080 Speaker 1: no one knows how to make Greek fire anymore. And 56 00:04:07,080 --> 00:04:11,680 Speaker 1: that's what happened after the Fourth Crusades sack of Constantinople. 57 00:04:11,960 --> 00:04:16,880 Speaker 1: After that, the Byzantines all remembered the weapon, but nobody 58 00:04:16,920 --> 00:04:20,919 Speaker 1: remembered how to make it anymore. So the lesson of 59 00:04:20,960 --> 00:04:25,799 Speaker 1: Greek fire is that critical knowledge can easily evaporate. And really, 60 00:04:25,839 --> 00:04:30,240 Speaker 1: when we look across history, we find that's a common story. 61 00:04:30,400 --> 00:04:35,839 Speaker 1: Things or organizations get started or invented, and eventually they 62 00:04:36,040 --> 00:04:40,120 Speaker 1: die out. So this kind of story arc how we 63 00:04:40,520 --> 00:04:43,839 Speaker 1: once knew something and then totally lost it has always 64 00:04:43,880 --> 00:04:48,120 Speaker 1: made me wonder how anything survives for a long time, 65 00:04:48,200 --> 00:04:53,760 Speaker 1: how anything persists because the universe is slipping away moment 66 00:04:53,839 --> 00:04:57,839 Speaker 1: by moment. But everywhere we look we do find systems 67 00:04:58,120 --> 00:05:01,680 Speaker 1: that come into existence that we fuse to let things go. 68 00:05:02,400 --> 00:05:07,360 Speaker 1: Brains hold on to a continuous sense of self, cultures 69 00:05:07,760 --> 00:05:13,160 Speaker 1: hold on to very old traditions, religion's last hundreds of generations. 70 00:05:13,520 --> 00:05:18,080 Speaker 1: The body plans of sharks has continued essentially unchanged for 71 00:05:18,480 --> 00:05:23,680 Speaker 1: hundreds of millions of years. So today's episode is about persistence, 72 00:05:24,160 --> 00:05:27,920 Speaker 1: about things lasting through time. This is something I've always 73 00:05:27,960 --> 00:05:30,920 Speaker 1: been fascinated with. So today we're going to weave together 74 00:05:31,040 --> 00:05:34,040 Speaker 1: several paths to get to the bottom of a question. 75 00:05:34,680 --> 00:05:38,480 Speaker 1: What makes things last from neurons to civilizations, and what 76 00:05:38,520 --> 00:05:43,600 Speaker 1: do very different lasting things have in common? And this 77 00:05:43,640 --> 00:05:46,400 Speaker 1: is part one. Next week, in part two, we're going 78 00:05:46,440 --> 00:05:50,520 Speaker 1: to talk about thinking about our current moment on a 79 00:05:50,600 --> 00:05:55,080 Speaker 1: ten thousand year timescale, we'll talk with Alexander Rose, the 80 00:05:55,160 --> 00:05:59,480 Speaker 1: former executive director of the Long Now Foundation, where we'll 81 00:05:59,480 --> 00:06:05,160 Speaker 1: hear about what organizations last through generations and why. For today, 82 00:06:05,200 --> 00:06:08,000 Speaker 1: we're going to start in the brain with very brief 83 00:06:08,040 --> 00:06:11,960 Speaker 1: windows of persistence, and then we'll move out to see 84 00:06:11,960 --> 00:06:17,200 Speaker 1: some of these same principles across decades and centuries and millennia. 85 00:06:17,720 --> 00:06:22,599 Speaker 1: So let's dive into the inner cosmos. Every sensory system 86 00:06:22,720 --> 00:06:25,800 Speaker 1: faces a fundamental problem, which is that the world arrives 87 00:06:26,240 --> 00:06:31,240 Speaker 1: in small brief pieces. The brain solves this by holding on, 88 00:06:31,400 --> 00:06:36,120 Speaker 1: by making the signals persist longer than they would otherwise. 89 00:06:36,520 --> 00:06:40,680 Speaker 1: For example, when you're hearing my voice, every sound I 90 00:06:40,760 --> 00:06:44,560 Speaker 1: make is only in the outside world between us for 91 00:06:44,680 --> 00:06:49,040 Speaker 1: some tens of milliseconds. But your brain holds onto those sounds, 92 00:06:49,279 --> 00:06:53,719 Speaker 1: it retains them internally, so they're around long enough to 93 00:06:53,720 --> 00:06:57,520 Speaker 1: be combined with whatever follows. So, in other words, all 94 00:06:57,560 --> 00:07:00,760 Speaker 1: that comes through your speakers at any moment is a 95 00:07:00,800 --> 00:07:05,040 Speaker 1: single phoneme, a little unit of speech. So you hear 96 00:07:05,480 --> 00:07:15,960 Speaker 1: one phoneme and then another, and then another and then another. 97 00:07:17,120 --> 00:07:19,480 Speaker 1: And the key is that it wouldn't mean anything to you, 98 00:07:19,560 --> 00:07:22,480 Speaker 1: except they are able to hold them in your brain 99 00:07:22,560 --> 00:07:25,800 Speaker 1: and replay them so that each little moment is heard 100 00:07:25,880 --> 00:07:30,400 Speaker 1: with the others kept in mind, so you hear speech. 101 00:07:31,480 --> 00:07:34,880 Speaker 1: So with these little sounds of speech, these phonemes, the 102 00:07:34,920 --> 00:07:37,440 Speaker 1: brain preserves each of them long enough to be able 103 00:07:37,440 --> 00:07:42,080 Speaker 1: to understand sounds in context. If the sound appeared and 104 00:07:42,200 --> 00:07:45,239 Speaker 1: disappeared as quickly as it does from the world, nothing 105 00:07:45,320 --> 00:07:48,600 Speaker 1: would have any meaning. And I'll give you another example 106 00:07:48,600 --> 00:07:53,080 Speaker 1: which will make this even clearer. Music Music also depends 107 00:07:53,160 --> 00:07:57,360 Speaker 1: on persistence in the brain. You can understand a melody 108 00:07:57,800 --> 00:08:02,120 Speaker 1: only because earlier notes remain present in your brain when 109 00:08:02,160 --> 00:08:06,600 Speaker 1: the later notes arrive. Without that, music would collapse into 110 00:08:06,760 --> 00:08:11,440 Speaker 1: isolated blips with no rhythm and meaning. Why Because a 111 00:08:11,560 --> 00:08:18,600 Speaker 1: single pitch is just an air compression wave. It arrives, 112 00:08:18,680 --> 00:08:23,360 Speaker 1: it vibrates the ear drum, and then it physically disappears 113 00:08:23,800 --> 00:08:26,760 Speaker 1: on its own. It doesn't carry much meaning. But when 114 00:08:26,880 --> 00:08:40,960 Speaker 1: notes arrive in sequence, a melody appears. But the melody 115 00:08:41,080 --> 00:08:45,840 Speaker 1: only exists because of the brain's ability to keep information alive. 116 00:08:46,440 --> 00:08:49,720 Speaker 1: As each note reaches your ears, the previous notes are 117 00:08:49,800 --> 00:08:54,560 Speaker 1: still present in neural activity. They remain in there shooting 118 00:08:54,600 --> 00:08:57,960 Speaker 1: around in the neurons, and they do this long enough 119 00:08:58,040 --> 00:09:01,760 Speaker 1: to overlap with what comes next. This allows your brain 120 00:09:01,840 --> 00:09:07,200 Speaker 1: to create relationships across time. What you hear at any 121 00:09:07,280 --> 00:09:10,920 Speaker 1: moment is shaped by what you heard in the moments before. So, 122 00:09:11,120 --> 00:09:16,360 Speaker 1: just like speech, music depends on this internal persistence. A 123 00:09:16,480 --> 00:09:21,480 Speaker 1: melody requires comparison, just like a word or sentence does. 124 00:09:21,840 --> 00:09:25,640 Speaker 1: When we talk about resolving tension in music, this is 125 00:09:25,679 --> 00:09:31,559 Speaker 1: only possible because the tension has been sustained. Even understanding 126 00:09:31,640 --> 00:09:36,239 Speaker 1: a simple beat depends on your brain's capacity to predict 127 00:09:36,360 --> 00:09:39,920 Speaker 1: the next thump by preserving the timing of at least 128 00:09:40,000 --> 00:09:43,679 Speaker 1: the last two thumps. The stuff of the outside world 129 00:09:44,040 --> 00:09:46,960 Speaker 1: has to be held onto and maintained. So let me 130 00:09:47,040 --> 00:09:49,800 Speaker 1: drive this point home with an example that I love. 131 00:09:50,280 --> 00:09:53,800 Speaker 1: A tune that you recognize has nothing to do with 132 00:09:53,960 --> 00:09:59,719 Speaker 1: the individual notes. Instead, it's all about the relationship between 133 00:09:59,760 --> 00:10:04,040 Speaker 1: the ne notes. And this is why you can transpose 134 00:10:04,160 --> 00:10:07,080 Speaker 1: a tune to any key, so that all the notes 135 00:10:07,120 --> 00:10:11,120 Speaker 1: are different, but their relationship is the same. So here's 136 00:10:11,160 --> 00:10:20,880 Speaker 1: happy Birthday and the key of C. But if I 137 00:10:21,000 --> 00:10:24,679 Speaker 1: transpose this to another key, you'll see that the individual 138 00:10:24,720 --> 00:10:29,160 Speaker 1: notes are different, but you have no trouble recognizing the melody. 139 00:10:36,760 --> 00:10:39,720 Speaker 1: Why can you still recognize the tune? Because it's not 140 00:10:39,960 --> 00:10:43,400 Speaker 1: about the individual notes. A piece of music is about 141 00:10:43,720 --> 00:10:49,200 Speaker 1: the relationship between the notes, and that relationship depends on 142 00:10:49,679 --> 00:10:53,440 Speaker 1: holding in mind what just came before. So here's my 143 00:10:53,520 --> 00:10:57,400 Speaker 1: assertion for today. I suggest that music would not be 144 00:10:57,720 --> 00:11:03,480 Speaker 1: understood by any the alien species that doesn't keep signals 145 00:11:03,640 --> 00:11:08,080 Speaker 1: alive internally in their nervous systems for a little while. 146 00:11:08,240 --> 00:11:11,600 Speaker 1: If some space alien didn't have a way to make 147 00:11:11,640 --> 00:11:15,800 Speaker 1: signals persist, but instead they were just reactive to the moment, 148 00:11:16,440 --> 00:11:20,440 Speaker 1: then they would have an immediate sensory experience for each note, 149 00:11:20,679 --> 00:11:23,920 Speaker 1: but they wouldn't be able to understand the notes as 150 00:11:23,960 --> 00:11:28,640 Speaker 1: having a relationship. Music only makes sense when you think 151 00:11:28,640 --> 00:11:31,960 Speaker 1: about it as the tension between what is being held 152 00:11:32,000 --> 00:11:35,200 Speaker 1: and what arrives next. Same with language, of course, if 153 00:11:35,280 --> 00:11:39,160 Speaker 1: the alien heard each of our phonemes and then forgot 154 00:11:39,200 --> 00:11:43,440 Speaker 1: it before the next phoneme arrived, language couldn't mean anything. 155 00:11:43,840 --> 00:11:50,720 Speaker 1: Everything would just be disconnected. Sounds. Meaning depends on holding 156 00:11:50,960 --> 00:11:56,120 Speaker 1: earlier signals in mind while later ones unfold, so the 157 00:11:56,200 --> 00:11:59,960 Speaker 1: brain is constantly retaining signals and giving them a law 158 00:12:00,040 --> 00:12:03,600 Speaker 1: longer life than they actually have in the outside world, 159 00:12:03,880 --> 00:12:07,400 Speaker 1: and we see this across all the senses. Take vision. 160 00:12:07,679 --> 00:12:10,400 Speaker 1: Imagine you're sitting in a dark room and I flash 161 00:12:10,440 --> 00:12:14,080 Speaker 1: an LED light on and off. It's so brief, let's 162 00:12:14,080 --> 00:12:17,840 Speaker 1: say twenty five milliseconds, that it's already gone before you 163 00:12:17,880 --> 00:12:20,360 Speaker 1: can register it. But here's the wacky part that my 164 00:12:20,440 --> 00:12:23,280 Speaker 1: lab has done a lot of experiments on. The flash 165 00:12:23,320 --> 00:12:26,960 Speaker 1: seems to you to last a little while, like one 166 00:12:27,040 --> 00:12:31,240 Speaker 1: hundred milliseconds. It's not existing in the world anymore, but 167 00:12:31,320 --> 00:12:36,199 Speaker 1: it exists for even longer directly in your conscious experience. 168 00:12:37,080 --> 00:12:42,560 Speaker 1: The neurons that represented in awareness continue firing. That brief 169 00:12:42,600 --> 00:12:48,600 Speaker 1: flash is stretched in time. This phenomenon is called visual persistence, 170 00:12:48,960 --> 00:12:51,040 Speaker 1: and it's a version of the same idea, but this 171 00:12:51,080 --> 00:12:56,320 Speaker 1: time directly in conscious awareness. That perception makes things last 172 00:12:56,400 --> 00:13:00,439 Speaker 1: longer than they exist in the world. And visual persistence 173 00:13:00,559 --> 00:13:04,160 Speaker 1: is the only reason that movies work, because we're able 174 00:13:04,160 --> 00:13:09,679 Speaker 1: to perceive continuity and motion from looking at still frames 175 00:13:10,120 --> 00:13:15,040 Speaker 1: flashed one after the other. In other words, because of persistence, 176 00:13:15,320 --> 00:13:21,280 Speaker 1: successive images overlap in the mind. The brain integrates pictures 177 00:13:21,360 --> 00:13:25,680 Speaker 1: over time, blending what just happened with what is happening now, 178 00:13:25,720 --> 00:13:30,840 Speaker 1: and the result is motion from flashed still frames. You 179 00:13:30,880 --> 00:13:34,800 Speaker 1: can see this visual persistence with a simple demonstration. If 180 00:13:34,840 --> 00:13:36,840 Speaker 1: you flash a light on and off, and on and 181 00:13:36,840 --> 00:13:39,560 Speaker 1: off and on and off really quickly, it will fuse 182 00:13:39,800 --> 00:13:43,320 Speaker 1: into what looks like a steady light. The speed at 183 00:13:43,320 --> 00:13:47,200 Speaker 1: which this happens is called the flicker fusion threshold, and 184 00:13:47,240 --> 00:13:50,719 Speaker 1: it's normally about fifty five hurts, meaning you do this 185 00:13:50,800 --> 00:13:54,320 Speaker 1: on off, on off fifty five times every second. So 186 00:13:54,520 --> 00:13:58,679 Speaker 1: if a light flickers fast enough, the flicker goes away 187 00:13:58,800 --> 00:14:02,200 Speaker 1: and it seems like a solid light now. One of 188 00:14:02,200 --> 00:14:04,200 Speaker 1: the things my lab has studied over the years is 189 00:14:04,240 --> 00:14:07,760 Speaker 1: the way we can leverage that as a diagnostic tool, 190 00:14:07,840 --> 00:14:10,880 Speaker 1: because it turns out that not all brains make things 191 00:14:10,960 --> 00:14:14,800 Speaker 1: persist for the same amount of time, and specifically, people 192 00:14:14,840 --> 00:14:20,160 Speaker 1: who are suffering with schizophrenia have a longer window of persistence. 193 00:14:20,880 --> 00:14:25,160 Speaker 1: A brief flash lingers in their brain for a longer time. 194 00:14:25,520 --> 00:14:29,960 Speaker 1: What this means is that the visual world has more overlap, So, 195 00:14:30,080 --> 00:14:34,560 Speaker 1: for example, a person with schizophrenia has a lower flicker 196 00:14:34,640 --> 00:14:37,840 Speaker 1: fusion threshold. What that means is that I can flash 197 00:14:37,880 --> 00:14:41,160 Speaker 1: a light on and off more slowly and the person 198 00:14:41,200 --> 00:14:44,880 Speaker 1: will see that as a steady light. In other words, 199 00:14:44,920 --> 00:14:48,400 Speaker 1: persistence lasts longer. Now I'm telling you that little detail 200 00:14:48,400 --> 00:14:52,360 Speaker 1: about schizophrenia because this allowed me to build a purely 201 00:14:52,640 --> 00:14:56,640 Speaker 1: visual test for diagnosing schizophrenia, and this is something I'm 202 00:14:56,680 --> 00:14:59,960 Speaker 1: currently running tests on at Stanford. If you're interested, I'm 203 00:15:00,120 --> 00:15:17,160 Speaker 1: linking my papers on this to the show notes. So 204 00:15:17,720 --> 00:15:20,520 Speaker 1: let's zoom out to what we've seen so far about 205 00:15:20,640 --> 00:15:25,040 Speaker 1: how the brain makes very short signals persist. The brain 206 00:15:25,120 --> 00:15:29,080 Speaker 1: constructs what the psychologist William James a century ago called 207 00:15:29,520 --> 00:15:34,320 Speaker 1: this specious present, by which he meant that the present 208 00:15:34,440 --> 00:15:38,360 Speaker 1: moment is not actually an instant, but it's a short 209 00:15:38,560 --> 00:15:44,120 Speaker 1: time window. This little sliding window of now causes the 210 00:15:44,160 --> 00:15:48,880 Speaker 1: present moment to have a certain thickness. Everything sticks around 211 00:15:48,920 --> 00:15:51,560 Speaker 1: for a bit in the brain so that events in 212 00:15:51,760 --> 00:15:56,320 Speaker 1: time can get compared. In other words, your neural circuits 213 00:15:56,400 --> 00:16:00,600 Speaker 1: refuse to let the world disappear too quickly. And we 214 00:16:00,640 --> 00:16:05,120 Speaker 1: see this kind of temporal integration in every sensory system, 215 00:16:05,120 --> 00:16:09,080 Speaker 1: which allows the immediate past to remain active in the present, 216 00:16:09,360 --> 00:16:12,960 Speaker 1: whether the brain is stitching notes into music or still 217 00:16:13,040 --> 00:16:17,640 Speaker 1: frames into motion. It relies on short lived memory to 218 00:16:17,880 --> 00:16:24,080 Speaker 1: transform fragments into meaning. So persistence is something we talk 219 00:16:24,160 --> 00:16:27,160 Speaker 1: about in neuroscience, but today I want to explore the 220 00:16:27,200 --> 00:16:32,640 Speaker 1: notion of persistence from animals to nations to civilizations. And 221 00:16:32,680 --> 00:16:35,720 Speaker 1: in thinking about this, I now think there are five 222 00:16:36,040 --> 00:16:40,160 Speaker 1: main ways that things persist. So let's dive in. The 223 00:16:40,240 --> 00:16:44,800 Speaker 1: First reason something persists is that it's optimized on some 224 00:16:44,960 --> 00:16:49,960 Speaker 1: fitness landscape, meaning that it can't really get better. So generally, 225 00:16:50,040 --> 00:16:54,960 Speaker 1: when we think about evolution, we picture a constant transformation 226 00:16:55,080 --> 00:16:58,720 Speaker 1: of species, with new forms replacing old ones, like the 227 00:16:59,080 --> 00:17:03,600 Speaker 1: cartoon of Homo sapiens evolving from a common ancestor with 228 00:17:03,680 --> 00:17:07,720 Speaker 1: great apes until he's walking upright. But what's interesting is 229 00:17:07,760 --> 00:17:11,800 Speaker 1: that some animals haven't changed in hundreds of millions of years, 230 00:17:11,920 --> 00:17:16,240 Speaker 1: like sharks. Little details have shifted, but the overall body 231 00:17:16,280 --> 00:17:21,639 Speaker 1: plan has persisted. Why Well, because they work, they're streamlined, 232 00:17:21,720 --> 00:17:26,480 Speaker 1: they have very efficient propulsion, and their sensory systems are 233 00:17:26,520 --> 00:17:31,320 Speaker 1: perfectly tunfoer hunting, so the design continues to succeed. So 234 00:17:31,480 --> 00:17:35,960 Speaker 1: this first reason is obvious. Some things persist because they 235 00:17:35,960 --> 00:17:39,600 Speaker 1: are at or near a local optimum, and if they 236 00:17:39,600 --> 00:17:43,240 Speaker 1: were to change, that would probably introduce risk, as in, 237 00:17:43,320 --> 00:17:45,679 Speaker 1: you're more likely to muck up the system rather than 238 00:17:45,720 --> 00:17:50,720 Speaker 1: to improve it. So stability becomes the winning strategy, and 239 00:17:50,840 --> 00:17:54,639 Speaker 1: persistence here is just a signal of success. And of 240 00:17:54,640 --> 00:17:58,840 Speaker 1: course we see this with everything. Technologies persist when they 241 00:17:58,920 --> 00:18:03,359 Speaker 1: meet needs efficiently. We can see these kind of technological 242 00:18:03,720 --> 00:18:08,680 Speaker 1: sharks everywhere. The bicycle rolling down the street still has 243 00:18:08,720 --> 00:18:12,400 Speaker 1: the triangular frame that was designed over a century ago. 244 00:18:13,000 --> 00:18:16,240 Speaker 1: It's a design that balances strength and weight and efficiency 245 00:18:16,640 --> 00:18:21,400 Speaker 1: so well that most attempts to reinvent the bicycle these 246 00:18:21,440 --> 00:18:27,240 Speaker 1: remain curiosities. Or look at steel shipping containers. These stack 247 00:18:27,320 --> 00:18:31,080 Speaker 1: and ports around the world. They have standardized dimensions and 248 00:18:31,119 --> 00:18:34,680 Speaker 1: that locks ships and cranes and trucks and warehouses into 249 00:18:34,720 --> 00:18:37,960 Speaker 1: a single, coordinated system that would only get messed up 250 00:18:38,000 --> 00:18:40,080 Speaker 1: if you tried to change it. Or look at this 251 00:18:40,240 --> 00:18:46,679 Speaker 1: screw with its spiral thread which translates rotation into linear force. 252 00:18:47,240 --> 00:18:51,800 Speaker 1: This solves its little mechanical problems so cleanly that its 253 00:18:51,880 --> 00:18:56,480 Speaker 1: basic form has persisted for centuries. Or take the four 254 00:18:56,600 --> 00:19:01,760 Speaker 1: tined fork. This is centuries. There are variations, with the 255 00:19:01,800 --> 00:19:05,479 Speaker 1: basic form of a fork stays the same because it 256 00:19:05,520 --> 00:19:11,120 Speaker 1: accomplishes spearing and lifting efficiently across lots of food types. 257 00:19:11,600 --> 00:19:15,600 Speaker 1: So these technologies persist because they reached a place of 258 00:19:15,640 --> 00:19:20,760 Speaker 1: alignment with performance and costs and usability. Just like this shark, 259 00:19:21,040 --> 00:19:26,359 Speaker 1: they solve their ecological problems efficiently, and once a form 260 00:19:26,480 --> 00:19:30,000 Speaker 1: settles into a valley where it works well, staying there 261 00:19:30,600 --> 00:19:35,800 Speaker 1: often proves a better decision than constant reinvention. So, like 262 00:19:35,840 --> 00:19:38,399 Speaker 1: I said, I think that's the obvious reason why some 263 00:19:38,440 --> 00:19:41,359 Speaker 1: things persist. But I was thinking about this, and I 264 00:19:41,400 --> 00:19:45,760 Speaker 1: think there are four more interesting angles that we could 265 00:19:45,760 --> 00:19:51,080 Speaker 1: take on this about other reasons why things last. For 266 00:19:51,160 --> 00:19:53,679 Speaker 1: the first one, let's turn to something I think about often. 267 00:19:54,119 --> 00:19:57,440 Speaker 1: Why you can recognize a friend that you haven't seen 268 00:19:57,480 --> 00:20:00,159 Speaker 1: in like ten years. This happened to me at my 269 00:20:00,400 --> 00:20:03,880 Speaker 1: high school reunion, where I saw people whose hair had 270 00:20:03,960 --> 00:20:07,359 Speaker 1: changed entirely, maybe they didn't have hair anymore, Their face 271 00:20:07,400 --> 00:20:10,639 Speaker 1: had changed shape a bit, their body had changed shape, 272 00:20:11,040 --> 00:20:15,240 Speaker 1: and yet I was able to mostly recognize them, so 273 00:20:15,359 --> 00:20:20,280 Speaker 1: something about them remained unmistakable. How do our brains allow 274 00:20:20,400 --> 00:20:24,760 Speaker 1: someone's identity to persist for so long? And, by the way, 275 00:20:24,800 --> 00:20:27,160 Speaker 1: I've wondered the same thing when I look at Google 276 00:20:27,359 --> 00:20:31,520 Speaker 1: or Apple photo albums. In these photo albums, they recognize 277 00:20:31,560 --> 00:20:34,720 Speaker 1: you or your kids through decades of time, even though 278 00:20:34,760 --> 00:20:37,920 Speaker 1: you can see that everyone's faces have changed a lot, 279 00:20:38,040 --> 00:20:44,119 Speaker 1: everyone's gotten older. So what's going on with brains and algorithms? Okay, 280 00:20:44,160 --> 00:20:46,480 Speaker 1: for both of these, it's simply that they're figuring out 281 00:20:47,040 --> 00:20:51,879 Speaker 1: which aspects of a face are worth carrying forward. They're 282 00:20:51,920 --> 00:20:56,719 Speaker 1: sensitive to deep structure rather than the surface detail, so 283 00:20:57,080 --> 00:21:00,639 Speaker 1: things like the spacing of features, how how far apart 284 00:21:00,680 --> 00:21:03,320 Speaker 1: your eyes are, and how that relates to your nose, 285 00:21:03,480 --> 00:21:06,560 Speaker 1: and the details of your mouth and your smile and 286 00:21:06,840 --> 00:21:10,119 Speaker 1: your filterum, that's the vertical groove that connects to the 287 00:21:10,119 --> 00:21:12,520 Speaker 1: bottom of the nose to your upper lip. All that 288 00:21:12,640 --> 00:21:18,399 Speaker 1: geometry remains stable enough to anchor down your identity even 289 00:21:18,440 --> 00:21:22,960 Speaker 1: while everything else changes. This is why airport security cameras 290 00:21:23,000 --> 00:21:26,240 Speaker 1: can recognize the face even if you dye your hair 291 00:21:26,680 --> 00:21:31,600 Speaker 1: or wear colored contacts or whatever. Recognition works because certain 292 00:21:31,640 --> 00:21:36,480 Speaker 1: information persists, and the algorithms can ignore certain kinds of change, 293 00:21:36,560 --> 00:21:39,560 Speaker 1: like shifts in the lighting or changes in the hairstyle. 294 00:21:39,840 --> 00:21:42,639 Speaker 1: They filter all that stuff away. And for your brain, 295 00:21:42,680 --> 00:21:47,320 Speaker 1: in particular, it's training on faces from infancy onward. It 296 00:21:47,400 --> 00:21:51,280 Speaker 1: sees them again and again, and it learns which features 297 00:21:51,720 --> 00:21:56,960 Speaker 1: persist across situations. Over time, it builds an internal model 298 00:21:57,359 --> 00:22:03,000 Speaker 1: that figures out what matters enough to preserve. This question 299 00:22:03,119 --> 00:22:07,960 Speaker 1: of what information to keep is asked by memory. More generally, 300 00:22:08,400 --> 00:22:12,040 Speaker 1: most of your world passes through you and you don't 301 00:22:12,040 --> 00:22:17,400 Speaker 1: remember it. Your memory only retains some experiences and decides 302 00:22:17,440 --> 00:22:21,359 Speaker 1: the rest just doesn't matter. And in fact, culture works 303 00:22:21,359 --> 00:22:25,240 Speaker 1: this way too, It repeats practices that continue to matter 304 00:22:25,760 --> 00:22:30,560 Speaker 1: while letting most others drift into obscurity. What these all 305 00:22:30,600 --> 00:22:35,439 Speaker 1: have in common is that persistence depends on abstraction. What 306 00:22:35,560 --> 00:22:39,080 Speaker 1: survives is something a little removed from the details. So 307 00:22:39,240 --> 00:22:43,240 Speaker 1: the brain's ability to recognize a familiar face after decades 308 00:22:43,800 --> 00:22:48,240 Speaker 1: is one example of a universal strategy, which is preserving 309 00:22:48,440 --> 00:22:54,200 Speaker 1: identity by learning what to ignore. In other words, persistence 310 00:22:54,440 --> 00:22:58,080 Speaker 1: is about knowing what to hold on to. And nowhere 311 00:22:58,160 --> 00:23:00,360 Speaker 1: does this apply more than to the notion of your 312 00:23:00,440 --> 00:23:05,040 Speaker 1: own self. You consider you you, even though you have 313 00:23:05,160 --> 00:23:08,560 Speaker 1: changed enormously over the years. You're not the same person 314 00:23:08,920 --> 00:23:11,800 Speaker 1: that you were five years ago, and certainly all your 315 00:23:11,960 --> 00:23:15,800 Speaker 1: cells have changed. I refer you to Inner Cosmos episode 316 00:23:15,840 --> 00:23:19,040 Speaker 1: eighty two, about the continuity of the Self and the 317 00:23:19,080 --> 00:23:21,560 Speaker 1: ship of theseus for more on this front. But The 318 00:23:21,600 --> 00:23:24,800 Speaker 1: point I want to emphasize now is simply that you 319 00:23:25,240 --> 00:23:29,320 Speaker 1: change through time, but you have certain things in common, 320 00:23:29,440 --> 00:23:32,000 Speaker 1: like your name and the stories you like to tell, 321 00:23:32,040 --> 00:23:34,160 Speaker 1: and where did you go to college and so on, 322 00:23:34,600 --> 00:23:38,280 Speaker 1: and you end up with the illusion of a stable 323 00:23:38,440 --> 00:23:42,800 Speaker 1: self that persists through time. Now here's the thing that 324 00:23:42,880 --> 00:23:47,120 Speaker 1: might come as a surprise. The persistence of your self 325 00:23:47,720 --> 00:23:50,719 Speaker 1: is something that you have to learn. This is what 326 00:23:50,760 --> 00:23:55,360 Speaker 1: we call a cognitive development. So look at very young children. 327 00:23:55,840 --> 00:23:59,600 Speaker 1: A young child can recognize her reflection in a mirror 328 00:24:00,040 --> 00:24:03,000 Speaker 1: pretty early in life. She notices that when she smiles, 329 00:24:03,040 --> 00:24:06,040 Speaker 1: the mirror image smiles. She notices that when she touches 330 00:24:06,080 --> 00:24:09,080 Speaker 1: her face, so does the mirror image. So that's good. 331 00:24:09,560 --> 00:24:15,560 Speaker 1: But this kind of self recognition doesn't yet work across time. 332 00:24:16,119 --> 00:24:19,280 Speaker 1: If you show that same child a video of herself 333 00:24:19,600 --> 00:24:23,960 Speaker 1: from a year earlier, something surprising happens. She might point 334 00:24:24,000 --> 00:24:26,879 Speaker 1: to the screen and say that's a girl, or she 335 00:24:27,000 --> 00:24:31,080 Speaker 1: calls the child by another name. The idea that this 336 00:24:31,359 --> 00:24:37,000 Speaker 1: earlier version of her belongs to herself hasn't yet taken hold. 337 00:24:37,760 --> 00:24:42,200 Speaker 1: So selfhood across time is something we have to figure out. 338 00:24:42,560 --> 00:24:45,359 Speaker 1: What happens over the first few years of life is 339 00:24:45,359 --> 00:24:50,920 Speaker 1: that children gradually assemble a sense of persistence. They come 340 00:24:50,920 --> 00:24:55,000 Speaker 1: to understand that the person who fell down yesterday is 341 00:24:55,040 --> 00:24:58,600 Speaker 1: the same one standing here right now. This depends on 342 00:24:58,840 --> 00:25:03,800 Speaker 1: memory and languag whig and narrative all developing together, so 343 00:25:03,920 --> 00:25:07,720 Speaker 1: the self becomes a story that can be updated while 344 00:25:07,800 --> 00:25:11,840 Speaker 1: retaining a central thread. Now, one consequence of this is 345 00:25:11,880 --> 00:25:16,119 Speaker 1: we all have what's called childhood amnesia, meaning we don't 346 00:25:16,359 --> 00:25:20,000 Speaker 1: remember anything from the first few years of our lives. Now, 347 00:25:20,040 --> 00:25:23,000 Speaker 1: it's not that you didn't have memory during those first 348 00:25:23,040 --> 00:25:25,639 Speaker 1: few years, because you were remembering a ton of stuff 349 00:25:25,640 --> 00:25:29,280 Speaker 1: and learning from it. But what was missing was a 350 00:25:29,440 --> 00:25:35,600 Speaker 1: durable narrative to scaffold everything. You had experiences, but there 351 00:25:35,680 --> 00:25:40,439 Speaker 1: was no persistent identity that you could stitch them into. 352 00:25:41,280 --> 00:25:46,480 Speaker 1: As you grew, you began to tag experiences as belonging 353 00:25:46,800 --> 00:25:50,800 Speaker 1: to you. You learned which details madded enough to carry 354 00:25:50,920 --> 00:25:55,320 Speaker 1: forward and which didn't really matter. So your identity emerged 355 00:25:55,400 --> 00:26:01,040 Speaker 1: as a stable pattern extracted from constant chain, just like 356 00:26:01,240 --> 00:26:05,159 Speaker 1: with face recognition algorithms. And now on to the next 357 00:26:05,200 --> 00:26:08,439 Speaker 1: way that some things persist, and that has to do 358 00:26:08,520 --> 00:26:12,800 Speaker 1: with self repair. So, for example, if you walk along 359 00:26:12,880 --> 00:26:17,520 Speaker 1: an ancient Roman Harbor, you can still find concrete structures 360 00:26:17,960 --> 00:26:23,000 Speaker 1: that have endured wave after wave for two thousand years. 361 00:26:23,480 --> 00:26:27,840 Speaker 1: These blocks were sitting in seawater, exposed to salt and 362 00:26:27,880 --> 00:26:32,680 Speaker 1: stress and erosion, but they only got better with time. Now, 363 00:26:32,720 --> 00:26:36,920 Speaker 1: how in the world does Roman concrete shrug off centuries 364 00:26:37,320 --> 00:26:42,760 Speaker 1: of battering while our modern concrete would spawl and corrode 365 00:26:42,880 --> 00:26:49,600 Speaker 1: under these circumstances, Roman concrete persists because it can self repair. 366 00:26:50,040 --> 00:26:53,560 Speaker 1: This is probably accidental, but it's baked into the chemistry. 367 00:26:54,040 --> 00:26:57,320 Speaker 1: In twenty twenty three, a team at MIT performed all 368 00:26:57,440 --> 00:27:01,320 Speaker 1: kinds of imaging and found that Roman builder hot mixed 369 00:27:01,400 --> 00:27:07,119 Speaker 1: their concrete with quicklime, which left tiny lime clasts scattered 370 00:27:07,160 --> 00:27:11,600 Speaker 1: through the mortar. And these act like self healing capsules. 371 00:27:11,960 --> 00:27:18,040 Speaker 1: So when micro cracks form and water seeps in, calcium 372 00:27:18,080 --> 00:27:24,280 Speaker 1: dissolves and reprecipitates this calcium carbonate, which seals the crack. So, 373 00:27:24,400 --> 00:27:29,440 Speaker 1: whether purposeful or not, the Romans achieved persistence of their structures. 374 00:27:29,680 --> 00:27:33,960 Speaker 1: So even though the concrete lasts, how did this knowledge fade? Well, 375 00:27:33,960 --> 00:27:39,680 Speaker 1: it's partly because history re optimizes, so various other recipes 376 00:27:39,720 --> 00:27:43,760 Speaker 1: for cement took over, most notably Portland cement by the 377 00:27:43,800 --> 00:27:48,240 Speaker 1: eighteen hundreds, mostly because Portland cement cures quickly and it 378 00:27:48,400 --> 00:27:52,680 Speaker 1: pairs well with steel. In other words, we switched goals 379 00:27:52,720 --> 00:27:57,120 Speaker 1: to speed and early strength and skyscrapers and so on, 380 00:27:57,440 --> 00:28:01,080 Speaker 1: and so the recipe changed. But just go stand in 381 00:28:01,119 --> 00:28:04,280 Speaker 1: an aqueduct and run your fingers over the mortar and 382 00:28:04,320 --> 00:28:08,840 Speaker 1: you'll see a material that knows how to persist across 383 00:28:09,240 --> 00:28:12,800 Speaker 1: centuries by self repair. And now I want to move 384 00:28:12,840 --> 00:28:15,800 Speaker 1: on to a fourth way that things persist. And by 385 00:28:15,800 --> 00:28:19,199 Speaker 1: the way, none of these methods are mutually exclusive. To 386 00:28:19,400 --> 00:28:23,080 Speaker 1: understand this, one walk into an old cathedral and look 387 00:28:23,119 --> 00:28:27,280 Speaker 1: closely at the windows. The glass often appears thicker at 388 00:28:27,320 --> 00:28:30,560 Speaker 1: the bottom than at the top. Why well, what was 389 00:28:30,560 --> 00:28:32,960 Speaker 1: explained to me when I was touring a cathedral in 390 00:28:33,040 --> 00:28:36,480 Speaker 1: Europe a while ago, was that the glass behaves like 391 00:28:36,560 --> 00:28:40,200 Speaker 1: a very slow liquid. You give it enough centuries and 392 00:28:40,240 --> 00:28:44,280 Speaker 1: it gradually flows downward under its own weight. What's cool 393 00:28:44,280 --> 00:28:47,360 Speaker 1: about this idea is that it invites you to imagine 394 00:28:47,440 --> 00:28:51,360 Speaker 1: time as a force so powerful that even solid matter 395 00:28:51,520 --> 00:28:54,720 Speaker 1: eventually yields. And by the way, I've heard this explanation 396 00:28:54,800 --> 00:28:59,640 Speaker 1: of cathedral glass a dozen times since then, But that 397 00:28:59,720 --> 00:29:05,880 Speaker 1: explanation is totally wrong. Glass at room temperature behaves as 398 00:29:05,920 --> 00:29:09,840 Speaker 1: a solid, and its viscosity is so high that any 399 00:29:10,000 --> 00:29:14,160 Speaker 1: flow would require time scales far longer than the age 400 00:29:14,200 --> 00:29:18,160 Speaker 1: of the universe. As it turns out, the uneven thickness 401 00:29:18,240 --> 00:29:24,240 Speaker 1: of old windows comes from manufacturing methods. Medieval glassmakers shaped 402 00:29:24,280 --> 00:29:29,280 Speaker 1: pains unevenly, and installers generally placed the thicker edge at 403 00:29:29,320 --> 00:29:33,160 Speaker 1: the bottom for stability. It has nothing to do with 404 00:29:33,360 --> 00:29:37,440 Speaker 1: gravity pulling on it, so the glass hasn't moved. But 405 00:29:37,520 --> 00:29:41,720 Speaker 1: the myth goes on, and the reason the myth persists 406 00:29:42,320 --> 00:29:46,000 Speaker 1: is more interesting than the myth itself. The reason the 407 00:29:46,040 --> 00:29:50,680 Speaker 1: myth persists is that ideas follow rules of survival that 408 00:29:50,800 --> 00:29:55,800 Speaker 1: differ from the rules governing matter. An idea can last 409 00:29:55,960 --> 00:30:00,280 Speaker 1: because it feels explanatory, or it resonates in some way. 410 00:30:01,120 --> 00:30:05,600 Speaker 1: Ideas persist not necessarily because they're true, but because they 411 00:30:05,600 --> 00:30:09,880 Speaker 1: have some other quality that makes them sticky. One might 412 00:30:10,000 --> 00:30:15,600 Speaker 1: hypothesize that the glass myth persists because vast spans of 413 00:30:15,640 --> 00:30:21,240 Speaker 1: time are awe inspiring, and or because the story translates 414 00:30:21,280 --> 00:30:25,239 Speaker 1: the abstract concept of time in something visible, and or 415 00:30:25,320 --> 00:30:28,960 Speaker 1: because it makes the person who says it sounds smart. 416 00:30:29,000 --> 00:30:33,360 Speaker 1: Whatever the reasons that it became sticky. Once an idea 417 00:30:33,520 --> 00:30:37,479 Speaker 1: is embedded in memory and culture, everyone starts to repeat 418 00:30:37,480 --> 00:30:41,080 Speaker 1: it and so it gets carried forward. So what this 419 00:30:41,280 --> 00:30:47,680 Speaker 1: tells us is that with human culture, persistence doesn't require accuracy, 420 00:30:48,400 --> 00:30:52,880 Speaker 1: just good transmission from brain to brain. And we see 421 00:30:52,880 --> 00:30:56,480 Speaker 1: this pattern everywhere, like with urban legends. With an urban legend, 422 00:30:56,800 --> 00:31:02,040 Speaker 1: a story begins somewhere, maybe with a little grain of plausibility, 423 00:31:02,360 --> 00:31:06,920 Speaker 1: and then it travels. There are urban legends about contaminated 424 00:31:06,920 --> 00:31:11,440 Speaker 1: Halloween candy, or vanishing hitchhikers, or a friend of a 425 00:31:11,480 --> 00:31:15,400 Speaker 1: friend who narrowly escaped disaster. And by the way, when 426 00:31:15,440 --> 00:31:18,480 Speaker 1: you travel around the world, you see that urban legends 427 00:31:18,840 --> 00:31:23,800 Speaker 1: shift their details according to local fears and cultural contexts, 428 00:31:23,800 --> 00:31:28,120 Speaker 1: but the core structure stays the same. Why do these 429 00:31:28,160 --> 00:31:32,520 Speaker 1: sorts of stories persist. In part, it's because urban legends 430 00:31:32,560 --> 00:31:37,880 Speaker 1: attach themselves to emotion, usually some combination of surprise and danger, 431 00:31:38,240 --> 00:31:42,760 Speaker 1: and this emotional component buffs up the recall and people 432 00:31:42,840 --> 00:31:47,480 Speaker 1: love retelling it, and the stories are shaped and refined 433 00:31:47,800 --> 00:31:51,600 Speaker 1: for memorability the way that good jokes are. So urban 434 00:31:51,680 --> 00:31:56,120 Speaker 1: legends unmask the same principle of persistence that the myth 435 00:31:56,280 --> 00:32:00,720 Speaker 1: about medieval Glass does, which is that ideas can survive 436 00:32:01,560 --> 00:32:06,240 Speaker 1: just by embedding in shared memory. Once a community starts 437 00:32:06,240 --> 00:32:11,760 Speaker 1: to repeat a story, that repetition becomes a stabilizing force, 438 00:32:12,120 --> 00:32:33,880 Speaker 1: and that lets it endure across time. Now beyond explanatory 439 00:32:34,000 --> 00:32:37,720 Speaker 1: stories about class and urban legends, this is true of 440 00:32:37,840 --> 00:32:42,760 Speaker 1: cultural beliefs of all sorts. Takes something like religion. Now. 441 00:32:42,800 --> 00:32:45,400 Speaker 1: I happen to be of the opinion that religions can 442 00:32:45,440 --> 00:32:49,280 Speaker 1: be very useful for people and communities. But I also 443 00:32:49,400 --> 00:32:52,120 Speaker 1: don't think, with two thousand religions on the globe, that 444 00:32:52,240 --> 00:32:54,719 Speaker 1: one of them is true and all the others are false. 445 00:32:54,960 --> 00:32:58,080 Speaker 1: But for today's purpose, what I think is fascinating is 446 00:32:58,160 --> 00:33:02,160 Speaker 1: the persistence of religions, the way their stories continue to 447 00:33:02,200 --> 00:33:07,440 Speaker 1: be told and retold across tens or hundreds of generations. Now, 448 00:33:07,720 --> 00:33:11,280 Speaker 1: like the glass or the urban legend, some of the 449 00:33:11,320 --> 00:33:15,600 Speaker 1: particular ideas and religions survive because they're well told stories, 450 00:33:15,840 --> 00:33:21,600 Speaker 1: and mostly because they strive to explain some meaningful mystery 451 00:33:21,680 --> 00:33:25,480 Speaker 1: in our lives. And I think there's a social flywheel 452 00:33:25,520 --> 00:33:28,880 Speaker 1: effect such that if your parents told you a story, 453 00:33:29,160 --> 00:33:31,880 Speaker 1: it feels very lovely to pass that on to your 454 00:33:31,920 --> 00:33:37,120 Speaker 1: own children. On top of that, you have ritual repetition, 455 00:33:37,200 --> 00:33:40,480 Speaker 1: you have texts, you have music, you have great architecture. 456 00:33:41,080 --> 00:33:46,200 Speaker 1: And religious systems also endure because they distribute information across 457 00:33:46,240 --> 00:33:51,360 Speaker 1: a huge swath of people and places, and this redundancy 458 00:33:51,960 --> 00:33:54,400 Speaker 1: creates resilience. We'll come back to that in a moment. 459 00:33:54,920 --> 00:34:01,000 Speaker 1: So put all this together, and religions are persistent engines. 460 00:34:01,480 --> 00:34:04,920 Speaker 1: They solve the problem of how to keep an idea 461 00:34:05,080 --> 00:34:10,920 Speaker 1: alive across centuries of change. In this way, cultural ideas 462 00:34:11,120 --> 00:34:15,480 Speaker 1: like religions can often look like a cultural habit, in 463 00:34:15,520 --> 00:34:19,720 Speaker 1: the sense that once they're established, they keep on trucking 464 00:34:19,760 --> 00:34:25,960 Speaker 1: along automatically even when their origins fade. Sticky religions persist 465 00:34:26,120 --> 00:34:30,200 Speaker 1: because they prove effective at capturing attention and moving from 466 00:34:30,280 --> 00:34:33,000 Speaker 1: one mind to another. By the way, the reason I 467 00:34:33,040 --> 00:34:36,920 Speaker 1: say sticky religions is because there are tens of thousands 468 00:34:36,960 --> 00:34:40,239 Speaker 1: of other religions that we don't even see anymore. They 469 00:34:40,320 --> 00:34:44,239 Speaker 1: lasted for a long time, and people gave up their 470 00:34:44,320 --> 00:34:49,080 Speaker 1: life for those deities, but the religions weren't sufficiently well 471 00:34:49,120 --> 00:34:53,919 Speaker 1: constructed to persist to the present. So we see that 472 00:34:54,080 --> 00:34:58,319 Speaker 1: religions and many type of cultural stories last because they 473 00:34:58,320 --> 00:35:03,760 Speaker 1: were optimized for transmit and that lets us revisit Greek fire, 474 00:35:03,840 --> 00:35:08,400 Speaker 1: which I mentioned at the beginning. Greek Fire illustrates the converse. 475 00:35:08,719 --> 00:35:11,400 Speaker 1: I told you that Greek fire was a weapon so 476 00:35:11,680 --> 00:35:14,680 Speaker 1: powerful that as shaped to the fates of empires. But 477 00:35:15,520 --> 00:35:21,279 Speaker 1: the formula vanished precisely because there wasn't enough redundancy for 478 00:35:21,480 --> 00:35:26,560 Speaker 1: carrying it forward. The bus factor was too small. Greek 479 00:35:26,640 --> 00:35:32,080 Speaker 1: fire was super powerful, but power didn't translate into endurance. 480 00:35:32,600 --> 00:35:38,240 Speaker 1: So persistence isn't just about impact, but it depends on transmission. 481 00:35:39,280 --> 00:35:42,240 Speaker 1: So back to the big picture. Some things persist because 482 00:35:42,719 --> 00:35:46,319 Speaker 1: they're optimal, or at least locally optimal, like sharks. Other 483 00:35:46,440 --> 00:35:51,160 Speaker 1: things endure because they're strong and self healing, like Roman concrete, 484 00:35:51,280 --> 00:35:53,839 Speaker 1: and we just saw that. Other things persist because they're 485 00:35:53,880 --> 00:35:58,759 Speaker 1: structured just right to be memorable. They're sticky, and once 486 00:35:58,800 --> 00:36:02,759 Speaker 1: an idea digs in, it can outlast the originators in 487 00:36:02,800 --> 00:36:05,040 Speaker 1: any evidence that got it started in the first place. 488 00:36:05,520 --> 00:36:10,399 Speaker 1: And the idea's mechanism of transmission is moving from one 489 00:36:10,520 --> 00:36:13,920 Speaker 1: brain to another. And now I want to turn to 490 00:36:14,000 --> 00:36:17,200 Speaker 1: the fifth reason why some things last, And for that, 491 00:36:17,680 --> 00:36:20,120 Speaker 1: think about when you were a kid and you played 492 00:36:20,239 --> 00:36:23,560 Speaker 1: rockstis or paper, and one of the rules was that 493 00:36:23,760 --> 00:36:28,120 Speaker 1: paper beats rock. I was thinking about this recently in 494 00:36:28,200 --> 00:36:32,680 Speaker 1: terms of the way that information gets written down. Until recently, 495 00:36:32,760 --> 00:36:37,280 Speaker 1: almost two thousand years ago, people used to carve proclamations 496 00:36:37,320 --> 00:36:42,800 Speaker 1: into stone, and stone has the advantage of feeling really permanent. 497 00:36:43,600 --> 00:36:47,160 Speaker 1: Then paper got introduced almost too millennia ago in China 498 00:36:47,200 --> 00:36:50,520 Speaker 1: and in Europe about a thousand years ago. The problem 499 00:36:50,560 --> 00:36:54,719 Speaker 1: with paper is that it's very fragile by comparison. It tears, 500 00:36:55,000 --> 00:37:00,320 Speaker 1: and it burns and it disintegrates. But what's happened cross 501 00:37:00,400 --> 00:37:06,600 Speaker 1: history is that paper has outlasted stone. Why it's because 502 00:37:07,160 --> 00:37:12,120 Speaker 1: monuments erode. Wherever you find any ancient inscription, you'll see 503 00:37:12,160 --> 00:37:16,600 Speaker 1: that time has softened it and it's nearing disappearance. You 504 00:37:16,640 --> 00:37:21,440 Speaker 1: see that ancient statues lose their faces and their name plaques. 505 00:37:21,880 --> 00:37:27,040 Speaker 1: So as material, stone persists, but the information carved into 506 00:37:27,120 --> 00:37:32,200 Speaker 1: it gradually fades away. Now, the surprising contrast is that 507 00:37:32,760 --> 00:37:39,160 Speaker 1: ideas written on paper have much longer lives. Why it's 508 00:37:39,200 --> 00:37:44,040 Speaker 1: because they're copied and recopied and translated and rewritten. Here's 509 00:37:44,080 --> 00:37:50,719 Speaker 1: the key. The individual fragile pages vanished, but the pattern survives. 510 00:37:51,040 --> 00:37:54,040 Speaker 1: It seems so crazy that paper could beat rock, but 511 00:37:54,120 --> 00:37:58,440 Speaker 1: it does because it can be easily replaced. So what 512 00:37:58,480 --> 00:38:01,600 Speaker 1: does this mean? It means that your ability turns out 513 00:38:01,640 --> 00:38:05,560 Speaker 1: to matter less than replicability. A medium that can be 514 00:38:05,920 --> 00:38:11,640 Speaker 1: reproduced easily travels farther through time than one that demands 515 00:38:11,960 --> 00:38:17,120 Speaker 1: physical permanence. What survives is what can be regenerated when 516 00:38:17,160 --> 00:38:19,880 Speaker 1: it breaks. And once you start looking for this principle, 517 00:38:20,160 --> 00:38:25,120 Speaker 1: you'll see it everywhere in nature. Take genes. The wild 518 00:38:25,160 --> 00:38:28,680 Speaker 1: part about the DNA code is how long it's survived 519 00:38:28,960 --> 00:38:33,719 Speaker 1: and how massively successful it is. This simple little codebook 520 00:38:33,760 --> 00:38:38,120 Speaker 1: has survived across billions of years, and the key is that, 521 00:38:38,360 --> 00:38:43,879 Speaker 1: just like with paper, its longevity doesn't come from physical toughness. 522 00:38:44,120 --> 00:38:49,719 Speaker 1: Individual DNA molecules are unbelievably fragile. They break easily when 523 00:38:49,760 --> 00:38:52,600 Speaker 1: you subject them to heat or other kinds of radiation 524 00:38:52,880 --> 00:38:56,800 Speaker 1: or chemicals they don't like. But the genetic code keeps 525 00:38:56,880 --> 00:39:02,080 Speaker 1: on trucking. Why DNA persists because it is copied. Every 526 00:39:02,160 --> 00:39:07,000 Speaker 1: time a cell divides, its genetic information is rewritten onto 527 00:39:07,080 --> 00:39:10,000 Speaker 1: new base pairs. And whenever you do that, you check 528 00:39:10,040 --> 00:39:12,120 Speaker 1: for errors and try to correct mistakes, and you scan 529 00:39:12,239 --> 00:39:14,879 Speaker 1: for damage and fix it. But the key is that 530 00:39:14,920 --> 00:39:19,560 Speaker 1: the original molecule doesn't need to survive as long as 531 00:39:19,600 --> 00:39:25,160 Speaker 1: the pattern survives. This makes DNA less like stone and 532 00:39:25,239 --> 00:39:30,440 Speaker 1: more like paper. The medium is expendable and the information 533 00:39:30,640 --> 00:39:33,719 Speaker 1: is what matters. Even though the molecule is fragile, the 534 00:39:33,920 --> 00:39:39,200 Speaker 1: information continues. And this is the same principle with memories 535 00:39:39,360 --> 00:39:43,280 Speaker 1: persisting in your brain. You can still remember, for example, 536 00:39:43,400 --> 00:39:47,480 Speaker 1: your third grade teacher's name. Now, why is that weird? 537 00:39:47,960 --> 00:39:51,359 Speaker 1: It's because every cell in your body is getting all 538 00:39:51,400 --> 00:39:54,760 Speaker 1: its pieces and parts replaced every moment of your life, 539 00:39:55,160 --> 00:39:58,919 Speaker 1: such that you're an entirely new person. Some years later, 540 00:39:59,480 --> 00:40:02,160 Speaker 1: none of the the molecules that you had in your 541 00:40:02,200 --> 00:40:05,200 Speaker 1: body in the third grade are there anymore. It's all 542 00:40:05,280 --> 00:40:10,200 Speaker 1: new stuff. But you still have your memories and your 543 00:40:10,239 --> 00:40:14,400 Speaker 1: knowledge and your perception because those are stored in the 544 00:40:14,560 --> 00:40:20,239 Speaker 1: constantly rebuilding structure that persists. Your whole brain is made 545 00:40:20,280 --> 00:40:24,160 Speaker 1: of very fragile stuff, just little cells, but it keeps 546 00:40:24,200 --> 00:40:29,040 Speaker 1: the structure even as the details get replaced again and again. 547 00:40:29,480 --> 00:40:32,880 Speaker 1: So back to religions and cultural stories. Even more generally, 548 00:40:33,160 --> 00:40:36,400 Speaker 1: I argued in a minute ago that cultural stories last 549 00:40:36,520 --> 00:40:40,920 Speaker 1: because they're useful, they're memorable. They propose answers to questions 550 00:40:40,960 --> 00:40:44,239 Speaker 1: that people have, and they give rituals and social coherence. 551 00:40:44,400 --> 00:40:46,439 Speaker 1: But the interesting part here is that you can think 552 00:40:46,480 --> 00:40:50,400 Speaker 1: of something like the Catholic Church living on top of 553 00:40:51,040 --> 00:40:55,719 Speaker 1: generations of humans that are born. They absorb the religious 554 00:40:55,719 --> 00:40:58,160 Speaker 1: dogma from the elders, they pass it to their children, 555 00:40:58,440 --> 00:41:01,759 Speaker 1: and they pass away. The Catholic Church is something like 556 00:41:02,160 --> 00:41:09,400 Speaker 1: information living on top of constantly recycling humans. Culture persists 557 00:41:09,520 --> 00:41:16,319 Speaker 1: because ideas are reenacted and recopied across generation, just like 558 00:41:16,360 --> 00:41:19,320 Speaker 1: the way your memory of your third grade teacher lives 559 00:41:19,400 --> 00:41:25,080 Speaker 1: on top of constantly changing neurons. So across the spectrum 560 00:41:25,160 --> 00:41:31,960 Speaker 1: from brains to civilizations, persistence sometimes depends on renewal. Unlike 561 00:41:32,000 --> 00:41:36,080 Speaker 1: something that lasts because it's optimized or strong, Persistence very 562 00:41:36,120 --> 00:41:41,040 Speaker 1: often emerges from something fragile, like paper or DNA or 563 00:41:41,200 --> 00:41:45,360 Speaker 1: memories or humans, and you keep passing the message along. 564 00:41:45,719 --> 00:41:48,920 Speaker 1: In other words, what you want is not necessarily the 565 00:41:48,960 --> 00:41:53,960 Speaker 1: thing that doesn't get damaged. You want fragile elements organized 566 00:41:54,120 --> 00:42:00,359 Speaker 1: into replicating patterns. So let's wrap up. Today is a 567 00:42:00,840 --> 00:42:05,319 Speaker 1: meditation on how things last. And what we saw is 568 00:42:05,360 --> 00:42:11,000 Speaker 1: that across all scales from neurons to cultures. Persistence follows 569 00:42:11,440 --> 00:42:14,400 Speaker 1: just a few patterns. Systems that rely on a single 570 00:42:14,440 --> 00:42:20,240 Speaker 1: instance those vanish easily, but systems that distribute themselves endure, 571 00:42:20,520 --> 00:42:24,719 Speaker 1: and systems that can be repaired or self repair outlast. 572 00:42:24,840 --> 00:42:29,520 Speaker 1: Systems that demand perfection. Now, coming back to the beginning, 573 00:42:29,840 --> 00:42:32,799 Speaker 1: what I find interesting is that we're always told to 574 00:42:33,000 --> 00:42:36,160 Speaker 1: live in the present, But the fact is that the 575 00:42:36,200 --> 00:42:39,920 Speaker 1: world you inhabit is not just the knife edge of 576 00:42:40,000 --> 00:42:45,600 Speaker 1: the present. It's a dense accumulation of held moments. Every 577 00:42:45,960 --> 00:42:49,560 Speaker 1: habit you have, every story you tell, every building that 578 00:42:49,600 --> 00:42:52,920 Speaker 1: you live in, the culture that you are embedded in. 579 00:42:52,960 --> 00:42:56,840 Speaker 1: These are all solutions to the same problem, how to 580 00:42:56,960 --> 00:43:02,760 Speaker 1: keep things alive for longer. Even podcasts like this one 581 00:43:03,320 --> 00:43:07,759 Speaker 1: participate in that process. Sounds held in your brain for 582 00:43:07,880 --> 00:43:13,319 Speaker 1: seconds become ideas held for years. A story that you 583 00:43:13,440 --> 00:43:17,040 Speaker 1: hear becomes part of your internal landscape, maybe for the 584 00:43:17,120 --> 00:43:22,320 Speaker 1: rest of your life. The cosmos outside continues to change 585 00:43:22,400 --> 00:43:26,440 Speaker 1: every second in its vast motion, but the job of 586 00:43:26,480 --> 00:43:31,319 Speaker 1: your inner cosmos is to stitch moments together to make 587 00:43:31,800 --> 00:43:35,560 Speaker 1: some things in the world persist. And now we're well 588 00:43:35,600 --> 00:43:38,279 Speaker 1: positioned for next week's episode, where I'm going to talk 589 00:43:38,320 --> 00:43:42,240 Speaker 1: with Alexander Rose, the former director of the Long Now Foundation, 590 00:43:42,719 --> 00:43:46,000 Speaker 1: and we're going to dive into his research on organizations 591 00:43:46,040 --> 00:43:49,680 Speaker 1: that outlast their founders might say over a thousand years. 592 00:43:50,160 --> 00:43:56,320 Speaker 1: What makes them last? Join me next week to find out. 593 00:43:58,239 --> 00:44:00,640 Speaker 1: Go to eagleman dot com slash pod cast for more 594 00:44:00,680 --> 00:44:04,000 Speaker 1: information and to find further reading. Join the weekly discussions 595 00:44:04,040 --> 00:44:06,799 Speaker 1: on my substack and check out and subscribe to Inner 596 00:44:06,880 --> 00:44:09,680 Speaker 1: Cosmos on YouTube for videos of each episode and to 597 00:44:09,760 --> 00:44:13,399 Speaker 1: leave comments. Until next time. I'm David Eagleman, and this 598 00:44:13,600 --> 00:44:14,759 Speaker 1: is Inner Cosmos