What can we know, and how can we know it?


Skepticism in the Enlightenment

We have already encountered skepticism in philosophy, when we read Socrates's Apology.  But now we are going to consider how skepticism was revived as a virtue during the Enlightenment.  And, like any virtue, skepticism can be a virtue that can be difficult to get right.

Who Is Descartes?


Rene Descartes (1596-1650) was a mathematician and philosopher writing in the beginning of the era we now call the Enlightenment. During this period, the "scientific method" came to be appreciated as a reliable way of knowing about the world, many new scientific disciplines were formed, and philosophers opened new questions about the role of religion in personal and civic life. At the heart of all of this was an ambition to both push the limits of human knowledge and discover the best ways of coming to have knowledge.

The selections below are from the beginning of Descartes' groundbreaking Meditations on First Philosophy, published in 1641. In this text, Descartes introduces a method for rooting out his biases and establishing a secure foundation for knowledge.

The Cartesian Method of Doubt (Meditation 1)

Descartes begins his first Meditation by laying out the reasons why he is choosing to doubt all his beliefs, and the method by which he will go about doing it.

Some years ago I was struck by how many false things I had believed, and by how doubtful was the structure of beliefs that I had based on them. I realized that if I wanted to establish anything in the sciences that was stable and likely to last, I needed—just once in my life—to demolish everything completely and start again from the foundations. It looked like an enormous task, and I decided to wait until I was old enough to be sure that there was nothing to be gained from putting it off any longer. I have now delayed it for so long that I have no excuse for going on planning to do it rather than getting to work. So today I have set all my worries aside and arranged for myself a clear stretch of free time. I am here quite alone, and at last I will devote myself, sincerely and without holding back, to demolishing my opinions.

I can do this without showing that all my beliefs are false, which is probably more than I could ever manage. My reason tells me that as well as withholding assent from propositions that are obviously false, I should also withhold it from ones that are not completely certain and indubitable. So all I need, for the purpose of rejecting all my opinions, is to find in each of them at least some reason for doubt. I can do this without going through them one by one, which would take forever: once the foundations of a building have been undermined, the rest collapses of its own accord; so I will go straight for the basic principles on which all my former beliefs rested.

Descartes feels he’s been wrong so many times, and is not entirely sure why he believes certain things he believes, so he resolves to, just once in his life, build up an entire new belief system on solid ground. To do this, he says that he must first doubt all his previous beliefs. In order to doubt all his beliefs, he seeks to locate the foundations of his beliefs. If he can doubt the foundation, then logically the whole set of beliefs resting on that foundation will also fall. This method is known as the Cartesian Method of Doubt: inquiring after the foundations of beliefs in order to cast doubt upon them, and by extension the entirety of one’s belief system. The goal is to tear down all your beliefs in order to build up a new set of beliefs with a more justifiable foundation.

Can I Trust My Senses?

We might think that beliefs based on perception—what we see, smell, feel, etc.—are some of the most trustworthy we have. We can be deceived about the beliefs we learn from other people or any beliefs that require a great deal of reasoning. But our eyes don't usually deceive us about our surroundings. And we need well-functioning senses in order to conduct scientific observations—the Enlightenment gold standard for being rational. So sense-perception is the first foundation Descartes attempts to doubt.

Doubt: Our senses can mislead us.

Whatever I have accepted until now as most true has come to me through my senses. But occasionally I have found that they have deceived me, and it is unwise to trust completely those who have deceived us even once.

Our senses are often fooled. Consider the optical illusion at the top of this essay, or give a blindfolded friend a cup of orange juice and tell them it’s milk. If our senses can be tricked that easily, why should we ever trust them?

Hope: Surely some sense-experiences are trustworthy.

Yet although the senses sometimes deceive us about objects that are very small or distant, that doesn’t apply to my belief that I am here, sitting by the fire, wearing a winter dressing-gown, holding this piece of paper in my hands, and so on. It seems to be quite impossible to doubt beliefs like these, which come from the senses. Another example: how can I doubt that these hands or this whole body are mine? To doubt such things I would have to liken myself to brain-damaged madmen who are convinced they are kings when really they are paupers, or say they are dressed in purple when they are naked, or that they are pumpkins, or made of glass. Such people are insane, and I would be thought equally mad if I modeled myself on them.

Doubt: I might be dreaming.

What a brilliant piece of reasoning! As if I were not a man who sleeps at night and often has all the same experiences while asleep as madmen do when awake—indeed sometimes even more improbable ones. Often in my dreams I am convinced of just such familiar events— that I am sitting by the fire in my dressing-gown—when in fact I am lying undressed in bed!

Hope: I know when I'm awake!

Yet right now my eyes are certainly wide open when I look at this piece of paper; I shake my head and it isn’t asleep; when I rub one hand against the other, I do it deliberately and know what I am doing. This wouldn’t all happen with such clarity to someone asleep.

Doubt: Dreams usually feel real.

Indeed! As if I didn’t remember other occasions when I have been tricked by exactly similar thoughts while asleep! As I think about this more carefully, I realize that there is never any reliable way of distinguishing being awake from being asleep. This discovery makes me feel dizzy, which itself reinforces the notion that I may be asleep!

The Dreaming Argument

Here is one way we might try to outline Descartes' Dreaming Argument:

  1. If I know something, it is because my senses are not deceiving me.
  2. When I sleep, my senses deceive me.
  3. I do not know for certain whether I am awake or asleep.

C. Therefore, I do not know anything (at least, anything sensory).

Is this a good argument? 

Global Skepticism

By raising the possibility that he doesn't know anything, Descartes is entertaining a form of global skepticism, the idea that we cannot know anything at all to be true or false. This is opposed to local skepticism, the idea that we cannot know anything about a particular aspect of the world. For example, you might be locally skeptical about whether or not God exists, but you’re not skeptical about whether or not you have a head.


Divine Deception?

Hope: Some of My Beliefs Are True Even in Dreams

Suppose then that I am dreaming—it isn’t true that I, with my eyes open, am moving my head and stretching out my hands. Suppose, indeed that I don’t even have hands or anybody at all. Still, it has to be admitted that the visions that come in sleep are like paintings: they must have been made as copies of real things; so at least these general kinds of things— eyes, head, hands and the body as a whole—must be real and not imaginary.

For even when painters try to depict sirens and satyrs with the most extraordinary bodies, they simply jumble up the limbs of different kinds of real animals, rather than inventing natures that are entirely new. If they do succeed in thinking up something completely fictitious and unreal—not remotely like anything ever seen before—at least the colors used in the picture must be real. Similarly, although these general kinds of things— eyes, head, hands and so on—could be imaginary, there is no denying that certain even simpler and more universal kinds of things are real. These are the elements out of which we make all our mental images of things—the true and also the false ones. These simpler and more universal kinds include body, and extension; the shape of extended things; their quantity, size and number; the places things can be in, the time through which they can last, and so on.

So it seems reasonable to conclude that physics, astronomy, medicine, and all other sciences dealing with things that have complex structures are doubtful; while arithmetic, geometry and other studies of the simplest and most general things—whether they really exist in nature or not—contain something certain and indubitable. For whether I am awake or asleep, two plus three makes five, and a square has only four sides. It seems impossible to suspect that such obvious truths might be false.

Doubt: A Deceptive God

However, I have for many years been sure that there is an all-powerful God who made me to be the sort of creature that I am. How do I know that he hasn’t brought it about that there is no earth, no sky, nothing that takes up space, no shape, no size, no place, while making sure that all these things appear to me to exist?

Some people would deny the existence of such a powerful God rather than believe that everything else is uncertain. Let us grant them—for purposes of argument—that there is no God, and theology is fiction. On their view, then, I am a product of fate or chance or a long chain of causes and effects. But the less powerful they make my original cause, the more likely it is that I am so imperfect as to be deceived all the time—because deception and error seem to be imperfections.

Having no answer to these arguments, I am driven back to the position that doubts can properly be raised about any of my former beliefs. I don’t reach this conclusion in a flippant or casual manner, but on the basis of powerful and well thought-out reasons. So in future, if I want to discover any certainty, I must withhold my assent from these former beliefs just as carefully as I withhold it from obvious falsehoods.

Descartes is in dangerous waters here. He was writing at a time when it was unthinkable to call God's existence into question. By writing about a "deceptive God," Descartes risked being discredited as a scholar. His solution to this problem is to suppose an evil demon is deceiving him.
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Doubt: A Deceptive God Demon!

So I shall suppose that some malicious, powerful, cunning demon has done all he can to deceive me—rather than this being done by God, who is supremely good and the source of truth. I shall think that the sky, the air, the earth, colors, shapes, sounds and all external things are merely dreams that the demon has contrived as traps for my judgment. I shall consider myself as having no hands or eyes, or flesh, or blood or senses, but as having falsely believed that I had all these things. I shall stubbornly persist in this train of thought; and even if I can’t learn any truth, I shall at least do what I can do, which is to be on my guard against accepting any falsehoods, so that the deceiver—however powerful and cunning he may be—will be unable to affect me in the slightest. This will be hard work, though, and a kind of laziness pulls me back into my old ways. Like a prisoner who dreams that he is free, starts to suspect that it is merely a dream, and wants to go on dreaming rather than waking up, so I am content to slide back into my old opinions; I fear being shaken out of them because I am afraid that my peaceful sleep may be followed by hard labour when I wake, and that I shall have to struggle not in the light but in the imprisoning darkness of the problems I have raised.


A Priori vs. A Posteriori Knowledge

In the absence of trust in his senses, Descartes asks himself if there's anything he can know without his senses. In other words, is there any knowledge that is independent of experience? This is what philosophers call a priori knowledge, as opposed to a posteriori knowledge, which depends on experience. Some candidates for a priori knowledge might be moral intuitions (e.g. "kicking puppies for fun is wrong") or mathematical truths (e.g. "1+1=2"), while you might know a posteriori that your room was cold yesterday or that this text box is gray.

Here’s Notre Dame professor Patricia Blanchette explaining this distinction:


The One Thing I Can Know (Meditation 2)

Lacking any certainty in things he knows a posteriori, or after experience, Descartes asks what we can know a priori, using pure reasoning, without any input from sensory experience. He arrives at one single a priori truth, in what has become perhaps the most famous argument in the history of Western philosophy.

Even then, if a demon is deceiving me, I undoubtedly exist: let him deceive me all he can, he will never bring it about that I am nothing while I think I am something. So after thoroughly thinking the matter through I conclude that this proposition, I am, I exist, must be true whenever I assert it or think it.

But this ‘I’ that must exist—I still don’t properly understand what it is; so I am at risk of confusing it with something else, thereby falling into error in the very item of knowledge that I maintain is the most certain and obvious of all. To get straight about what this ‘I’ is, I shall go back and think some more about what I believed myself to be before I started this meditation. I will eliminate from those beliefs anything that could be even slightly called into question by the arguments I have been using, which will leave me with only beliefs about myself that are certain and unshakable.

The Cogito

This passage presents what is known as the cogito (an abbreviation of the Latin sentence, cogito, ergo sum—"I think, therefore I am"). This is the idea that the act of doubting itself—or any other act of thinking—presupposes a thinking self. Thus, the act of thinking is the indubitable foundation of knowledge that Descartes has been searching for, serving as undeniable proof of his own existence.

The cogito raises a number of challenging interpretive questions. Scholars have debated whether it is meant to be an argument, or instead a kind of "performance" that leads to certainty of one's own existence. Those who interpret it as an argument have disagreed over whether it is a good argument, and over what problems, if any, it faces. (How would you map and evaluate the cogito as an argument?)

There is also a question as to exactly what conclusion the cogito leads to. What is this "I" that Descartes thinks is established by his act of doubting? Descartes takes up this question next.


What Am I?

Well, then, what did I think I was? A man. But what is a man? Shall I say ‘a rational animal’? No; for then I should have to ask what an animal is, and what rationality is—each question would lead me on to other, still harder ones, and this would take more time than I can spare. Let me focus instead on the beliefs that spontaneously and naturally came to me whenever I thought about what I was. The first such belief was that I had a face, hands, arms and the whole structure of bodily parts that corpses also have – I call it the body. The next belief was that I ate and drank, that I moved about; and that I engaged in sense-perception and thinking, which I thought were done by the soul. If I gave any thought to what this soul was like, I imagined it to be something thin and filmy – like a wind or fire or ether – permeating my more solid parts. I was more sure about the body, though, thinking that I knew exactly what sort of thing it was. If I had tried to put my conception of the body into words, I would have said this:

By a ‘body’ I understand whatever has a definite shape and position, and can occupy a region of space in such a way as to keep every other body out of it; it can be perceived by touch, sight, hearing, taste or smell, and can be moved in various ways.

I would have added that a body can’t start up movements by itself, and can move only through being moved by other things that bump into it. It seemed to me quite out of character for a body to be able to initiate movements, or to be able to sense and think, and I was amazed that certain bodies – namely, human ones – could do those things.

But now that I am supposing there is a supremely powerful and malicious deceiver who has set out to trick me in every way he can – now what shall I say that I am? Can I now claim to have any of the features that I used to think belong to a body? When I think about them really carefully, I find that they are all open to doubt: I shan’t waste time by showing this about each of them separately. Now, what about the features that I attributed to the soul? Nutrition or movement? Since now I am pretending that I don’t have a body, these are mere fictions. Sense-perception? One needs a body in order to perceive; and, besides, when dreaming I have seemed to perceive through the senses many things that I later realized I had not perceived in that way. Thinking? At last I have discovered it – thought! This is the one thing that can’t be separated from me. I am, I exist – that is certain. But for how long? For as long as I am thinking. But perhaps no longer than that; for it might be that if I stopped thinking I would stop existing; and I have to treat that possibility as though it were actual, because my present policy is to reject everything that isn’t necessarily true. Strictly speaking, then, I am simply a thing that thinks—a mind, or soul, or intellect, or reason, these being words whose meaning I have only just come to know. Still, I am a real, existing thing. What kind of a thing? I have answered that: a thinking thing.

What else am I? I will use my imagination to see if I am anything more. I am not that structure of limbs and organs that is called a human body; nor am I a thin vapour that permeates the limbs – a wind, fire, air, breath, or whatever I imagine; for I have supposed all these things to be nothing because I have supposed all bodies to be nothing. Even if I go on supposing them to be nothing, I am still something. But these things that I suppose to be nothing because they are unknown to me – might they not in fact be identical with the I of which I am aware? I don’t know; and just now I shan’t discuss the matter, because I can form opinions only about things that I know. I know that I exist, and I am asking: what is this I that I know? My knowledge of it can’t depend on things of whose existence I am still unaware; so it can’t depend on anything that I invent in my imagination. The word ‘invent’ points to what is wrong with relying on my imagination in this matter: if I used imagination to show that I was something or other, that would be mere invention, mere story-telling; for imagining is simply contemplating the shape or image of a bodily thing. That makes imagination suspect, for while I know for sure that I exist, I know that everything relating to the nature of body including imagination· could be mere dreams; so it would be silly for me to say ‘I will use my imagination to get a clearer understanding of what I am’—as silly, indeed, as to say ‘I am now awake, and see some truth; but I shall deliberately fall asleep so as to see even more, and more truly, in my dreams’! If my mind is to get a clear understanding of its own nature, it had better not look to the imagination for it.

Well, then, what am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wants, refuses, and also imagines and senses.

The Cogito and the Body

Descartes thinks he can know, with absolute certainty, that he exists now as a thinking thing. But he cannot know any facts about his body – after all, you can have mistaken beliefs about your body while dreaming.  So has Descartes proven he is something (a mind) distinct from his body?

This is the origin of the so-called mind-body problem: Are the human mind and human body two separate entities? And if so, how do they interact?

This problem vexed philosophers for centuries after Descartes. Any answer provided ended up mired in objections, paradoxes, and contradictions.

Is the mind separate from the body? Then how do they interact? If they interact through neurons or something physical, isn’t the mind at least partly physical?

Is the mind not separate from the body, but simply neurons firing and chemicals flowing? Then why do we have any subjective experiences at all? How can a bunch of chemical reactions amount to something with vivid sensations, emotions, thoughts, and desires?

Philosophers continue to debate this question, which has taken on new significance in the era of artificial intelligence.




This digital essay was prepared by Paul Blaschko and Meghan Sullivan from the University of Notre Dame, and edited by Justin Christy and Sam Kennedy.


Blaschko, Paul and Meghan Sullivan. 2022. "Descartes' Meditations: Doubt Everything." The Notre Dame Philosophy Commons. Justin Christy and Sam Kennedy (eds.).