We are beginning with René Descartes and his Meditations on First Philosophy, which is undoubtedly one of the most important books in philosophy, and indeed one of the most important books in the history of Western civilization. It is based on the events that happened to him on a snowy evening, on November 10th, 1619, when he was alone in a cabin and began thinking about knowledge, which is our topic.
Descartes was extraordinarily brilliant, and he received the best education available at the time. He studied at the finest schools and learned everything there was to know. Yet he emerged deeply disappointed with his education. He felt that none of it was certain, none of it was solid, and, most importantly, none of it did what he believed knowledge should do: help us live better.
He felt that the body of knowledge that had accumulated, the ideas on which there was broad consensus, simply was not in the service of human life. People could not cure diseases. There were no machines to transport us more quickly or connect us through communication. Medicine was poor. All the things that would genuinely improve life were beyond the reach of the education and learning of his day.
What Descartes decided to do in response was extraordinarily audacious: he would start science anew. He would reboot Western civilization. He would return to the very beginning, and build knowledge from scratch, placing it on the right foundation. His ambition was to reconstruct science in a form it had never taken before, so that it could finally deliver the benefits that human life deserved.
We have to realize just how different things were in the early seventeenth century from how they are today. When we think about how to do science, it seems quite obvious to us. You put on a white coat, you run experiments, you make observations, you form a hypothesis and test it. All of this feels self-evident, but that's only because we have been learning these methods our whole lives. We now have centuries of successful science behind us. We have a vast body of accumulated knowledge, and generations of scientists who have been refining the method with demonstrable success, producing the technology that proves how productive it is.
But the reason we have all of this is precisely because of thinkers like Descartes, because of books like this one. Science began at a specific moment in history. It is a human creation, not something we discovered ready-made in the world, but a method we invented. Which means someone had to invent it. And the person who invents something, by definition, does not have it beforehand. Descartes did not know how to do science, because science did not yet exist.
Even Isaac Newton, arguably the greatest scientist who ever lived, illustrates how unsettled things still were. When scholars went through his papers after his death, they found that the man who discovered the law of gravity and unraveled the nature of light had written ten times as much on alchemy and Biblical prophecy as he had on what we would recognize as science. It simply was not yet clear which kinds of inquiry could produce reliable knowledge. Descartes, then, is starting from nothing. His task is to create, from the ground up, a way of knowing things.
If you don't yet know how to know things, you can't know how to learn how to know things. And you can't simply start by observing. That seems like an obvious place to begin: just look around, make some observations. That's roughly what we were taught in eighth grade when we did experiments: you just make some observations. But you can't simply open your eyes and look at the world. It's not that simple.
Consider a thought experiment. Imagine you wake up very early in the morning, climb a mountain, and look to the east as the horizon begins to brighten. What would you say that you see? You would probably say that you see the sunrise. And if someone pressed you — "Exactly what do you mean by that?" — you would clarify: of course the sun isn't literally rising; what I'm seeing is the effect of the Earth's rotation lowering the horizon and revealing the sun.
But suppose we could bring someone from the twelfth century, a twelfth-century farmer, to stand next to you and look at exactly the same thing, look at the horizon at six in the morning from that mountain. Ask her what she sees, and she would say she sees the sun rise, but she would mean it literally. She would say she sees the sun actually moving up the sky, not the Earth moving. The philosophical question is: are the two of you seeing the same thing or not?
In one straightforward sense, of course you are. You're seeing the same photons striking your retinas at the same time. But in another very important sense, you are not seeing the same thing at all. You are seeing the light through a conceptual lens: a heliocentric model of the solar system, in which the sun sits at the center and the Earth turns on its axis while orbiting around it. That is why you interpret what you see as the Earth rotating and the sun being revealed.
Our twelfth-century farmer — call her Frida — sees the same light through the lens of a geocentric universe, in which the Earth is at the center and the sun moves around it. Her beliefs are shaping what she sees just as much as your beliefs are shaping what you see. We tend to assume that all we do is open our eyes and the world is presented to us, unmediated. In fact, our perceptions are always mediated through beliefs, ideas, and prior commitments.
We say that seeing is believing, but in a very important sense, believing is seeing. What you believe determines what you see and what you understand what you see to mean.
Descartes couldn't simply begin collecting observations, because if you hold the wrong beliefs, if you have the wrong framework within which you are interpreting your perceptions, then you will be seeing things incorrectly. Frida was making the same observations as you were when she looked to the east, but she was seeing, as we would say, the wrong thing, because she held other beliefs that were mistaken. Descartes therefore felt that before he could start looking, before he could form hypotheses, before he could do anything of that kind, he had to make sure that his conceptual framework, his overall view of the universe, was correct. Because if it was wrong, then everything filtered through it would be distorted, and everything he saw or thought on that basis would be wrong.
So before he goes tinkering with this belief or that belief, before examining this object or that object, he first had to get his entire worldview right. That is an extraordinarily difficult task, because it is the worldview itself that tells us how to do anything, including how to correct that very worldview. Even the attempt to fix or replace one's worldview is itself shaped by the worldview one already holds, which makes it incredibly hard to break free of. And if you cannot break free of it and verify that the worldview you have is correct, then nothing else you do can be certain. Everything else remains suspect, because it could all amount to seeing the sun rise rather than seeing the earth rotate.
This is what Descartes undertook to do. He decided he was going to get all the way down, beneath these frameworks, under them, to the absolute bottommost layer. He had to find ideas from which to build what he called his foundation. That foundation, an analogy he uses throughout the book, had to be absolutely solid and absolutely certain.
The beliefs he placed in his foundation, the beliefs he began from, had to be not merely true, but so true that they could not be false. There are many true beliefs, many things we hold to be correct, but almost all of them could in principle be wrong. I've been wrong about quite a lot of things. What Descartes wanted were ideas so right that, no matter what, they must be true. That is an incredibly difficult standard to meet, but it had to be that way so that he could be certain he was starting from the right beginning.
Consider what happens in an operating room. When a surgeon opens up a patient's body, that person becomes extraordinarily susceptible to infection. Any germ that enters can cause serious illness. That is why surgeons scrub their hands in that extreme way for seemingly ten or twenty minutes. That is precisely what Descartes is doing to his mind. He is scrubbing it of every last possible germ of error until he arrives at a perfectly pristine, sterile epistemological environment where he knows there is no infection. Whatever he builds from that point forward, he can rely on completely.
This is what Descartes is doing in his cabin on that snowy evening in November of 1619. He writes it all up in The Meditations, and he takes us along with him. It is wonderfully written because he is not merely reporting his conclusions, he is showing us the journey, inviting us to think these things through alongside him and to question the ideas ourselves as he proceeds.
We will start with the First Meditation. Descartes is searching for beliefs that can serve as his foundation, and his standards are ridiculously, absurdly high. A belief must be completely and absolutely certain under any circumstances, under any conditions whatsoever. He only needs to do this once, in order to establish the foundation, and then he can trust it going forward.
This is not the method he intends to use in science for the rest of his life. It is simply the way he is going to begin figuring out how to do science at all. The question, then, is how he is going to find beliefs that meet such an extraordinary standard.
How do you find something like that? Consider how you would find a bulletproof material, something like Kevlar. You would wrap a variety of materials around a watermelon and shoot them. If the bullet passes through, the material is not bulletproof, and you set it aside. You keep shooting through materials until you find one that repels the bullet — and there you have it: that one is bulletproof.
That is precisely what Descartes is going to do with his beliefs. He is going to try to find ways in which they might be false — any possible way, no matter how bizarre, no matter how unlikely. He is not trying to prove them false. The point is not that the beliefs he holds are false, but that they might be false. What he needs to find is a belief that cannot be false, one that is indubitable, meaning it cannot be doubted under any conditions whatsoever.
And so, just like you test the bulletproof material by shooting at it, he will test his beliefs by trying to doubt them. He will go through everything he believes and search for any scenario, however strange or improbable, in which a given belief could be false. If he can conceive of any situation, in any possible universe, where it might fail, he sets it aside. Not because it is wrong, but because it cannot serve as a foundation. The foundation must be absolutely certain, and anything that is even remotely dubitable cannot qualify as indubitable.
He wants to make no assumptions whatsoever. He does not yet know what the universe is actually like, whether it is heliocentric or geocentric, and that uncertainty extends to everything. Nothing is exempt from this process of doubt.
Descartes is going to perform what has come to be called methodological doubt. This is not organic, natural doubt. It is not that he genuinely wonders whether these things are true. He is intentionally trying to doubt his beliefs, deliberately constructing situations in which doubt can be manufactured, so that the beliefs in question will be set aside. He may return to them later, but for now they will not enter his foundation. He intends to do this for every belief he holds, working through all of them until he finds something that resists doubt, and whatever survives will serve as his foundation, if anything can.
The remainder of the First Meditation is Descartes implementing this methodological doubt. He has one additional rule that will make the process considerably faster, because he holds an enormous number of beliefs. If he had to examine them one by one — this shirt is blue, the sun is shining, and so on through everything he believes — it would take forever. But if he can group beliefs together and doubt them as a group, the work goes much more quickly.
The way he groups beliefs is by their source — where a belief comes from, what it is based on. If he can show that a whole group of beliefs derives from one particular source, and then show that source to be dubious, he can set aside that entire group at once. This simplifies his task considerably. The rest of the First Meditation is therefore organized around grouping his beliefs into three clusters according to their source, and for each cluster he will produce a reason to doubt it.
These are called skeptical arguments, after an ancient Greek philosophical school, the Skeptical School, which was a group of philosophers who believed that knowledge was impossible for human beings. They developed a series of arguments to try to demonstrate this, and Descartes draws on these traditional skeptical arguments to cast doubt on each of his groups of beliefs.
The first group is the most obvious, the most immediate, the most apparent. Right here, right now, how do I know this shirt is blue? How do I know the sun is shining? How do you know that I'm here? How do I know that I'm here? The answer, of course, is the senses: I see the shirt, you hear my voice, I see the sun. Beliefs we acquire through the senses are called empirical beliefs — anything we get through sight, hearing, smell, touch, or taste. This is an enormous category, since we get most of our knowledge through the senses in some way, and we trust them quite a great deal.
So what could possibly be the problem? Once Descartes has identified this group — beliefs based on the senses — he needs a skeptical argument that will render them dubious, that will show how they might be false. Not that they are false, but that they might be, because that is all he needs to disqualify them from serving as his foundation. It is not that he disbelieves them, it is that they are not certain enough to serve as a starting place.
The argument he uses is called sensory deception. The senses lie to us. You are in a crowd and you hear your name being called, but it wasn't really your name. You see your friend in the distance and wave, only to realize it is someone else entirely. You feel your phone buzzing and pull it out, and nothing is there. Our senses trick us. We rely on them, we trust them, and yet they are sometimes wrong. And if they are sometimes wrong, they are dubious. We cannot trust them at all, and therefore we cannot trust anything we get through the senses.
But not so fast. The Meditations is written in an unusual style: Descartes is not simply presenting conclusions, but taking us through his own thought process. The text is a back-and-forth in which he contradicts himself, because he is conducting a dialogue with himself, leading himself through these ideas. And so he comes back and objects. When someone is far away, or when there is noise and confusion, the senses may mislead us, but what about this hand, held directly in front of my face? I see it clearly. It is not far away. There is no fog, nothing in between. There is no way this is an optical illusion. If I hold an empirical belief under good conditions — close by, unobstructed, clear — it cannot be the same kind of sensory illusion as those other cases.
He is therefore forced to revise his earlier claim. It turns out that the first skeptical argument does not eliminate all empirical beliefs; it only eliminates empirical beliefs held under bad conditions. There remains a distinct category: empirical beliefs held under good conditions, such as seeing my hand directly in front of my face. These survive the first skeptical argument. I can still believe this is my hand, even knowing that my senses sometimes deceive me, because this situation is simply not the kind in which sensory illusion occurs. Whether these beliefs can be admitted into the foundation, however, requires a different and more powerful skeptical argument to make them dubious.
Of course, Descartes has a response. The second skeptical argument concerns dreaming. Consider: I am looking at my hand and I firmly believe it is right here in front of me. But what if I am dreaming? What if I am actually in bed with my hand behind me, merely dreaming that it is in front of me? I would still believe it was there, and every empirical condition would appear to be perfectly satisfied — nothing obstructing my view, good lighting, my hand directly before me — and yet the belief would be false.
Am I dreaming right now? I don't think so, but do I know? Can I be certain? Descartes argues that there is no specific feature that can tell us with certainty whether we are dreaming or not. All the criteria you may have heard of — black and white versus color, clarity, vividness — could themselves be simulated as part of the dream. They are not knowledge; they could all be elements of the dream itself.
Dreams can perfectly replicate empirical data while everything they present is entirely false. The dream argument therefore eliminates all empirical beliefs. Any belief you acquire through the senses could be simulated within a dream and thus be false, even when the conditions seem perfect, even when you think you are carefully checking. Anything derived from the senses cannot, on these grounds, be placed in the foundation of knowledge. That is a great deal to lose, given how many of our beliefs come through perception. The question then becomes: is there anything left?
Descartes believes there is, and this will be one of the central questions throughout the entire course. In addition to empirical information, Descartes holds that there is an entirely different category of knowledge called a priori. A priori is a technical philosophical term meaning independent of the senses, knowledge we arrive at without consulting sensory experience. The things that qualify as a priori include logic and mathematics.
Descartes was one of the greatest mathematicians who ever lived. He invented Cartesian coordinates in geometry, Cartesian being simply the adjective form of Descartes. He came up with the idea one morning while lying in bed, a habit he was deeply committed to; he preferred to stay there until noon. On that particular morning, he was watching flies walk across a screen window and realized that if you could plot their positions, you could represent shapes geometrically and develop a full coordinate system from it. So if anyone tells you that lying in bed is unproductive, you can point them to Cartesian coordinates.
And if anyone insists that rising early never hurt anyone, you can also point them to Descartes because he was hired by Queen Christina of Sweden, who enjoyed getting up early to philosophize outdoors in the cold, and he was dead within six months of entering her service. The lesson, it seems, is to stay in bed.
Now, a priori knowledge is exemplified by mathematics. Descartes argues that two plus two equals four whether you are dreaming or not. The dream argument establishes that you cannot rely on anything you perceive while dreaming, but it has no bearing whatsoever on the truth of mathematical claims. Two plus two equals four regardless of your state of consciousness, which means mathematical and logical truths survive the dream argument. Because they survive it, Descartes needs an entirely different and more powerful skeptical argument to cast doubt on them.
Here Descartes pulls out all the stops and introduces the nuclear bomb of skeptical arguments: the evil demon, or evil genius argument. He asks us to imagine an extraordinarily powerful demon whose sole purpose is to deceive us. Could such a demon have made something in our brains defective, so that when we add two and two, our minds arrive at four with complete certainty, and yet four is simply wrong? Perhaps the true answer is five, or ten, or the color blue, or something our faculties cannot even compute.
When the first Pentium chip was released, it had to be recalled at enormous expense because it produced one specific mathematical error. The chip was absolutely certain its calculation was correct, but that certainty was the product of a flaw, a fault in its design, some defect in its construction. What if the human brain is like that? Either God created our minds with such a defect, or evolution did. And evolution does not design us for mathematics; it designs us to survive long enough to pass on our genes, for which advanced mathematical reasoning is hardly essential.
It is therefore possible that even our a priori beliefs, the ones we thought were immune to doubt, are mistaken. This is essentially the scenario of The Matrix, but it goes deeper than that film suggests: not only could our perceptions be fabricated, but our very ways of thinking could be flawed. In that case, we cannot even be certain that two and two equals four. So even our a priori beliefs are out.
And that is where Descartes ends the First Meditation. He leaves us with no empirical beliefs we can trust and no a priori beliefs we can trust — stripped of every foundation on which knowledge might rest.
Descartes seems to be leaving us in an epistemological void, a wasteland where we can't trust anything and have no beliefs with which to begin again. And yet he observes that if doubt is the only thing we can be sure of, then at least we can be sure of that. This is where he begins the Second Meditation, where he attempts to start building things back up. And he gets there with what is almost certainly the most famous sentence in all of philosophy.
He asks himself: under these incredibly hostile epistemological conditions, where it is so difficult to rely on any knowledge or information whatsoever, what can we possibly still claim to know? Is there a world around us? It could be a dream, it could be the Matrix. Are there other people? Is there history? Is there a God? With the three skeptical arguments in place, none of this is knowable. He cannot even be certain that he himself exists. After all, he knows his body only through his senses, and those senses are no longer trustworthy.
But wait. He is doubting whether he exists. He is doubting whether God exists, whether the world exists. And the question immediately arises: who is doing the doubting? If he is doubting that he exists, he must be present in order to do the doubting. He has to exist in order to be able to doubt whether he exists. This one belief — his own existence — is unlike all the others, because whereas every other belief is rendered dubious by methodological doubt, this one is actually proven by it.
Descartes performs a kind of conceptual judo flip on the evil demon. The demon can trick him about the world, about God, about his body. But Descartes has to be there in order to be tricked. Is he wrong about his own existence? He has to exist in order to be wrong about it. Whatever you propose — that he is deceived, that he is fooled, that his belief is false — he must exist in order for any of that to be the case. He turns methodological doubt against itself, and it is a brilliant move.
This is the source of the famous phrase: cogito, ergo sum: I think, therefore I am. Doubting is itself a form of thinking. To doubt is to think that something might be wrong. And for him to think anything at all, he must exist, because thinking requires a thinker. Doubting is a thought, so doubting presupposes a doubter. Therefore, not everything can be doubted. The very act of doubting guarantees that there is someone doing it. His existence is not rendered dubious by doubt; it is proven by doubt.
And so Descartes has his first belief, the first piece of the foundation. It's not much. He does not yet know what he is, he does not know what the world is, he does not know anything else. But he knows, at minimum, that he exists. One thing. One small, seemingly meager belief, but it is something.
With his existence established, Descartes can begin rebuilding. The next question he asks is: "I exist, but what am I?" This brings up a very important philosophical distinction between existence and essence. Existence is that something is; essence is what it is. He has proven his existence — "I think, therefore I am" — but he does not yet know what he is, because everything he used to believe about himself is now dubious: that he is human, that he has a body, that he belongs to a particular society, that he is a Frenchman. None of that can be known.
If you do not know what you are, it is very difficult to proceed. Even if you affirm that you exist, you barely know what you are affirming. Descartes must determine his essence, but he has only one tool at his disposal: methodological doubt. He will go through all the beliefs he once held about himself, attempt to adopt each one, and when he reaches a belief he cannot doubt, that will be his essence. This does not mean it is the only thing he is, only that it is the indubitable one, the belief he can count on while constructing his foundation.
He used to think he was a human being, that's gone. That belief came through teaching and empirical experience. He used to think he had a body, that's gone. All of that is far too uncertain. What, then, is left? The very act that secured his existence in the first place was doubting, and doubting is a form of thinking. He knows for certain that he is thinking.
Everything else remains questionable. He may think he is walking, but that perception comes through the senses and could be a dream or an illusion. Thinking, however, is indubitable, because doubting is itself a form of thought. In order to doubt that he is thinking, he must think. To doubt is to entertain the thought that one might not be thinking, which means one already is thinking. That he is thinking is therefore the one thing he knows with certainty. He may well be other things, but for now those cannot enter his foundation. The only thing he can be sure of is what he himself declares: he is a thinking thing.
Descartes proposes that everything in the universe can be divided into one of two categories, which is why the theory is called Cartesian Dualism. Every thing in the universe can be classified into one of two classes: thinking things and material things. That is all there is. Minds and bodies.
We are, fundamentally, minds. We have some strange connection to a body, and that connection proved to be a serious problem, the mind-body problem is one of the great difficulties his framework generates. Minds are defined by thinking, while bodies are defined by what Descartes calls extension, meaning that they take up space.
Descartes has one last belief that survives methodological doubt. He has established his existence (that he is) and his essence as a thinking thing (what he is). Now he adds a third certainty, one that has the same structure as the other two. That is, rather than being undermined by methodological doubt, it is actually made certain by it. That certainty is simply that he has ideas.
Consider the perception of sunlight coming through a window. He does not know whether it is true, whether there actually is a sun out there. But in order to be fooled into thinking there is a sun, in order to have the illusion he must have the perception of a sun. The people in the Matrix perceived a world around them, even though that world was not real and nothing outside their brains corresponded to those perceptions. They had to have those perceptions in order to be deceived by them. You cannot believe a perception is real unless you first have the perception.
Descartes finds himself in exactly that position. He has this entire sensory array, and he simply does not know what is causing it. It could be an evil demon, a dream, or the Matrix. But he must have it, if only to be wrong about whether it is real. He calls these ideas.
So his foundation consists of three things: he exists, he thinks, and he thinks ideas. He does not know whether the world exists, whether God exists, whether other people exist, or whether he has ever taken a single step. He knows nothing beyond himself. Yet from that tiny, almost absurdly meager beginning, he's going to rebuild his entire knowledge base. He's going to bring back the world, he's going to prove that God exists, and he's going to establish the foundations of science.