 Thanks very much everyone for coming to this installment of the podcast live here at King's College London as part of the Arts and Humanities Festival. And I'm very grateful for you to showing up to hear me talk about my podcast and to give you an example of the kind of thing that I'm doing in the podcast. The podcast is something that I've been doing for about a year. It started in October 2010 and since it's a weekly podcast and since I skipped August, it's now published about 50 episodes. Actually 51, the 51st episode just went up on Sunday. And the idea of the project is to cover the entire history of philosophy without any gaps, that's sort of the slogan. So I want to say a little bit about what that means and the motivation of the project. And then like I said, I want to give you an example. First, just out of curiosity, how many of you actually listen to the podcast? So for the benefit of people listening to the later recorded version of this, there's about I would say maybe four or five hundred people here. And they all raise their hands. Right, so at the beginning of the podcast, obviously, I started with the very earliest philosophers, so the Presocratics. And then I moved on to the only person you can really move on to from the Presocratics, namely Socrates, almost by definition. That was supposed to be funny. Presocratics, Socrates, no? You have to laugh audibly, or the people listening to the recording. Thank you. Thank you. Please laugh audibly at all the jokes. And then obviously Plato and Aristotle, and just now I'm moving on to what happens in philosophy after Aristotle in the Hellenistic period. The podcast that just went up this past Sunday kind of illustrates what I'm trying to do in the whole project. And the point of the without any gaps slogan, which is that if you're studying philosophy, say in an undergraduate curriculum, like the undergraduate curriculum at King's. Then normally you would kind of visit the highlights, sort of like going around Europe and only visiting Paris, Berlin and London. And actually at King's we're unusual in that we offer modules on a very wide range of topics in the history of philosophy. But even at King's you can't study absolutely everything in the history of philosophy because there's just too much to cover. Whereas with a podcast, especially a weekly podcast, so it can cover a lot of ground very quickly. I thought I would be able to deal with basically the entire history of philosophy without leaving anything out. So this 51st episode that just came up is a good example of this. It deals with the immediate followers and students of Plato and Aristotle, the so-called Old Academy and Theophrastus. The Old Academy is basically a way of referring to these two philosophers, Spusippus and Xenocrates. And these are three people that I assume none of you would ever have heard of. Had anyone heard of these people before they got to that episode of the podcast maybe? One person. But she's cheating because she's a PhD student in philosophy. So that doesn't count. So these are clearly not household names. But not only are they interesting in their own right, but I think that if you leave out these minor figures, clearly you're going to miss something about the development of philosophy. So if you take, for example, a philosopher who's much, much later, like St. Thomas Aquinas, he's 13th century AD. Plato and Aristotle are 4th century BC. So if, as would often be the case, you're doing the history of philosophy, say as a student or as a casual reader or something, if you skip from Plato and Aristotle to Aquinas, and actually of course you might skip even further than that, you might skip to Descartes, but suppose that you skip to Aquinas. You just skipped over the better part of 2000 years. And to understand where Aquinas is coming from, you have to remember that he's not immediately right after Aristotle. So he's not immediately engaged with the works of Aristotle. He is engaged with the works of Aristotle, but as mediated by almost 2000 years of philosophical history, including, for example, the history of Platonism, which means the whole medieval history of the reception of Platonism, late antique Platonism, and then you can't understand late antique Platonism without understanding the immediate followers of Plato. So there's a sense in which to understand what a giant like Aquinas was doing, you have to understand what minor figures like Spusippus and Sinocrates were up to. So that's the idea of the project. In terms of scope, the idea is to sort of go through without leaving anything out. But there's also another kind of motivation, which is maybe in terms of breadth. So I tend to have a very generous, broad understanding of what philosophy is. And I think that to understand the history of philosophy, but also philosophy today, you need to pay attention to fields that we would now consider non-philosophical, and even fields that were considered non-philosophical at the time. It's kind of a truism, everybody knows this, that knows anything about the history of philosophy, that ancient and medieval philosophers considered disciplines that for us are not philosophy to be part of philosophy. So for example, physics or chemistry, the study of how one element changes into another, these are topics covered in the works of Aristotle, so they count as philosophy. And they would have been considered as a type of philosophy, natural philosophy, until very recently. But even such disciplines as mathematics and astronomy and medicine, which is something I'll be getting onto later in this talk, have always been intimately related with philosophy. Here at King's we actually have projects looking at the relationship between philosophy and medicine. And there have always been very fruitful interchanges between the physical sciences and philosophy. So this is something that goes way, way back, all the way through the history of philosophy. Later in this talk I'm going to be discussing the contribution that the most important person in the history of medicine, I would say Galen, who lived in the second century AD, made to the philosophical discussion of void, which is what I'm going to be talking about for most of the time. So one of the other goals of the podcast then is to look at the relationship between philosophy and some of these other disciplines, something I've already done a little bit because when I was going through the pre-Socratic material, I spent one episode talking about the Hippocratic corpus and how Hippocrates or the works ascribed to Hippocrates relate to ancient philosophy. And in later episodes I'll be doing that again with medicine, also with areas like rhetoric and astronomy and mathematics. Now obviously this project to cover the entire history of philosophy is not something I could sort of distill into a one hour lecture. So what I wanted to do is come up with a topic that would allow me to display some of the motives of the podcast in one nice little package. And I guess this fits into the theme of the Arts and Humanities Festival in that it basically means I want to tell you a little story. And the story is the story of an idea. The idea is the idea of nothing. So that's why the talk is called A Brief History of Nothing. And of course this is a cheap gag, right, because the slogan of the podcast is without any gaps. And as anyone who listens to the podcasts regularly or even casually knows, I cannot resist a cheap gag. So I sort of thought of the gag and then I wrote the whole talk to go around it, which is pretty much how I operate. So what I'm going to be trying to do is take you through the history of philosophical discussions about void. So what we might think of as empty space. Although as I'll be saying in a second, the first ancient philosophers and even some of the late ancient philosophers didn't necessarily conceive of void as empty space. And I'm going to try to follow this problem or topic, the topic of void, all the way from the pre-Socratics up to the 10th century AD in the work of a philosopher who I'm actually working on at the moment whose name is Abu Bakr Razi. And he lived in Iran and Iraq, like I say, in the 10th century AD. So this lecture is going to take us from before Plato and Aristotle, so from the 5th century BC all the way up to the 10th century AD. So one and a half thousand years. Ready? Are you excited? Yeah. So, maybe the first thing to say about the topic of void is that the Greek word for it is tōkennon, so the empty. And that already maybe shows you the way that they're thinking about it. They're thinking about it as a kind of area or we might say space where there's nothing in it. And our kind of default assumption that space is mostly empty is certainly not one that was universally shared by ancient philosophers or for that matter medieval philosophers. It's a very, or comparatively, recent phenomenon that philosophers and scientists think that void should be integrated into our picture of the natural world. And in fact, probably the very first time that anyone starts to discuss this topic of void or the empty is from the point of view of someone who's trying to say that it's not a real phenomenon. It's kind of hard to say that void doesn't exist, right? Because the whole point of void is that it's nothing. So to say that it doesn't exist sounds kind of paradoxical, which will be important in a moment. But just bear with me if I say that void doesn't exist, according to some of these philosophers. And in fact, it turns out to be a majority view among ancient and medieval philosophers that there's no such thing as void or that void doesn't exist. There are no empty spaces anywhere in the cosmos. Although there are exceptions, as we'll see. So the first person to take this up, as far as I know, is a philosopher named Melissa. So how many of you have heard of Melissa? Hmm. Not a household name. That's kind of depressing, actually, since all 500 of you have supposedly been listening to my podcast and I have covered Melissa's already. He's one of the followers of Parmenides, which makes him a so-called Eliatic. Parmenides of Elia, hence Eliatic, is the father of metaphysics, sometimes people say, and is well known for the view that being is one. There's no differentiation and no such thing as non-being. And it immediately follows from that. You might think that there's no such thing as the empty, right, because the empty would be a kind of non-being. This is made explicit by his student, Melissa, who is actually trying to argue for something much more surprising than the view that there's no such thing as void. He's trying to show you that there's no such thing as motion. And the reason for this is that he's a follower of Parmenides. Parmenides has taught that there's no such thing as differentiation or non-being or change, because they think of change as something that involves going from having a property to not having a property, which is a kind of non-being. So he says, look, the only way that something could move is if there was emptiness around it, because otherwise it will be blocked. So imagine if everything around you was completely full, kind of packed tight like the tube in rush hour. So suppose that everything is full. So suppose, as it were, that the air, suppose it's made of particles, suppose the air particles are completely packed together like cubes stacked against each other with no spaces between them. Well, given that the air on the far side of it will be pushed up against the walls, it seems obvious that you won't be able to move, right? So the only way you can move is if there's some emptiness, but of course there can't be any emptiness, because emptiness is a kind of non-being, and to say that non-being exists is clearly a contradiction. So this is his argument against emotion, and notice that it invokes as an assumption the impossibility of void. This shows, I think, that at least for some people in the ancient world, the very idea of void is kind of clearly absurd, because to assert the so-called existence of void is to assert that non-being exists, which doesn't make any sense. It's just a self-contradiction. Now, obviously matters did not rest there, and there's a kind of immediate reaction to the Eliatic philosophy on the part of some more famous people named the Atomists, well not named the Atomists, named things like Democritus. So have you all heard of Democritus? Yes? Alright. So Democritus, along with another philosopher named Leukippus, pioneers this new atomistic philosophy. And you could almost think of what they're doing as grasping the nettle of Melissa's argument. So they say, well look, if you look around you at the world, clearly there is motion, so if Melissa's is right that there needs to be emptiness in order for there to be motion, then so be it. So be it. There's emptiness. There's void. The idea that non-being's existence is contradictory is just something we'll have to kind of ignore or work around in order to preserve the obvious fact that motion does genuinely exist. And so notice that we in a way have a choice here between two apparently absurd assertions. One is the claim that motion doesn't exist, and the other is the claim that non-being exists. So either there's no such thing as motion, or there's such a thing as void. Now we in the modern world have no problem with this, because we also agree with the Atomists that bodies are made of particles moving around in empty space or void. But clearly this is something that they were having a lot of trouble with at the time, so they thought it was a really challenging idea. And to a large extent what was radical about Atomism was not just their contention that bodies were made of uncuttable parts, that's what the Greek word atoma means, is uncuttable. What was radical about their view was also that space, although that's not a word they use, can be empty. Now the reason that we have a different conception of void than the ancient Atomists is in part precisely because we have this notion of space, which they don't. So they're really stuck with calling it the empty non-being and things like that. Maybe the first notion of space starts to be articulated in ancient philosophy by Plato, who's also the first to do many other important things. And he takes up the topic of space in a dialogue called the Timaeus, which is very, very influential and becomes a kind of standard text for people interested in natural philosophy for many centuries to come. And in the Timaeus, he says that the natural world arises when a kind of god, who he calls the demiurge or craftsman, imposes form or determination on something that Plato calls the receptacle. And at one point he compares the receptacle to space. So one thought that you might have, and that later philosophers as we'll see did have, is that the receptacle could be sort of empty space. But that doesn't seem to be quite what Plato has in mind because he thinks that what the demiurge really does is to impose geometrical shape on the receptacle. And the result of that is that you start to be able to build bodies. So for him, bodies at their atomic level or their particular level are made up of things like pyramids and cubes. So for example, he says the element of fire is made of tiny, tiny little pyramids which are themselves made up of little triangles. So the triangular faces of the pyramid. And he even says, for example, that the reason fire can burn things is because it's got very sharp angles. Because the tips of the pyramid have more acute angles than other three-dimensional shapes. Whereas, for example, Earth is made of cubes. Now he's not committed to the view that there's void. And in fact, the idea that you could get bodies just by imposing shape on the receptacle, I think tends to suggest that whatever the receptacle is, it's not just empty space. Because it's not clear why imposing shapes on empty space would give you faces of three-dimensional objects. But Aristotle, who's very critical of Plato's views in the Timaeus, points out that Plato maybe accidentally winds up being committed to void. Because if you shove together things like pyramids and optihedrons and cubes, so the kind of three-dimensional shapes that Plato is envisioning. Obviously they don't nestle up against each other nicely so that they all fit with no spaces in between. They'll be more like a bunch of dice in a box which are all kind of lying on top of each other at angles. And Aristotle then says, well, clearly there will be spaces in between the three-dimensional geometrical shapes. And so you'll wind up with void. And so Aristotle then identifies criticisms of Plato that Plato is committed to void when maybe Plato didn't mean to be committed to void. Of course that's only a telling criticism if there's no such thing as void. And Aristotle is one of the majority of ancient philosophers who does think that there's no such thing as void. So in fact, what he thinks is that the cosmos is a sphere, this is what Plato thinks as well by the way, with the outermost heavens surrounding more planetary spheres that have the moving planets seated upon them, and in the middle of the earth where we live. Now you might naturally think, well if the whole cosmos is a sphere, then there must be something outside the sphere, right? So that's got to be empty space. And that is an objection that some people raised against Aristotle. So one of the classic arguments was, if I go to the edge of the cosmos and stick out my arm, then if I'm right at the edge, either I'll be able to stick out my arm, so there must be some empty space for me to stick out my arm into, or there must be something outside the cosmos that's stopping me from sticking out my arm. In which case I wasn't at the edge of the cosmos after all, because there's more stuff outside it to block my arm from moving into this so-called space. So Aristotle's actually committed to the view that there's nothing outside the cosmos, not even empty space. So you might say, there isn't even non-being outside the cosmos, that's how much nothing there is outside the cosmos. There's not even nothing, as it were. So he has various arguments against the possibility of void, of which I think the most clever is this. And I leave to you as a homework assignment to figure out why this argument is wrong. So, answers on a postcard. Here's the argument. Things move hither and yon at various speeds. So imagine a stone drop through the air, or imagine a stone drop through water. Now clearly the speed that the stone falls is going to be inversely proportional to the density of the medium. In other words, it will fall faster in air than water. So the thinner the stuff it's moving through, the less dense the stuff it's moving through, the faster the object will move. Right? Everyone buys this? Yes? They're all nodding, listeners. Well void has a density of zero. So the speed of something in void would be infinite. Whoops. Clearly things can't move infinitely fast. If there was a void, they would move infinitely fast, therefore there's no such thing as void. Isn't that a great argument? I love that argument. Because it's so plausible and so wrong. One of the best things about history of philosophy, right? It's sort of wrong arguments that sound right. And that's certainly one of them. At least I think it's one of them. So that's one of the arguments he makes against the possibility of void. Now obviously though Aristotle doesn't want to assert the Eliatic doctrine that there's no such thing as motion. So he has to explain how there can be such a thing as motion, even though there's no emptiness to move around in. So what he does is, he says that body is not atomic, it's continuous, but it can vary in density. So what he would say is that the air around me is full, so there's no gaps or empty space in the air around me. So if I move my hand through the air, then what I'm doing is not taking advantage of emptiness that's scattered in amongst the air particles. So that I can sort of scatter the air particles as I move my hand. Rather, what's happening is that I'm pushing the air and moving it somewhere else. And he even has a theory that the reason why things continue to move through air, for example, if you fire an arrow through air. He thinks that what happens is that the point of the arrow pushes away the air from in front of it. And the air is kind of cycled back to the back of the arrow and that pushes it on from behind. So that the arrow is literally pushing itself through the air by using the air that it's passing through as a kind of pushing mechanism. Which is also very clever, although it does raise the question why things don't fly through the air forever once they're fired. And he can't really explain that because he doesn't have the notion of momentum. But the notion of momentum would be the topic for another lecture. So I won't get into that any further. But anyway, Aristotle does realize that he needs to explain why Melissa's is wrong. And why motion is possible even in a world with no void. And he makes these attempts to explain it. So really what that sets up for the rest of the ancient tradition is a kind of opposition between some philosophers who are atomists. So they think that bodies are made of uncuttable atoms at the bottom. So if you cut and cut and cut a body, you'll get to a smallest particle that can't be cut anymore. They're the atomists, right? Atomists just mean something uncuttable. And these atomists think that the atoms move around in void. So this is not only the ancient atomists but also the Epicureans who think this. So there's those people on the one hand, people who believe in atoms and void. And on the other hand, there are people who don't believe in void and think that if you keep dividing bodies, you can keep dividing as much as you want. So if you have a sharp enough knife, you can cut any body no matter how small in half. That's Aristotle's view. So he thinks that bodies are continuous, not atomic. And he denies the existence of void. And most philosophers fall into one or the other of these categories. The exception is the Stoics. The Stoics are Aristotelians in the sense that they think that body is continuous. So they reject the Epicureans' atomism and say that you can cut bodies indefinitely small. But on the other hand, they want to say that there is void, just not here. So what they say is that the cosmos is finitely big, just like Aristotle said, so it's spherical. And then they're the ones who use this sticking out the argument. So they say, look, there has to be space or void outside the cosmos. In part because they think that the cosmos actually gets bigger and smaller for various reasons. So they think there's a cosmic cycle where the cosmos kind of implodes into flame and gets much bigger. And then it reduces back to its current configuration. And of course that would be impossible unless there was empty space around the cosmos for it to explode into. So what that means is that they are committed to a continuous physics rather than an atomist physics. But also the existence of void. And in fact they agree with the Epicureans that void is infinite. So you could go off to your left if you like and just keep going. And for them eventually you would have reached the edge of the cosmos. And then in theory at least you could just keep going and you'd go through more and more empty space. And you would never run out of the empty space because there's an infinity of void surrounding the cosmos. Interestingly, even in the peripatetic, in other words the Aristotelian tradition, there are some figures who think that void is something you could contemplate. So in particular yet another non household named Strato. I'd be very impressed with. Has even our PhD student heard of Strato? Yes she has. She's too good. She's heard of everybody. But okay, for the rest of you. Strato was the leader of the Aristotelian school after Theophrastus who's Aristotle's immediate successor. And he had some very innovative ideas about physics. And in particular he at least flirted with the possibility that void exists. So this goes to show you how much room for argument there was in the ancient world between people who accepted the existence of void and people who didn't accept the existence of void. And this is where we kind of get to the main event. So the main event and the thing I kind of really wanted to talk about. I know I've been talking for half an hour. But it's only now that I'm getting to what I really wanted to talk about. Are you excited? Excited all over again. It's like the beginning of the talk. But better because it's half over. Okay, so Strato seems to have been an influence on a doctor named, ready? Erasistratus. And if you don't think that one of the problems with doing a podcast on the history of philosophy is that you have to say words like Erasistratus without stumbling over them. Then you're wrong. So Erasistratus was a doctor who worked in the city of Alexandria on the coast of Egypt. Or what we now call Egypt. And he has a kind of mechanist understanding of the human body. So he thinks that he can explain anatomical features of the body. By the way, some of this, just to warn you, is going to get kind of nasty and gross. And I know that you're glad. Even if you pretend you're not. So he has a kind of mechanist understanding of human anatomy. And he wants to explain why things move around inside the body by invoking the kind of mechanical processes that we could also see, for example, in machines. And one of the processes that he wants to invoke is what's sometimes called by the Latin name of horovacui. Which means something like abhorrence of vacuum. So the thought here is not necessarily that void ever actually exists or comes about inside the body. But rather that in order to prevent a void from forming, things will move around in various ways. So for example, if you have a vessel full of water and you evacuate the water by, for example, drilling a hole in the bottom. Then air will rush in from above in order to fill the space. Because a void has to be prevented from forming. So it's almost like nature is a referee who will step in and shift things around at all costs to make sure that no void can form. And that's why we never experience it. So that's why it never happens that you empty a jug of wine or water and look in the jug and see. Not that you can see this. But see that there's void in the jug rather than air. So how would you use this to explain anatomical phenomena? Well here's an example. He says that when you breathe in, everyone breathe in with me. It's like a yoga class. Did you notice that when you did that your chest expanded? So when you breathe in your lungs get bigger. This is why according to Eris Isrides you take in air when you breathe. So your chest cavity expands, the lungs get bigger and air has to be drawn in because otherwise a vacuum would form. Because the air that was filling your lungs when your lungs were small wouldn't be sufficient to fill the lungs once they're big. So when you breathe in it's actually this mechanism of avoiding the vacuum that prevents a void from forming in your lungs and thus draws in air and allows you to breathe. Another example is that he thought the reason why the bladder takes in urine from the urinary tract. This is the gross part by the way. So when your bladder is evacuated, and we all know how that happens because we're all adults here. What that means is that more urine has to be drawn in from behind from the urinary tract because otherwise a void would form inside the bladder. Another example, he thinks that air is pushed from the heart through all the blood vessels. So he has this view that Galen criticizes and in fact mocks which is that your blood vessels are not full of blood but rather full of air. An obvious objection to this, which any of us could think of, is if I cut one of my blood vessels I don't hear pssshhhh. What happens is that blood spurts out. But Eris Isrides in fact seems to have claimed, according to Galen, that in fact although you don't hear a kind of hissing. It is in fact the case that air is escaping from your blood vessels and that blood is then pulled in from the surrounding body in order to prevent a void from forming and then the blood leaks out. So the first thing to come out of your blood vessel if it's punctured is air. Because he thinks that your entire circulatory system is designed to move air around your body. So that's why the lungs are connected to the heart. The reason you see blood is simply because blood gets sucked into the picture in order to stop a void from forming. So that brings us to Galen. Galen is really the source of our information about Eris Isrides's view. He brings up Eris Isrides in order to criticize him and as I say even to make fun of him. So one of the things he says which I quite like is that on Eris Isrides's theory if you punctured a blood vessel in your arm all the air should drain out of your body in one go. Because it's all connected. So you would just crumple to the floor having been completely evacuated of air or pneuma as they call it, breath. And you would literally die just from having a small puncture in one of your blood vessels. And that's clearly not the case. Similarly he wants to argue against Eris Isrides's view about the bladder. And the way that he does this is by invoking a kind of natural power which is something that Galen is quite keen on. So his view is something like this. The bladder doesn't draw in urine to prevent a void from forming. It draws in urine because the bladder is just the kind of organ that has a natural power for attracting urine from the kidneys. So there's urine forming in the urinary tract and the bladder pulls it in because it has a natural power for attraction of urine. And he thinks that without these natural powers of attraction and also repulsion and there are some others you would not be able to explain human anatomy. One way of thinking about this is that he's using a kind of idea that also turns up in Aristotle which is that the natures of things have to be understood in terms of their purposes. So for example the purpose of an oak tree is to grow big and tall, make acorns, drop the acorns and thus create more oak trees. The purpose of the bladder is to attract urine and then expel it from the body to prevent a harmful build up of urine. So he thinks of the body as a well designed, for lack of a better word, organism. Where every organ has a specific role which is given to it by nature. Who he thinks of as a kind of providential, almost divine force. And he doesn't like Eris Sistratus' view because it invokes these kind of mindless, purposeless, mechanical forces to explain things like the bladder and breathing and so on. Rather than these natural powers of attraction. So you might think that Galen then would have no use at all for this whole principle of avoiding the formation of a void. But in fact there are contexts where he says, well actually Eris Sistratus is right for example, that the lungs work like a bellows and that they draw in air to prevent void from forming. So he's quite happy with that as long as he can also say that the lungs have the natural purpose of furthering the process of breathing. As long as he can say that blood has such and such a purpose, the bladder has such and such a purpose. So he wants to hold on to the idea that the body and everything in it is, to use a technical term, teleological. In other words, it has a goal or a purpose. And once he's secured that by arguing in various ways against Eris Sistratus and other anti-teleological views. He's then happy to say as a kind of caveat, oh yeah but there is also this phenomenon that if a void would form somewhere, then nature will kind of step in and prevent it from happening. So like Eris Sistratus, he thinks that any situation where void seems in danger of occurring will somehow be thwarted by the sloshing around of fluids or other bodies in order to fill whatever space is being evacuated. And he gives an example of this which is sucking up dirty water or sandy water through a straw basically or a cylinder. And he says that what happens if you do this is that since the water can move more easily than the sand, you'll draw mostly water out of the mixture. So if you imagine sort of putting a straw into a fish tank that's got sand in the bottom and sucking through it, you'll get some nasty water. But at least you won't get any sand in your mouth. And that's some comfort. So he does agree that, he seems to agree at least that nature will usually act in such a way as to prevent void from forming. But there's another context in which he discussed the topic of void. And this complicates matters considerably. And this is going to take us, and here I am going to make a bit of a jump because Galen is 2nd century AD and I'm going to have to jump 8 centuries to the 10th century AD to explain this. Okay, so he wrote a work called On Demonstration. And the reason why the 10th century Arabic tradition is relevant here is that the Greek version of On Demonstration is lost. And we only know about it through later reports. Some of which are in later Greek authors, as I'll say in a second. And some of which are in Arabic texts, like texts by this guy, Arazi, who's one of the main sources for our knowledge of Galen's work on demonstration. So this is kind of unfortunate actually, especially for us philosophers. Because I think probably if everything Galen had written was still surviving, one of the two or three most interesting texts from our point of view would have been this lost work on demonstration. And the little that we do know about it is sufficient to show us that it was really, really interesting. And it also was very influential, especially actually in the Arabic tradition where philosophers who were interested in medicine took their cue from Galen's ideas about demonstration to form their epistemology. So what was this work? Well, as the title sort of implies, it was about demonstration. And what Galen was saying was that there are certain standards of proof that we need to meet in order to take ourselves to have demonstrated something. And basically, it looks like at least a lot of this work was devoted to showing that other philosophers and maybe other doctors had given arguments which they thought were convincing, but which aren't demonstrative. In other words, they fall short of these standards of proof. And Galen was basically telling them off for not being sufficiently rigorous in their argumentation. Now, we do know from several sources that Galen discussed the topic of void in this work on demonstration. And the reason we know about this is from both the Greek and the Arabic tradition. So on the Greek side, we have two later commentators on Aristotle who in their physics commentaries, when they're talking about what Aristotle said about the void, say, oh yeah, and there's this guy Galen who you've heard of. He's a doctor. And in his work on demonstration, he said the following thing about void. And the two philosophers I have in mind here, just in case you're curious, are Themistius and Simplicius. So two ancient, late ancient commentators on Aristotle. So both of them tell us that according to Galen, void must be possible because if you imagine a full vessel, like a vessel that's full of water, for example, or wine. And then you imagine that the fluid is drained out of it without anything rushing in to fill it. Then there would obviously be emptiness and nothing but emptiness inside the vessel. And thus there would be void. So void does exist. Right? That seems to be the argument. That's at least the way Themistius and Simplicius presented. And Themistius and Simplicius say, well that's a rubbish argument because you actually haven't made it happen. Right? So in fact what would happen is if you prevented air from coming in as the fluid was draining out, the walls of the vessel would collapse. And in fact you can see this happen. So for example if you have a wine skin, right, that's only open at one end, and you force the wine out, then what you'll get is a collapsed wine skin. You won't get a wine skin that's empty, and sort of full of emptiness if you see what I mean. So you won't get wine skin that looks like it's inflated but has nothing inside. What will happen is the walls of the wine skin will get closer and closer until they touch. So there won't be any empty space inside the wine skin. And in fact nature will just not allow this to happen. You can never have an empty vessel. This is of course interesting in that this is precisely the point that Erisistratus had been invoking over and over in his anatomy. And precisely the point that Galen also invokes in his anatomical works. Right? Because remember that Galen, although he wants to stress that anatomy works through these natural goal-oriented processes, he does want to say that in some context things move around in such and such a way in order to avoid a void from forming. So this is very hard to explain. Right? Because we seem to have Galen saying exactly what Themistius and Simplicius say he should be saying in other works which are not lost. And then in this lost work on demonstration he's saying, no, no, a void could occur if the vessel didn't become full of something else. I think what Galen was probably saying here was not that void might actually occur, but rather that void is conceptually possible. So he's trying to explain to you that this old Eliatic idea that emptiness is a kind of non-being and that it would be absurd for non-being to exist. I think he's trying to explain to you that that's just a wrong-headed idea because clearly it's conceivable that void could form. Just imagine this case. Imagine the case where you have a full vessel which is emptied where nothing else comes in. What you've just imagined is a void. That doesn't mean it ever happens. It just means that it sort of could happen in principle. Now that would be a very natural thing for him to be doing in on demonstration. Because remember that on demonstration is always attacking other people for giving arguments that aren't demonstrative. So I think, I'm speculating here, that I think what he must have been doing is attacking someone who argued against the possibility of void. And he said, look, you haven't proven that void cannot occur because it's conceptually possible. That doesn't prove that it ever happens. But it proves that it's not logically or physically or metaphysically impossible. You can argue maybe about what kind of impossibility is at stake here. He's just saying that it's conceivable that void could occur. But again, that doesn't commit him to the view that it ever happens. So that's the Greek side of on demonstration. Now things get even more complicated. And I'm coming to the end. Because I'm now finally getting to the 10th century. It gets more complicated when we look at an Arabic source. This is a work with the self explanatory title, Doubts about Galen. Shukuk alla jalinus. By this philosopher, Razi. And Razi is basically doing to Galen what Galen had done to other philosophers in his on demonstration. Actually in this work, the Doubts about Galen, he says this is one of the best books that's ever been written. It's maybe Galen's greatest work. In fact he says this is sort of the best book you can read along with the Quran. Is this work on demonstration by Galen. So he really really likes it. But he also thinks there are mistakes in it. And he's taking his cue from Galen's on demonstration by being really critical towards Galen himself. And he says at the beginning, hey this is exactly what Galen tells us to do. Galen attacks his own predecessors with irreverent sarcasm. And so I'm going to do the same thing to Galen even though I'm a huge fan of Galen's. Galen would have wanted it this way. So as he's sort of kicking Galen in the head as Galen lies prostrate on the floor. He's saying, Galen told me to do this. That's sort of the tone of the work. So he eventually gets around to this topic of void. And he quotes on demonstration and what on demonstration has to say about void. But sadly he doesn't seem to agree with Themistius and Simplicius about what Galen said about void. Instead he said this. And this text is on your handout. He's characterizing Galen's position. And he says, Galen said void is not sensible but without showing whether or not it exists. When discussing this he went on to say that air is a body that rules out the existence of void. But then how could he first claim that one cannot know whether void is existent or not. After he showed that air is a body he supported his claim that void does not exist in the interstices. That's another difficult word to say. Void does not exist in the interstices. In other words the gaps of the air. With the fact that the plunger cannot enter the syringe as long as the hole at its orifice is not open. So here's what he's saying. Imagine a hypodermic needle. And imagine that the needle is blocked. So you can't sort of spritz anything out of it. And the plunger is back and there's nothing but air inside the needle. Obviously they didn't have hypodermic needles but they had similar syringe like constructions. So you try to force the plunger in and it won't go anywhere. So Galen says look this proves that there's no void in the air. There's no emptiness kind of scattered in amongst the air in the syringe. Because if there were you could push the plunger in until all the air was packed tight. So you could sort of exploit the emptiness inside the normal air. By pushing until all the air particles were forced together and there was no void left. And then the stopper of the or the plunger of the syringe was finally stopped. But you should be able to push it in to some extent. But instead you can't push it in at all. This by the way is interesting right? Because we believe that there's void in air. And yet we can observe the same phenomenon. Namely that if you have a hypodermic needle full of air and the needle is blocked. You can push pretty much as hard as you want. And you won't be able to push the air out. Or you won't be able to push the air together inside the needle. And Razi makes an interesting objection to this. Which is that there is void in the air. Because he was a believer in void. Although that's a whole other story. And the reason why you can't push the plunger in is that the face of the plunger is just too broad. So it can't exploit the tiny tiny little gaps in the air. It can't move around the particles in the air because it's just a big face. So it just pushes against what's effectively a wall of air. Because it can't sort of insert itself into the tiny little gaps in amongst the air particles. Okay, but what we're really interested in at least for the moment. Is what Galen was trying to say about void in on demonstration. Or at least that's what I'm interested in. Are you interested in it too? Do you want to know what I think? We now have all the evidence that I have. So the first thing I want to say about this is that it's very puzzling. So we have not only all the stuff about Erisistratus and whatever. Let's ignore that for the moment. Just think about what happened in on demonstration. Which is lost. So we can't see the original discussion. We have Simplicius and Themistius, the Greek commentators telling us that Galen argued that you can have void. All you need to do is imagine an emptied vessel that has nothing flowing back into it. And then we have Razi saying that Galen argued with this syringe example. That there is no void in air. Right? So on the one hand he was arguing that void is possible. And then the other he was arguing that it's not possible. This seems very, very puzzling. Well the first thing to say about this is that if you think about the purposes of on demonstration. It may be that in on demonstration he was completely neutral about whether void exists or not. Because on demonstration is basically a critical work. He is attacking other people's views. And he may well have attacked people who did believe in void. And then people who didn't believe in void. Or the other way around. Without asserting a view of his own. He may have been just trying to show that both sides lacked full demonstrative rock solid proofs for their position. That would be totally consistent with what else we know about this work on demonstration. And in fact if you look back at the passage that I quoted from Razi. You'll see that Razi is saying that Galen was claiming that you cannot know whether void is existent or not. So that actually gives us some independent evidence that, I mean Razi had read on demonstration. Which we haven't. And he is telling us that Galen's position in on demonstration was actually that you can't tell whether void exists or not. So that I think gives support to the possibility that Galen was arguing against both sides. So he is saying the anti-void camp has this problem about the emptied vessel. Where you can imagine it staying completely empty. And the pro-void camp has this problem about the syringe. They both got a problem therefore neither side is demonstrative. And I think some support for that thought can be found elsewhere in the extant, the surviving works of Galen. Because there is a passage, a great passage actually where he says. The thing about philosophers as opposed to doctors like me. Is that philosophers waste a lot of their time arguing about issues that cannot be resolved. And he gives us an example of this. The question does void exist outside the cosmos? So the Stoics think as I said that void does exist outside the cosmos. Aristotle thinks that void doesn't exist outside the cosmos. And Galen's response to both parties is, well do you have a way of going to look and see? You know, is there a bus where we can buy tickets to the edge of the cosmos? Because if not, I don't think we are going to be able to resolve this issue. And maybe we should sort of crack on with you know, healing people's illnesses and other useful things like that. Like I have been doing. So he has this nice kind of diatribe against philosophical authors. Who are sort of pie in the sky airy fairy time wasters. As opposed to him, Galen, who only does philosophy when it is actually useful. And when the questions can be answered with verifiable sense experience. I think that is why in this passage from Razi he says that void is not sensible. In other words it can't be perceived by the senses. I think the reason he was saying that there is because he wanted to say, well look we just don't have any sense experience to tell us whether void exists or not. So who knows? Now having said that, that then seems to be in tension with this syringe example. Because it looks like he then went on to say, but look void doesn't exist because here is an example. If it existed then you could push the plunger in. And Razi in fact tells him off for contradicting himself here. He says first you say we can't know whether void exists or not. And then you give us this syringe example with the plunger. Which suggests that we can know that void doesn't exist. But I think Razi was just very tendentiously willfully ignoring the nature of Galen's discussion. Because I think what Galen was trying to do was just show that there were arguments on both sides of the question. And that should again make us doubt our ability to resolve the question of whether there is void or not. But one last possibility that I think we could entertain is that if we think back to the context of the medical works. And the discussion about anatomy, the argument with Erisistris. It looks like maybe Galen was committed to the view that you don't ever have actual void occurring inside the body. Like in your lungs or your bladder or whatever. So he agrees with Erisistris about that. But of course that leaves it completely open whether there might be void somewhere else like outside the cosmos. Because that wouldn't be an important thing to consider when we are doing anatomy. So as a doctor he might really care whether there is void for example in the air in your lungs. Because if there were void in the air in your lungs then you wouldn't be able to use this idea that the lungs are like bellows. So they get bigger and they have to suck in air to fill the space otherwise a void would form. Because if there was void sort of scattered in amongst the air particles then the particles could just spread out to fill the lungs. And you wouldn't need to have any air come in from the outside. So it may be that he wasn't just sort of saying, oh a pox on both your houses we can't solve this void thing. Maybe he was trying to say with the syringe example that you can show that there is no void inside air or other actual bodies around us. But then he was trying to say with the empty vessel example that for all we know there could be void outside the cosmos or whatever is conceivable. So you can't have void inside bodies as it were like little bits of void. But you could have lots and lots of void if you wanted like emptiness. And just to leave you with one last thought. That philosopher that I've been talking about, Razi, in fact thought precisely this. So he says that God creates the world with the help of what he calls the soul in empty space. Or actually he calls it place. And he says that this kind of empty place is like the vessel into which God puts the world. He calls it absolute place. And says that in order for there to be somewhere for God to put the world there has to be place eternally and infinitely just like God. So he says that empty place or absolute place is one of the principles of the universe and thus is in a sense on a par with God. And for this and other reasons other people rather than calling him Razi just called him the heretic. And he's the guy I'm mostly working on these days. And part of the reason I'm working on him is that he stands at the end of this whole tradition that I'm interested in. So he and Galen show us that to study the history of philosophy you need not only to look at all these smaller figures. I mean you can see that I've mentioned many many people in this lecture who you might never have heard of before or who you might not know very much about. But they're part of the story of void from the ancient world into the medieval world. And they also illustrate the other point I was trying to make which is that to study the history of philosophy you need to look at other disciplines like medicine. Because Galen and Razi if you ask them both like what is your job they wouldn't have said philosopher they would have said doctor. So this hopefully was giving you an example of what I mean by doing the history of philosophy without any gaps. Thanks for coming.