Descartes 1: The Method of Doubt

okay welcome back to this series of lectures today we're going to be starting to talk about Descartes and is actually going to be two lectures on Descartes because he occupies such an important place in the history of philosophy especially at the beginnings of what we call modern philosophy so let's just quickly look back at our timeline so you may recall this timeline from the beginning of the course we started way back in the 3500 to 3 300 BCE talking about the origins of writing the beginnings of civilization we fast forward it a bit to the Year 600 before year 1 talked about the origins of Western philosophy bailey's and the pre-socratic philosophers spent a lot of time on Socrates Plato and Aristotle and now we're going to be moving ahead to begin to talk about Descartes any rights his most famous philosophical piece in 1641 the meditations on First Philosophy he wrote many other things and so we're going to be focusing on this one particular piece of his writing now you notice there's quite a bit of a gap there between 600 between 600 and 200 and between 1641 and that's because during the Middle Ages the medieval period so to speak medieval which is a fancy word for middle as I've mentioned before so during the medieval period lots of interesting things happen but one can sort of view what goes on there as a Christianisation or a muslin demonisation of the views of Aristotle and or Plato now that's not to say that there weren't interesting things happening in this period in fact many contemporary scholars are looking back at this period and rediscovering the work of these medieval philosophers and also finding out that they anticipate a lot of the things which we in the modern contemporary period are interested in so it's sort of an oversimplification to say that this period was predominantly focused on either Christianizing or muslim nything the work of Plato or Aristotle but we're going to generally give it that gloss will be skipping a lot of this stuff and focusing on Descartes okay so Rene Descartes of course he's French and he's writing a lot of his stuff in Aristotle now Descartes is in an interesting period in time because there is just the beginning of this rebellion against Aristotle in the Western world and so there's a group of people who are associated with the rejection of Aristotle so we have Galileo who very famous I'm sure everyone at least has heard of Galileo then Descartes and this quart of culminates with Newton and Newton was the younger of the bunch he wasn't contemporary of either Descartes so you can see he was born right right just seven years before Descartes dies and he's born one year after Galileo dies well Galileo Descartes knew each other they'd have met each other and Descartes credits Galileo with getting him interested in the kind of philosophic philosophical views and debates that he ends up being involved in for the rest of his life in fact they kept in correspondence with each other and they disagreed about some things but agreed about a lot of stuff as well so this is the beginning of the end of Aristotle's hold on Western philosophy and at this point in time the dominant view would have been in the West a Christianized version of Aristotle's physics and his philosophy so if you were going to Oxford in the sixteen hundreds of the 1500s you would have studied Aristotle's views as though they were the truth not in a philosophy class but you would have studied his physics in a physics class in the very same way that people who study physics at the undergraduate level in our time are introduced to Newton's mechanics you have been introduced to Aristotle's mechanics such that they were since people thought that they were generally correct now at this time generally people start attacking this view on empirical grounds and that's what we're going to be getting into it's the rise of the empiricist movement and of course one of the main empirical grounds that they're attacking Aristotle on is based on the work which is coming out of the invention of the telescope now galileo is often associated with the invention of the telescope although he it's a bit of a misnomer to say that he invents the telescope the principle of optics have been well-known since the medieval period the early Muslim philosophers and scientists had discovered that you could amplify an image by directing light through a lens of a certain sort what Galileo really does is to perfect a certain kind of grinding method which enables people to manufacture these kinds of lenses with a lot more detail and care and therefore generate telescopes which actually magnified things hundreds of times their size okay and of course if you turn a telescope around what you get is a microscope so at this point in time people are starting to look around with these new tools of the new Sciences and of course one of the one of the most startling things that people discover is that if they point the telescope at the moon for instance what they find up there seems to be much like what we find around here so it's a giant rock floating in space and of course this contradicted Aristotle's notion and the received notion at this time in history that the planets were made of a completely different kind of material that resembled nothing here on earth that they were perfect immutable which means non changeable and all they did was move in perfect circles around the Earth when you look at the moon what you see are craters pock marks various hills and valleys much like the landscape on earth but more barren so that was quite a shock to people now of course the other big change at this time was Galileo was also implicated in is the rise of the heliocentric view of the universe now heliocentric means putting the Sun at the center so as you're probably familiar with this what they call the Copernican revolution in science which is the denial of Aristotle's views that the earth is at the center of the universe and that everything moves around it so this new view comes out the view that the Sun is at the center and that the earth is actually moving around it and this was so anti common sense and also this common sense view had been incorporated into various religions so the Catholic Church declared that this view was heresy and famously Galileo was almost excommunicated he was put on house arrest for publishing his views about the heliocentric nature the university was forced to retract them and then of course he had a tract published posthumously after he died which says that he never really retracted them and so on now interestingly a pope john paul ii who just recently passed away officially apologized to galileo on behalf of the Catholic Church and the Catholic Church has since reformed its ways saying that faith and reason don't necessarily clash so that's one of the famous stories about the battles between science and religion which we won't dwell on here now another major attack on Aristotle's views which you might recall from the end of the last lecture Aristotle claimed that heavier objects fall faster than lighter objects and this is an accord with common sense not many people had bothered to test this there are some indications that perhaps some early Muslim philosophers and scientists were trying to test this and so perhaps this idea is not completely original with Galileo but the way the story goes in the West at least is that Galileo is one of the first to actually decide to empirically test this and so there are famous stories about him dropping things out of the Leaning Tower of Pisa as far as we know that is completely apocryphal and not true but what we do know that Galileo did was to take objects and roll them downhill and he would time them with his own heart rate and measure how many heartbeats it took for these objects to complete their path down the hill and what he discovered was that objects no matter their size accelerate at a uniform rate which we now know to be 9.8 meters per second squared what you can go and study in physics and learn all about that now of course this is the discovery of the the beginning of the discovery of the difference between momentum and acceleration so what Galileo noticed is that heavier objects hit the ground harder than lighter objects but they will be going at the same speed so acceleration or speed is constant and doesn't depend on the weight of the object in Galileo hypothesizes that if it weren't for external factors like the wind and so forth that if you took a very light object in a very head heavy object and dropped them they would hit the ground at the same time now he had no way of testing that of course but this is a hypothesis that he makes on the basis of these kinds of empirical results and this is the beginning of the empiricist movement we're going to be turning to empiricism after we're finished talking about Descartes Descartes is still a rationalist now the empiricists the famous what's so-called British empiricist John Locke George Berkeley and David Hume and of course David humans a Scottish philosopher but he's often called a British empiricist because of political beliefs on the part of Britain that Scotland is a part of their empire we won't dwell on that but it is a bit ironic that a Scottish philosopher is known as a British empiricist now here we have a picture which you probably can't see that well but this actually comes from the website of NASA and this is a picture of an astronaut on the moon you can see the lunar rover in the background and the American flag that they planted and what he's holding here is a 10 pound weight and a 1 pound weight and what he's going to do is drop them and you can actually go to NASA's website and watch the video of this experiment and they are trying to carry out the experiment proposed by Galileo Galileo said well look if it wasn't for the wind and if their everything was calm you could drop a heavy thing a light thing at the same time and they would hit the ground at the same time with different impact of course and so they did this experiment the astronaut makes a very funny comments about Galileo and how he hope he doesn't falsify Galileo's theory let's go of the two items and you can see them slowly but steadily falling to the ground keeping pace with each other and they hit the lunar surface at exactly the same time where the only difference being that the heavier object creates a larger puffs of dust as it hits the ground empirical confirmation in 1969 of Galileo's prediction in the 1500s so that's quite an amazing and goes to show you the power of the new methods that the empiricists and empirical science bring a bear and so we're going to start to see a fundamentally radical shift in the way people think about reality which is ultimately sweeping out Aristotle's views and ushering in the new views of the modern science now of course as I mentioned just a second ago Descartes is still a rationalist this is one of the things that Descartes and Galileo thought about Galileo was a complete empiricist who thought that the only way you could really know anything was to go and do an experiment whereas Descartes who still holds rationalism as the fundamental view of how knowledge is acquired so the interesting thing about Descartes is he's on the cusp of the rejection of Aristotle and in one sense has his feet in both traditions so he's very critical of Aristotle's views and we'll look at some of the ways that he's critical of the views of Aristotle but at the same time he's still very much a product of the Aristotelian way of looking at the world he uses a lot of the same terminology that you would find in the Scholastic's which it would be the philosophers of the schools of this period who are predominantly Aristotelian so it's interesting to look at descartes as someone who's right on the edge of the complete eradication of Aristotle's views from the traditional mainline Western thought but as someone who even though he's very angry about these views and very much wanting to reject them can't get out of the way of thinking that Aristotle had handed down completely so we'll keep our eye open for that okay so Descartes is a champion of what was known at the time as mechanistic philosophy and sometimes it's called corpuscular e'en philosophy because of the view that the atoms are little corpuscles and this is the view which he's first introduced to by Galileo so Galileo is an Italian he comes to France the Descartes meets him becomes very impressed with the ideas that the Italians are working on and the guiding metaphor of mechanistic philosophy is the idea that the world is a giant machine so simple hydraulics were coming online and oh sorry let me just whoops don't know why you can't just remove one bullet point okay so Bob simple mechanics had just been discovered hydraulic machinery animatronic kind of artwork you might find in the the lawn at the Palace and the in the in France where you would see so if you've been to Disneyland for instance and been on the Pirates of the Caribbean ride you will see these figures which move around but are just mechanisms that well a pirate is chasing a woman around a fire another pirate is raising a jug of liquor to his mouth and singing a song and so forth and these are just completely mechanisms and these kinds of simple devices would were known to the world at this time just barely having been discovered and of course the the main thing that people would have really been familiar with is the idea of clockwork and the idea of clockwork is really what you should be thinking of as what they mean by a mechanism so not the kind of watches or digital watches that we have but we mean the kind of clocks that you find in a clock tower for instance think about Big Ben giant clocks in towers that you could walk into and actually see the gears and levees moving in their things ticking things pushing on each other and the amazing thing about these things is that there's a lot of movement going on inside these kinds of mechanisms but there's no movement which is initiated on its own all of the movement is caused by movement of another part so think about how a cog is interlaced into another cog and as the one cog turns its gears connect to the other cog and push it down so one thing is moving and it connects to another thing which starts moving which is connected to another thing which is then moving and all the movement in this system is connected to each other and you have to go and wind it up and then the thing starts ticking away and now this was the view that they had about the world this is the view that they're developing that the world is a machine in this kind of way and the main objection that they have to Aristotle is that objects do not move themselves so remember that Aristotle held that when an object falls to the ground for instance that that is an instance of an object doing something the object is moving itself from the position where it started to the ground so an apple falling from a tree the Apple is of its own accord moving towards the center of the universe which happens to be the earth now these modern philosophers reject that idea the apple is not being moved excuse me the apple is not moving itself from the tree to the ground but rather is being moved by something else and of course you know whether something else is already it's gravity gravity is thought of as a force which is pulling the object to the ground and so the object is inert it doesn't have the object itself has no efficient cause it has no formal cause which is part of the explanation of its moving and it has no final cause there is no purpose for it to be on the ground it's just a mechanistic process very different kind of view than Aristotle had and this is what Descartes and his fellows think is that Aristotle is putting mind into nature so they really want to separate the material in the formal causes which they're fine with in which they interpret as matter and shape for the most part which are out there in the world and then efficient of final causes which are properties of minds only so the Apple has no final cause as no purpose it's trying to fulfill and the Apple has no efficient cause there's nothing in the Apple which is performing any kind of action so you can really see these modern philosophers as taking Aristotle's four causes and splitting them in half material and formal go to the natural world efficient and final go into the mind and it was a mistake on Aristotle's part according to Descartes to think that all four of these were the explanation for any natural phenomena at all okay so that's just a kind of a brief summary so we want to get clear on what Descartes thinks he's doing and they can't really responsible for focusing philosophy on epistemology on the study of search for knowledge and what he really wants is to find a foundation on which knowledge can be built so Descartes is often thought of as a foundationalist as opposed to other kind of theory of knowledge foundationless thinks that there's got to be some kind of bedrock where things bottom out and on which the other things that we're interested in can be founded on so he's very much in line with the tradition started by Plato that knowledge requires certainty but he rejects Plato's idea that the physical world is not knowable so you recall that Plato argued that the physical world was changing constantly and that real knowledge requires non change and so therefore posited a realm of forms that were eternal and unchanging which were the real objects of our knowledge but knowledge of the physical world was not really to be had rather while you had knowledge of was these eternal and unchanging things Descartes wants to have both he wants to show that knowledge requires certainty but he wants to reject the idea that we don't have real knowledge of the physical world but have knowledge of something else so he wants to connect these two ideas together turns out to be rather difficult okay so let's start at the beginning Descartes wants certainty as well but what does it mean to be certain well for Descartes what it means to be certain is being unable to doubt and this is an important move that Descartes moves here makes here so notice that Plato interprets the certainty criterion metaphysically which is to say that it creates a criterion according to which objects must be unchanging so that's talking about the metaphysics of objects the way reality is if knowledge is to be possible then the things we know must be unchanging Descartes is here putting more emphasis on the epistemology side of things he's not making a metaphysical claim he's saying look real certainty means that the knower is unable to doubt the thing that they know so why does he think this well if you can't doubt the thing and notice if it's not even possible that you could doubt it that it's literally impossible to have any doubt about the thing then that thing would have to be certain so his strategy is to adopt what has been come to be called the method of doubt now the method of doubt it's important we stop for a second I'll say this again as we go through but we can say at least start by saying that the the method of doubt is simply a method it's a strategy a way of coming up with something which is real knowledge and the method roughly works as follows so we are to take our beliefs and subject them to doubt and if it is possible to doubt those things then we treat that particular belief or set of beliefs as false and we repeat this until we find something that we are unable to doubt so that's the method in a nutshell now it's according it's a it's very important to take some caution here Descartes never thinks that these things are actually false so for instance to take a simple example when we were talking about my car and whether I know where my car is we found we were able to doubt that I knew where my car was and so we said that's not real knowledge right so the car is parked on the street I'm inside the room do I know that the car is where I left it well it's possible for us to doubt that that the car is where I left it because we can imagine it being towed stolen destroyed or whatever now that doesn't mean that I really think that it's false that my car is out there it just means that I don't know it and that's des cartes attitude here he's not going to say that these things are literally false but only that they cannot serve as a foundation so we want to treat them as though they're false while while we're engaged in the method until we find something which is undoubtable and then we use that thing which is undoubtable to build everything else back up so he's gonna go through several stages of doubting and he's going to get to the point where he wonders if you know maybe nothing is undoubtable but his ultimate goal is to try to show that all the stuff which we doubted all the stuff which we are treating us false can eventually be reclaimed once we find a foundation which is secure and will allow us to bear the weight of knowledge now we need to talk a little bit about what it means to be possible to doubt so Descartes it's very liberal about this and he has to be liberal about it in order to accomplish his goals so by possible to doubt Descartes literally means any possibility whatsoever so what we're trying to do here is we have a belief and we want to think of any scenario in which that belief could be false we're to try to think of the most far-fetched the most wacky the most way-out scenario that you can come up with and the reason for this is that if we allow that that'll guarantee that whatever can't be doubted really a certain so if we could find something that was truly undoubtable in this sense we could use it in a valid argument like a syllogism and the conclusion of that argument would be that the world exists in the way that we think that it does so this is roughly Descartes strategy he agrees with Aristotle that the theory of the syllogism is a good way to build knowledge if you start with true premises in a valid syllogism you're guaranteed true conclusions Descartes sees as a problem that we always are forced to be using premises which we don't know if they're true or not so the way to get rid of that is to find something which is truly undoubtable which no rational person could ever call into question and to use that as the premise in a syllogism and if the conclusion of that syllogism was that the world exists in the way that we think that it does then we would show that we have real knowledge of the physical world okay so that's the goal is to use the method to find something which is knowable with certainty that can't possibly be doubted and then to use that thing to build back the bridges that we burned all right so I'm going to break this down into three steps and this roughly follows the order that Descartes presents them in in the meditations which is a series of six meditations and I'm going to be roughly following the order that they're presented in in that book so the first thing that Descartes does is to point out that our senses are unreliable so here are some examples of the unreliability of the senses straight things look bent in water round things look oval when held at an angle you can take that one and practice it by taking a quarter and tilting the quarter a bit sideways and then if you were to draw the shape that you see you would draw an oval but the circle is this excuse me the quarter is really a circle in shape and it's not oval ER and of course no object can be both since a circle is defined as an object where every point on the circumference is equidistant from the center and an oval is an object that lacks that property some points on an oval are further away from the center then other points so that no thing can be the same a Descartes also points out that square things look round and this is really a product of his time so if you were far far away from some tire tower and you are approaching it on horseback for instance from 20 miles away the tower would look roundish and as you get closer to it you would notice that it was actually square so that's misleading now one from our own national anthem we say purple mountains Majesty in our national anthem but of course there are no purple mountains mountains only look purplish from far away because of the haze and the tint or which they acquire because the atmosphere in between us and there now here's another one which is date that Dakar likes a lot the Sun actually looks very small when in fact we know by reason that the Sun is many times the size of the earth and this is a surprising thing for instance that psychologists have verified if you take a small child and you ask this child what is bigger a house or the Sun what will all answer immediately the Sun is bigger but of course children are mistaken and a child will say a house house is giant the Sun is like this big and they make this the circle of about the size of the Sun appears to be from the earth so here's a case where the senses are presenting us with information that we can independently come to show is false and Descartes very same famously says that where something has deceived us once it is wise never to trust that thing again and this is his parting shot at any kind of empiricist claim to knowledge if you think that the senses are going to give us knowledge then you have to account for the cases where there's these rampant kinds of disconnects between what the senses of present to us and what reason later tells us right so these things according to Descartes cannot be the foundation of our knowledge because they don't give a certainty we can always doubt that the senses are presenting us with the way things actually are because we know for a fact that they deceive us on occasions so on a given occasion where you sort of think the senses are being accurate how can you be certain excuse me how can you be certain that they're accurate given that it's very easy for us to imagine a case where there's some kind of illusion going on so for instance you're looking at a red apple and you say how can I be misled about this well how can you be sure that there isn't is in a white Apple redlight under some kind of illusion going on now of course that doesn't mean that he thinks that this stuff is going on but what he's looking for certainty and what he's trying to show here is that you don't get certainty from the census because it's really easy to doubt that the way the census present things are the way they really are okay now here's an example from modern psychology this is something known as the muller-lyer illusion you can look at this and ask yourself with a question which of these lines is longer and if you're like a normal individual you'll say the middle line seems to be the longest line here but we can show that these lines are the same length so these are the same diagrams now just so you can see what the length of the line is and it's the arrow tips at the end which present the illusion that these lines are excuse me of that these lines are of differing lengths so there are plenty of instances of this and what modern cognitive psychology has discovered is that people seem to assume that their senses present them with this very reliable picture of the world outside of us and it just isn't that way so there are things called inattentional blindness change blindness and I recommend normally if this were an in-class presentation I would show you a video on YouTube of something called the amazing color changing card trick and I can't show that to you here but I recommend that you google this the amazing color changing card trick and watch this video for a demonstration of just how unreliable the senses really are okay so well you might say yeah but come on so maybe I don't know if the stick is bent I can imagine that it's you know it looks straight but it's really bent or it looks bent but it's really straight I can imagine the Apple has a different color I can imagine that the shapes I see may be miss presented to me in some sense but surely they've got to be generally right I mean even though the quarter which appeared to me round in one instance and oval er and the other instance and so I can doubt what the real shape the quarter is I can't doubt that there is a quarter there or more generally I may not know where my car is but I can't doubt that there is a car when I'm looking at it when I'm standing there in front of the car right when when you have that kind of direct sense experience it certainly seems that there's got to be for the most part accurate even though there may be this kind of room for inaccuracies so for instance you're standing here looking at a table and the table is my favorite example because this is usually presented in a classroom and I'm in front of a table so I pointed a table surely this table here is got to be here right well Descartes says not so fast we can doubt that the table in front of us is really there and he does this in two steps first by pointing to an ordinary everyday experience which we have all the time which suggests that you can have the table experience without the table and then he's going to ramp that up using a fictional character which you've probably heard of who's called the evil genius and some translations for the evil demon in other translations so well for instance you have a dream and in the dream there is a table and for instance I spend a lot of time lecturing about this stuff I often have table table like dreams where I'm sitting here banging on a table saying how do you know this isn't in a dream you wake up you go that's particularly particularly ironic because in the dream I was saying I know there's a table here when in fact there was no table there so you've seen tables in your dreams and tables in your dreams while you're dreaming appear to be just like tables which are visible and awake so how do you rule out that what you're seeing right now isn't the product of a dream what de cartes argument is that you can't rule it out at least and this is important at least not on the basis of sensory experience and the reason for that is pretty simple anything that you would cite as evidence would be something that would also be true in the dream so so if you say I see the table I hear it I feel it I smell it why you'd smell a table but let's say it's a mahogany table for instance so or you taste it I don't recommend this but you could go and lick a table you say look I have all this evidence for my senses I see it I hear it I smell it I taste it I feel it how could I doubt any of that well all of that is something that would be exactly the same in a dream so you could imagine yourself dreaming and saying look I can hear the table and knocking on the dream table and hearing a certain auditory sound and being convinced that you're really standing in front of an actual table but of course in reality you're laying there in your bed or on a couch or maybe on the subway or wherever you happen to be napping or sleeping and there are no tables anywhere present so it's incredibly easy to doubt that the table you're looking at right now is really there if in fact this were a dream then there would be no table so that's what I just said and dreams there are no tables at all just my experience as of a table now des cartes takes this to an extreme by imagining that there is an evil genius the evil genius is not the DJ Green Lantern for those of you who are in New York but he is someone who has the powers of God but is not good so God is by definition supremely good so we can't imagine God deceiving us and that's going to play an important role in the argument later but the evil genius could be deceiving us and evil genius is determined to deceive you into thinking that there is a physical world when there isn't one now again Descartes doesn't really think there is an evil genius he doesn't really think that there's somebody out there trying to deceive us into thinking that there is a physical world when there isn't one but what his point is is that there's no way to rule that out merely on the basis of our sensory experience and of course the situation that he's imagining here is something akin to the movie The Matrix or if you think the Matrix is terribly outdated you can update this and think about Inception so in the movie Inception you have people who are in these dreamlike states and you have other people who are manipulating the kind of data that they're being fed so they're making the person think that they're really in Rome or Paris or wherever and really the person is asleep on an airplane so then in this case Leonardo and his crew would be the evil geniuses in the matrix it would be the machines which are the evil geniuses it doesn't really matter who the evil geniuses are they are simply supposed to be somebody something which has the power to make you believe or excuse me the power to produce these kinds of experiences in you in the absence of the actual objects so in the matrix the machines are the evil geniuses but of course des cartes version of this is a bit more radical because in his version what he's imagining is that there is no outside to this matrix suppose that you are just in a dreamlike state and all there is is your consciousness of the things in the world and no things in the world at all what if there are no physical objects no tables no chairs no bodies no Sun no stars no moons no planets what if all of that stuff is the product of the evil genius you're just feeding you this sensory information without there being anything physical at all so you can certainly conceive that there are just these non-physical Minds who have experiences in the absence of anything physical whatsoever so you can kind of imagine this at the plot of a movie if you like and then everything would seem to you exactly like it does right now but none of it would be there none of it would be real now again Descartes doesn't believe this is actually true he thinks there are tables and chairs he's trying to prove that there are tables and chairs but of course the point is you cannot prove that simply by knocking tables over you cannot prove that tables and chairs exist merely by putting your soda on the table and saying look it takes more than that since you cannot rule out this massively skeptical scenario his point is that we can't use any kind of sensory evidence as a foundation for knowledge because there is this my new possibility that we cannot rule out okay now step three at this point you may say well gee okay so maybe I am a disembodied mind who has experiences as of a body but with no body whatsoever surely it's still the case that one plus one is two I mean right this is where Plato and the ancient Greeks looked for certainty certainty was to be found in mathematics but whether or not the physical world is really real we can be really confident absolutely certain that the mathematical truths at least in basic arithmetic are the way that we think they are right again that's where Plato looked for certainty now Descartes thinks while sadly for a Plato that he can imagine a scenario in which one plus one does not equal two and again just as before he's going to start with a common experience one that we're all familiar with and then ramp it up with the evil genius so the common experience that we're all familiar with is making mistakes in mathematics so for instance you take a math class you do some problem you get some answer to that problem which from your point of view seems absolutely correct and you turn it in and you get your paper back and there's a big red X on it and you go oh man I thought that answer was correct but clearly it's not correct so in fact you can also check your work doing the same problem getting the same answer and yet finding out that very same answer is still wrong now what explains this how are we able to do these mathematical problems checking our work over and over again only to get the same wrong answer well of course the answer is that you're making some mistake but not noticing it and in fact just a word to the wise this is what many standardized tests pray upon so there are some simple basic errors which people statistically tend to make in various kinds of math problems and so they always make sure to include one of these common wrong answers in the list of answers you can choose from and so people do the problem they get this wrong answer and they see the answer there and they go out well yeah that confirms that it must be right because why would it be there ah but of course you're making this mistake that you're unaware of and they're capitalizing on that now couldn't it be the case that for instance maybe one plus one equals three and it doesn't matter what you think about this could be 5 could be 123 the point that it's not 2 and that as we do our simple arithmetic maybe the evil genius is tricking me into thinking that 1 plus 1 is 2 so it might be the case that I have this feeling of certainty about one plus one being two I just feel that it's got to be true and yet in reality maybe some other claim is true maybe one plus one is three and the evil genius is just making it the case that I can't notice it right and this is again the same idea we're making some mistake but the evil genius who has unlimited power simply makes it the case that we never notice this mistake now in my experience people are reluctant to accept this part of des cartes argument you say well I can't even imagine that one plus one is some other thing than two so what I want to do right now is present a brief kind of evidence of what this could mean so I'm going to show you a proof that one plus one is three and proofs are different than solving equations it's a simple proof it's one that I lifted from WVO Quine and to have ad modified slightly to make the point that I want to make so this is not original with me but I think it's a useful way of trying to illustrate what des cartes point is now the difference between solving an equation and doing a proof is that when you solve an equation you don't know what the variable stands for so you know 5x equals 30 you don't know you don't know what X is equal to so you divide both sides by five and you find out that x equals six here we're going to know what x is equal to i stipulate that x is equal to one but the same rules apply and the fundamental rule of doing a proof is that end of solving equations that this is based upon is that as long as you do the same thing to both sides you never change the Equality right if you have one equals one you add 11 you have 12 equals 12 you divide by 6 you get 2 equals 2 you add a million you get a million in one equals a million 1 so as long as you do the same thing to both sides of the equal signs you always get something which is still equal on the other end so this is a very simple proof let's start so we stipulate that x equals 1 well then since x is equal to 1 it's going to be true that x squared equals x and you can verify that 1 times 1 equals 1 now let's subtract 1 from both sides so x squared minus 1 is going to be equal to X minus 1 and you can verify that again that says basically those are still the same numbers now notice though what's interesting about this is that we have a polynomial on the left hand side x squared minus 1 so we can factor the left-hand side and those of you who are familiar with algebra should know what the factors of this are X minus 1 times X plus 1 equals x squared minus 1 and of course you can verify that by doing the foil method first outer inner last and I won't take you through that but this you can go through and verify that this is still the nothing has changed all we've done is factor x squared minus 1 and we have something which is perfectly equal here now but notice we did this now we have X minus 1 on both sides so we can divide by that and we get X plus 1 equals 1 this is true because on the left hand side the X minus 1s cancel out and on the right hand side any number divided by itself is equal to 1 so we have X minus 1 divided by X minus 1 that equals 1 so now we have X plus one equals one you want to add one to both sides now and then we get X plus two equals two but now since we started out by saying that X is equal to one we can put one back in there and what we've ended up with is three equals two now that's weird but of course another way of writing two is as one plus one and so we've concluded that one plus one equals three now this is an excellent ploop proof works very good on drunk people if you happen to be at a party and you want to amaze some people as a Mathemagician you can show them this proof but of course I'll let you in a little secret there is an error in this proof one of the steps that we just went through is a mistake and if this were actually being done in class I'd wait around for you to try to figure out what the mistake is but since we're not actually doing it I'll point out where the mistake is so look at the step where we say now let's divide each side by X minus 1 well if you were keeping track of this you would notice that X minus 1 is equal to 1 minus 1 so that's actually equal to 0 so what we're saying right there is let's divide by 0 and if you know your basic arithmetic division by 0 is not allowed in fact it's undefined you can get any calculator and take a number divided by 0 and it will say error undefined you're not allowed to divide by 0 in simple arithmetic but of course I cleverly deceived you by having X minus 1 figure there instead of 0 so you wouldn't notice that X minus 1 was equal to 0 and even if you did notice you can certainly conceive of an all-powerful evil genius stopping you from noticing every time you're about to realize fill in the 1 there and get 0 somehow the evil genius simply distracts you such that you're never able to realize that there's that mistake there now imagine a group of people living in a world where they never noticed this mistake they might all be convinced that one plus one equals three now this was the mathematical truth now of course I'm not an evil genius I'm not evil and I'm not merely mildly malevolent and semi not dumb so but yet still I fooled many uh people with this proof by simply disguising zero as X minus one it's incredibly easy to trick people in the kind of multi-step processes and this is de cartes real insight here is that when we do arithmetic it always involves steps and every time there's a step there's a there's a room for error and we can imagine an evil genius deceiving us at just the crucial step so that we might be making mistakes that we never noticed and this is something that happens to us all the time in our everyday life so we can certainly imagine a scenario where the mathematical truths that we think are out there aren't out there that we could be wrong about this and so Descartes things well this cannot be our foundation we've got to look further so let's take a bit of time just to sum up what we've just been through so Descartes developed this method the method is designed to deliver certainty the method is known as the method of doubt for the purposes of the method if we can doubt a belief then we treat that belief is false he applies this method systematically he doubts that the world is the way that it appears he doubts that there is a physical world at all finally he doubts that there are that excuse me finally he doubts the mathematical truths and that the way we think arithmetic and so on the basic mathematical truths are the way that they really are but there is one thing that cannot be doubted and that is that while he's doing all this doubting Descartes must exist and this is some famously summed up in his Latin phrase cachito ergo zoom cogito is sort of how you pronounce now but it really is cachito in the original and that's irrelevant but you can see cachito means I think ergo means therefore assume is a conjugated form of the verb to be so what that says is I think therefore I am and that's what of des cartes more famous phrases there so he what his point here is that look you know allow that there's an evil genius remember he doesn't believe it but just just give it to the person right so there's an evil genius and suppose this evil genius is deceiving me into thinking that there's a physical world when in fact there isn't one and that math is different than we really think it is well this evil genius still couldn't make it be the case that I think that there is a physical world when there isn't one if there is no me so in order to be deceived there's got to be somebody who's being deceived so Descartes concludes that I know I am thinking and I know that when I'm thinking I exist that can't be doubted now you can you can't play any games with that you can't get the evil genius in there somehow to mess things up because you can allow the evil geniuses doing all this stuff and it still has to be the case that if you are doing the thinking and you are being deceived then there must be a you who is actually being deceived so he says ah now we've achieved the product of this method now we know with absolute certainty that whenever I am thinking I must exist this is the foundation now this is what he has discovered with certainty that he exists as a thinking thing whether there is a physical world or not he cannot doubt that he exists that's the foundation on which he plans to build back all of the stuff previously doubted now at this point he makes another move he says well you know this piece of knowledge has a certain quality what she calls being clear and distinct clear and distinct ideas are the foundation of Descartes epistemology so an idea is clear if it is quote manifest to an attentive mind that is if you are awake and alert well fed not being distracted no one's snapping their fingers annoyingly in the background on the cell phone or poking you with a needle or anything like that so if you're in this position and you focus the rational faculties of your mind on this fact you see that it must be true there's no way you can prove that cogito ergo su you simply come to understand what the phrase means and you see that it must be true with the power of reason an idea is distinct if it contains nothing but things which are themselves clear so there's no part of it which is unclear and we can leave that to the side the distinctness part it's really the clarity of the idea that matters so now Descartes makes his famous move here that the truth that he has discovered I think therefore I am is clear and distinct because of its self evidence you see once you think about it you see that it has to be true that's being self-evident it's mainly the evidence for its truth is the very thing itself and that's what makes it clear and distinct now he concludes that all clear and distinct ideas are true and this gives him a way to try to build back the things that we doubted earlier so what we need to do is look for ideas that are clear and distinct those things are necessarily true clear indistinct ideas are self-evident necessary truths according to Descartes and their truth is apprehended by rational intuition which is a kind of seeing with the mind now this of course is the way that he's in the Aristotelian tradition Aristotle was also into clear indistinct ideas though he didn't use that terminology Aristotle does think that there's a set of axioms which are known to be true which then we use to prove everything else and this is exactly the picture that Descartes has arrived at so he now has a foundation I think therefore I am in a way to verify it you look for things which are clear and distinct so the next step is to see how we extend what he has got so far so what has he got well he knows that he exists and he knows that his essential characteristic is thinking and that's because he thinks it's possible that he exists without a body but that is not possible that he exists without thinking he's very clear that when if there is no thinking then there is no Descartes but of course the whole method of data showed that it was certainly possible that he just be a disembodied mind so he doesn't need a body he could still be Descartes but if he's not thinking then there is no Descartes now of course what's a bit odd and something we'll spend some time talking about later is that by thinking de current means any kind of mental activity seeing tasting feeling pain believing that two plus two equals four so thinking is a generic catch-all term for Descartes which is supposed to mean anything that takes place in the mind all right so I said that already includes feeling pains in blue and etc and a lot of people resist the idea that feeling pain is something mental and Descartes leans on some empirical results at this point those known as phantom limb pain so if you're unfamiliar with phantom limb pain let me briefly familiarize with it so this is something has been known for hundreds of years that there are cases where people who have an amputated limb for instance say that a right arm has been amputated these people report feeling pain in that right arm the one that's not there so a common thing that they might report is that they feel as though they're clenching their fists very tight and they say very detailed things like they can feel their fingernails digging into their palms they can feel the pressure on the knuckles the tendons are tight they they it hurts a lot and you can perform this experiment take your fist and clench it as hard as you possibly can for say a minute and you'll start to feel the pain and if that were going on 24 hours a day you could see how this would be very excruciating now of course these people have no right arms so they have no fist they have no Nell's they have no palm but at the same time they say I feel the pain of the fingers digging into my palm now what are we to make of this doctors do not think that these people are crazy they do not think that these people are faking it doctors think this is real pain this is every bit as real as the actual pain you would feel from digging your fingernails into your palm in the way that I suggested you try out a moment ago so Descartes takes this as evidence that the normal person thinks that pain is located in the body part that hurts so you think the pain is in your palm the pain is in your knuckle but Descartes is arguing that the pain is really mental and it's important to realize that he does not mean that that means it's fake or unreal so we sometimes say in English it's all in your mind and we mean by that you're imagining it it's not real but Descartes doesn't mean that he means normal pain even the one where you have the hand isn't located in the body but is rather something that's purely mental that happens in the mind of the perceiver in other words these experiences are representational in the sense that they represent the body as having a pain at that location when really the pain is located in the mind of the person

Channel: Richard Brown

Views: 146,118

Keywords: René Descartes, method of doubt, a priori, epistemology

Id: ncEoWxsnXyc

Length: 57min 5sec (3425 seconds)

Published: Mon Oct 17 2011