The Man of Many Worlds, Part 1

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S1: Missing from lately.

S2: Sometimes you’re in the right place at the right time to provide the last moments of joy in someone’s life. And you don’t even know it. In October of 2001, Jay Wallace was scheduled to give a talk at Princeton’s philosophy department. He’s a philosophy professor at Berkeley and Princeton was where he got his Ph.D.. So in some ways it was a homecoming for him. It had already been postponed because of the terrorist attacks a month prior.

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S1: I think it was one of the first days that flights were possible across the country again.

S2: That’s. The talk was called the publicity of Reasons. He was arguing against the view that if you have a reason to do something, other people have a reason to help you do it. Jay didn’t think morality was that demanding. Instead, he wanted to argue that if you have a good reason to do something, other people have a reason not to interfere. You can think about that for a little bit. But whether you agree or not is not that relevant to the story. It’s about what happened during the talk. In his paper, Jake used the Greek letter Phi to stand for any kind of action. So his thesis would read in the paper, If you have a reason to fly, then other people have a reason not to interfere with you flying. But when he gave the talk,

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S1: did my cluelessness and excitement about being back in Princeton and talking to some of my former teachers and so on. I read it as my reasons to be, our reasons for other people not to interfere with my being.

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S3: I was oblivious to the scatological reading of Wallace’s thesis.

S2: That’s my friend, the philosopher Mark Schroeder, who was in the room at the time also.

S3: I was just really interested in the content.

S1: And of course, this provoked images in the audience of people standing at urinals tempted to interfere with the peeing of the people next to them.

S3: If you want to argue against the idea that if your friend has a reason to do something, you automatically have a reason to help them pin is pretty good counter-example. It’s actually.

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S1: And there was, you know, understandable kind of widespread kind of snickering in the audience

S2: in particular two of the most senior and eminent members of the department. Well, they got the giggles.

S1: Yes, they had the giggles. Exactly. And I particularly remember both Dick Jeffrey and David getting caught up in the moment.

S2: Dick Jeffrey was Richard C. Jeffrey, who was retired by then. He was famous for devising a rule for changing your mind rationally when you’re not quite sure whether your evidence is 100 percent true. And David was David Kellogg Louis reputed to be even then the greatest system building better physician since Gottfried Lynette’s in the 17th century.

S3: I remember raising my hand as high as I could at the very beginning of the question period to try to get into the questions early and keeping my hand up after every question who was called. And finally, it got down to the last four or five minutes of the question period after an hour of me holding my hand up. And finally, he points directly at me and I blasted into my question. I was like seven or eight sentences as I was going fast because I was so eager to ask this question. And Joie says, I’m sorry, I was calling on David. I was so embarrassed. I turned around to look at David. He was just giggling, giggling during the talk. He he said, That’s OK. Let Mark and ask this question. And David never asked this question.

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S2: Friday talks at Princeton start around 4:00 p.m. with questions and answers ending by sex. Then there’s a reception that lasts about an hour after which a group goes out to dinner in town.

S1: It was a pretty large dinner party. Actually, we were seated at a

S3: table restaurants in Princeton. They’re often BYOB, bring your own beverage. And so the group had brought along a couple of six packs of bottles of beer

S1: and ended the usual clumsy way. It took us a while to kind of figure out what to order, but we placed our orders. And then there was this just epic delay in getting any food to eat. In my memory, I think of it as being nearly an hour before any food came

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S2: at this point, we’re probably talking about 8:30 nine p.m., which wasn’t just annoying but dangerous for David Lewis. He was a diabetic. So without some carbohydrates in his system, he could go into an insulin shock. In fact, at that point, David was in trouble.

S3: I remember not being aware of David’s distress at first and then it coming up as a topic as he had another beer and was trying to get some calories and eventually the food was taking so

S1: long to get there. It just wasn’t showing up. Toward the end of this period, before the food arrived, he needed to go. David abruptly stood up. He walked out of the restaurant, and as far as I know, that’s the last time anyone who knew David saw him.

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S2: We don’t know how the rest of the night went for David Lewis. He didn’t drive. His house was another maybe 20 minute walk from the restaurant Steffi. His wife of many years had just left town that Friday for a race. It was a

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S4: bicycle. It wasn’t a race. It was a tour that Steffi.

S2: I interviewed her before she too passed away in 2019. It was a century bike tour that lasted all weekend

S4: and came home and discovered him stone dead. And it turns out that what he died of is what a lot of diabetics die, of which is diabetic induced sudden coronary heart failure.

S1: I was shocked, of course, you know, as I reflected on it in the days following, somehow it was salient, obviously to me that my talk had been the last philosophical event that David had attended. And I felt honestly a bit bad about that. I mean, David just lived for philosophy. That’s what he did. And, you know, I think he he respected me and I got along well with him. But I kind of wish that he’d been able to attend the talk that would have been more interesting or important or inspiring.

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S2: That’s selling yourself way too short. I mean, come on, he he seemed to be really engaged.

S1: Well, I think he found by malapropism incredibly funny. I don’t know how philosophically he gave us from Slate.

S5: This is high. Five Nation Philosophy in story form recording from Vassar College. Here’s Barry Lam

S2: On this season of High Five Nation, we’re beginning with a miniseries on the life and work of David Kellogg Lewis on the 20th anniversary of his death. You just heard about his last moments in this world, what he called the actual world. You’ll hear about some of his first. He was 60 years old. And on his view, there is a version of him who is still alive right now in another world. That version would have worked to solve a few more philosophical problems and would then retire to Australia and ride trains. His favorite pastime in his favorite country in both of these worlds, he’d remain an important and influential figure in 20th century thought that most of the public hasn’t heard about. It’s because he wasn’t a celebrity. He wasn’t political. And he had a slightly otherworldly quality to him, just like his philosophy. But he didn’t lack ambition. Right, some of the 17th century figures before him, he sought to unify math, science and human consciousness into a coherent whole. In four episodes through his friends, his colleagues, his students and his family. We’re going to hear about how an awkward, quiet man born into a body that never stopped betraying him. Ended up with the audacity to advance a theory of how everything fit together.

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S6: My name is Meghan Sullivan, I am the Woolsey Family College professor of philosophy at the University of Notre Dame.

S2: How did David Lewis contribute to our understanding of time travel and philosophy?

S6: Here’s why philosophers and some other people love David Lewis

S2: David Lewis came to a significant number of his philosophical views through thinking about time travel and the many paradoxes of thinking that a person could travel into the past and change it.

S6: That is,

S2: it was also his preferred way of teaching students philosophy. His undergraduate course at Princeton was centered around the paradoxes of time travel.

S6: So if you watch a lot of time travel movies, which I do and you’re even remotely awake during the movies, they almost always hit a big plot hole where you’re just, like, very confused about how the whole plot is working. I just watched the Tomorrow War. They’re traveling to the future to fight aliens, and there’s this question about what happens if you take people from the past and you send them to a future war and then they get killed, or are they not going to live in the present to create the future people?

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S2: And of course, you have the famous grandfather paradox, which is whether it’s possible to travel back in time and kill your grandfather so that you will never have been born. But if that’s true, you will never have traveled back in time to kill your grandfather, so you will be born.

S6: So that’s why people love David Lewis, is he? He kind of takes our concern for coherent, interesting, backward time travel seriously and then tries to use some tricks in analytic philosophy in the 20th century to make those stories work out. But other it’s really interesting about Louis is in the process of trying to explain to you how you might be able to go back in time and have an adventure. He also asked some interesting questions about what it is to be a person through time, and those have implications for our ordinary life.

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S2: David Lewis thought about all things in the world in four dimensions. It’s how he thought about people to. One of his signature views was that the four dimensional south was the only south there was. Imagine that you’re filming someone with a stationary camera, as she walked slowly from left to right from one end of the room to another. Then instead of running the film, you take each frame and lay them on top of each other. So in the end, you have a still picture of every moment of that person as they start from the left and end up on the right side of the room. What you’re seeing is a person in four dimensions all at once, maybe about 10 seconds of them. It looks like a streak through space, a worm. What you’re seeing is the four dimensional south. For David Lewis, a person isn’t an individual you meet in front of you in a room, in three dimensions. That’s only a part of them. The whole person is the entire streak, the entire worm that they inhabit in space and time from the moment of their birth to the moment of death. This is going to be the theory of the South that solves one of the paradoxes of time travel.

S7: Remember this day failed September

S2: 28, 1941, it may be a history making day. All eyes are on this man Ted Williams gunning for a war on racism. He’s getting it even. It’s September 28, 1941. The number one song is Blue Champagne by Jimmy Dorsey in a sign, and all of a sudden, Ted Williams stepped up to back on the final day of the regular season in a doubleheader, going six four to end the season four four oh seven batting average, a record which hasn’t been surpassed since. In Oberlin, Ohio, a town of 4300 people. David Kellogg, Lewis was born.

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S8: He, the eldest child of Ruth Hewitt, Kellogg, Louis, and she was a distinguished medieval historian. And John Lewis, he was a professor of government at Oberlin College.

S4: David is the eldest of three. He has a brother, John, who runs an auto repair shop in Austin, Texas, and who, like all of the Lewis, has all the brains in the world.

S9: Good folks. A little bit of academic snobbery there.

S2: Donald Lewis. David younger brother.

S9: More of my mother than my father. But our attitude totally passed by David, even with this incredible success academically. You once told me that if he traveled the. We’d like to sneak off and talk to the janitor where they were saying because it was more interesting,

S4: David sister Ellen was the baby of the family.

S6: Well, he was definitely my best, most nurturing family member of all. I’m Ellen Lewis. I am David Lewis sister. I was six years younger than him. Her parentage was highly academic and rigorous. We always had this little five fold seating at the dinner table every night, and I just remember being totally out lost. I was the kid and I was the dummy of the family, and the three of them were talking amongst themselves constantly and obviously viciously and understanding one another and sort of sometimes sparring. I just remember being totally left out, but in awe. And then when David and I were together, he talked at my level, which I just thought was incredibly kind of him. But I just felt like I had an ally in David. I was just the kid

S4: we went David was a boy. He had he had polio when he was, I don’t know, eight or nine years old.

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S9: It affected how we walked and that was all his life.

S6: And so he walked with a funny sort of a toes out step.

S9: We had one lazy eye that never worked very well. We were supposed to practice it by wearing a patch over the good eye. He hated to do that, and mother tried everything. You know, punish him for not doing a bribe and for doing it and everything, and he just wouldn’t do it. So his vision never was very good.

S4: So David was home home from school with polio, with Ewart being his his tutor. But while he was home, getting to the point where he could go back to school worked on medieval history with him, David was interested in history.

S6: He was not one, but two years younger than the kids in his class after he excelled by reading during his whole year in bed with polio. He probably just felt very, sort of physically, you know, not at their level.

S9: The or his field of science was chemistry. There was a room in the basement where he had a chemistry lab. We were in gas in there for his Bunsen burner and

S6: one of the most precious memories I have is watching him turn test to colors from blues to reds or the other way around. And just his love of learning and discovery was something that I I just always thrived with. I was very, very curious about science and about what we could observe in the world. And so was he, and I just felt like that was a special little world where we had a lot in common.

S9: Every Wednesday night, when my father has political science seminars upstairs, he would like to do something so that made a big stink, some kind of a big sulfur smell. You know, they come up from the basement to the living room. So he had, you know, some degree of mischief and. And.

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S2: The first paradox of time travel that David Lewis solves is the paradox of personal identity. Logic says that nothing can have contradictory properties at the same time, and Apple can’t be ripe and also not ripe simultaneously. David Lewis can’t be a chemist and not a chemist at the same time. And ordinarily, that’s fine. Apples and people change over time. They’re never contradictory at any given moment. But if David Lewis travels back in time as a 50 year old philosopher, he can walk into his childhood basement and look right at his younger self. And now David Lewis is both a chemist and not a chemist at the same time. He’s both 14 years old and 50 years old. He’s also standing and not standing. He’s one person, but he’s also two, and the contradictions multiply. The solution is simple once we see people as fundamentally four dimensional and not three dimensional objects, one part of David Lewis is standing next to another part of David Lewis. One part of David Lewis is 14. The other is 50. One part is standing, the other sitting. So there’s no contradiction. There’s only one person with two temporal parts. Just like one part of you is a neck and is straight. And another part of you was a finger that’s bent. The implications of this view are many. The 14 year old and the 50 year old David Lewis don’t have a thing in common, an essence that is three dimensional and that makes them two ages of the same person. No one has such an essence because if they did, the contradictions come back is the essence in a 14 year old who’s sitting or in a 50 year old who’s standing is the essence. One thing or two things. According to David Lewis, the possibility of time travel forces us to see the self as a four dimensional space time warm. At any given moment, you are just part of a person. Your younger and older selves are part of that same person. Only once your life is complete is the whole person complete from beginning to end. Birth to death. When all is calm down, come along with me, it’s spring 1957, the dawn of rock and roll for the second half of Dwight Eisenhower’s terms in Oberlin, Ohio. 15 year old David Lewis is about to drop out of high school

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S9: junior year of high school. He was taking half college courses. My mother wanted me to be more a little more of a normal, mainstream high school student who went on dates and had girlfriends and stuff dressed better and so forth.

S2: So I know this, but people out there don’t know it. David was not a suave dressing. Ladies, man, type person.

S9: No, not at all.

S2: Right? As far as you know, growing up, he didn’t express any interest in that kind of thing.

S9: None.

S4: She was one exam short of the regents requirement.

S9: He would have had to have taken civics that was taught by an athletic coach and think it was a worthwhile class. You would have had to have taken that to graduate from high school starting over there.

S4: So David Lewis is not a high school graduate.

S9: He went straight to Swarthmore and never did his senior year of high school.

S4: His first choice of major at Swarthmore was chemistry. He was a future scientist type. David father had a Fulbright one year and all of them spent the academic year of 1959 60 in Oxford.

S8: And while he was at Oxford, he also attended various lectures by, you know, some of the big names

S2: Alan Hajek friend and colleague.

S8: People like Grace and Austin and Straus. And while

S10: he was supervised by Iris Murdoch

S2: John Bigelow retired philosopher and she was

S10: a little annoyed with him that he wouldn’t read other philosophers much, he would just try to work out his own ideas on free will. But he was doing it by reason and logic, not by a leap of

S2: faith, by any means.

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S4: She took him in hand and. Made sure that he did of a lot of philosophy, of reading and took very great care over his papers in the way the tutorial system worked, that made a huge difference to him.

S2: That you’re in Oxford during the height of Oxford, philosophy was transformative for the teenage David Lewis. He returned to Swarthmore and switched to philosophy, where he joined a remarkable class of students, but ended up being giants themselves in the field and future colleagues. At the same time, other things happened that would also have lifelong consequences.

S9: Diabetes hit hard when you was like around 18 or 19, maybe 30 was home at Oberlin for Christmas vacation, eight a lot of rich food and stuff, and they just passed out. That’s why we went into a diabetic coma and we had a family doctor that everybody loved. There was a total idiot. Well, Fool didn’t figure it out very quickly. He always made wrong calls, you made wrong calls. It was me. He made wrong calls with everybody.

S4: Not with much in the way of lasting effect, though. It turned out life shortening cardiac consequences.

S2: Imagine a complete timeline of the world from the first moments of time until the last. Like any other timeline, you draw segments on it to tell you when events happened and for how long. That complete timeline. David Lewis called external time. It’s the fourth dimension of the universe. Every single event in the history and future of the universe will be on it. It’s a representation, a complete map of our entire world. The actual world. In our world, there isn’t any time travel. So everyone who was ever existed will be a smooth, continuous segment on that timeline. Dwight Eisenhower 1890 to 1968 David Lewis 1941 to 2001 But in a world where there is a time traveler, their appearance on the external timeline will not be one, but two maybe more segments. Meghan Sullivan Notre Dame

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S6: II next year invent a time machine and decide to go back to the year. My parents got married in 1980 and hang out with them through their early adulthood. So I live 40 years, then I get in the time machine and I go back to 1980. And let’s suppose I decide to never time travel again. After that, I just live out the rest of my life through the 80s and 90s. From the standpoint of like the manifold, if you look at like where’s the earliest Meghan worm part? It’s 1980. It was the latest Meghan worm part. It’s twenty twenty two. So from external time, I look like I took up forty two years. But in fact, in Meghan time, I lived 40 years the normal way. And then I probably lived another 40 years after getting out of the time machine in the 80s. So I lived in Meghan time. I got like 80 good years.

S2: Personal time is the length of an individual’s entire space-time worm. Every individual in the universe will have a worm of a certain length, depending on how long in time they’ve lived. But time individuals will have a spacetime worm that looks well caught up in the external timeline.

S6: My worm starts off normal, it starts off like a little baby grows, grows, grows until 2022, and then you look back in space time and you’ll see that another part of my worm just kind of appearing in that part of the spacetime manifold and then a big part of my worm could sit next to a smaller part of my worm the same way you could take an earthworm, you know, cut it in half and move the front part of its body to sit right next to the backrub. It’s a mean thing to do to a worm. Time travel is violent and brutal on its run because it literally takes part of you and just dislocates it or makes it discontinuous in a way that’s weird,

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S2: but it helps to solve the dreaded grandfather paradox.

S6: Suppose I want to go back in time and I want to kill baby Meghan. Like, you know, I want to commit suicide in a very interesting way. Not possible on David Lewis framework. This is where the philosophical fisticuffs start.

S2: The way Lewis thought about worlds in four dimensions is a lot like the way we think about worlds in three dimensions in common sense. We don’t think of spaces as appearing or existing simply because we inhabit them right now. That would be ridiculous. Space in the world with no people in it exists before people get to it and will continue to exist after people move out of it. The past and future are just dimensions in time, like east and west or dimensions in space. The future exists as much as the present, as much as the past. None of these times are waiting for current people and things to inhabit them to start existing

S6: for somebody like David Lewis. Nothing ever comes into or out of existence or like pops into existence or pops out of existence.

S2: In particular, facts about the world don’t pop into or out of existence in three dimensional space. If your house is painted white on the inside you, walking into the house doesn’t make the space and its color pop into existence. Similarly, in four dimensions the future and all of the facts that are true about the future don’t just pop into existence just because you come to live in it. Those facts were always there. David Lewis thought about worlds as being determined by all the facts that are true in them past, present and future. We live in a world where the Mongol empire happened in the 13th century, where Joe Biden is the president of the US today, a world in which the Mongol empire never came to power in Asia and where Joe Biden lost the election of 2020. These are alternate universes where the facts are different from our world, and this is true of future facts as much as past ones, every future fact. That is true of our world. To define our world as much as every past fact does. So if in 2024 Biden wins re-election, though, that future fact is part of our world. Any alternative fact is only true in an alternate universe, so back to time travel. If it’s a fact about our world that Meghan was born in 1982 and it’s a future fact that Meghan a 40, we’ll travel back in time and try to kill baby Meghan. Then it’s already determined in the world that Meghan survives to travel back in time to try to kill herself. Both facts exist in our world and have always existed and in fact defines the world that we live in. That’s why adult Meghan can’t kill baby Meghan. It’s not that she can’t. She doesn’t. And we know she doesn’t, because it’s already a fact that she survives long enough to travel back in time to try and kill her baby self. You traveled back. You have a gun, you’re pointing it at baby Meghan 1983 David Lewis is next to you. You’re about to shoot. Here’s what David Lewis is going to say. OK, you can fire, but you’re not going to die. Yeah, because here you are. So that’s what he’s going to say. That’s the David Lewis movie, the David Lewis movie. It could be one where you fire and then it backfires on you, right? And it doesn’t kill the baby and you try to fire again. And maybe like, I don’t know, everything can happen in that movie, but you dying a bird can fly in front of the bullet.

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S6: Yeah, there are all these like just miracles like really, really the way bullets normally fire out of a gun at close range. It works, but in this particular movie, we know that it doesn’t.

S2: Now here’s another movie, and this is the Looper movie. Now I’m battling my future self, and then I shoot myself. And then the shoot yourself poofs disappears in front of me. And boom, that’s it. Like, that’s not a David Lewis movie. That’s another movie. Now, is there an argument that that’s actually the coherent story and that David Lewis is wrong?

S6: Yes. And don’t imagine time is this like static spread out spacetime and instead think about time this way. Time is a growing block.

S2: This is the view advocated by Peter Van in Waggon. The past and present exists. But the future doesn’t until it too becomes the present. The growing bloc view says the future comes into existence, but it doesn’t exist eternally as a dimension extending into the whole of the future as on David Lewis view. This view implies that once someone travels into the past into 1980, since 1980 is now the present. Everything from 1980 to 2021 disappears.

S6: The whole block of spacetime goes poof, and that stops existing, which remember, is like the cardinal rule of four David Lewis is nothing ultimately stops existing.

S2: Now, you can’t change the future because the future doesn’t exist yet. But remember, the future was your past. You lived it before you traveled back in time. No, not on this view. You have these memories. These are now false memories. You have no past. There’s no time worm anywhere in the world that’s continuous with you. Now, you annihilated your history when you time traveled. David Lewis has a point on this alternative view of time. Time travel is genocide. It’s more than genocide. You’ve annihilated the entire universe for a period of 40 years in all of the ways it had evolved over that time. As for the grandfather paradox, you can kill the man that would be your grandfather. And once you have. You won’t be born in the future. But then how could you be here having time travel? Because you’re not the person who will have been born? You’re just some entity living in the present. Having killed another man? You have no history, that history was annihilated doesn’t exist anymore. You’re a free floating entity in time with no grandfather, no father, no mother. The present and past doesn’t contain anything in existence. Continuous with you. Your worm was annihilated when you time travel. That’s the price you pay for time travel in a growing block universe.

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S6: Now, is that the right view, is that what time really is and what people are know even when and wagging doesn’t believe that. But one of the things the philosophers are trying to do is just say, All right, let me give you ways it could make this logically consistent. But you’ll think about them and whether you want to make a movie about them. But you’ll also wonder, like, what does it mean to have control over your future?

S2: On David Lewis view, any time travel story where someone changes the past or future is actually an alternate universe story. There’s no annihilation, no genocide of people and times in the actual world. Given the four dimensional view of time and of persons and worlds, if you wanted to travel back into the past or into the future and stay in the same world, you’re going to have to live with the fact that everything you planned to do. You’ve already done. And everything already done can’t be undone. Every fact about the world, even your own time travel in it has been written into the very world itself. Change anything about the story of the world, and you’ve entered an alternate universe, not a different time in the same universe. So get it straight, is it time travel in the same world you’re interested in or travel from one universe to an alternate universe? Because those are very different things, and David Lewis has a lot to say about alternate universe is also. Next time on High Fine Nations miniseries, The Man of Many Worlds, the Harvard Years,

S4: he seemed attracted to me. I had known a lot of fairly strange people, inarticulate people in high school, so I was used to that.

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S2: David Lewis learns about love and language and comes close to failing out of his Ph.D. program.

S5: Hi Fi Nation is written, produced and edited by Barry Lam, associate professor and chair of philosophy at Vassar College. Executive producer of Slate Podcast is Alicia Montgomery, editorial director for Slate Podcast is Gabriel Roth, senior managing producer for Slate Podcast. Is June Thomas, managing producer for Slate Podcast is Asha Saluja, editor of Slate Pleases Me Channel two production assistants this season, provided by Jake Johnson with hyphenation. Talk for complete transcript show notes and reading list for every episode that AHIP Nation DaUg follow hi fi nation on Facebook and Twitter, and at the website for updates on stories and ideas.