Cartesian Skepticism

Rend Descartes (1596-1650)


1. The Project of the Meditations.

The announced project of Descartes' Meditations is to establish a "foundation" upon which to build something "firm and lasting" in the sciences. In order to appreciate the point of this effort, it helps to try to imagine how you might proceed if you suddenly came to doubt the reliability of the numerous authorities you have trusted as sources of information about the natural world. This was easier for Descartes that in it is for us, since for him it was obvious that the various "authorities" disagreed so completely with one another and were so often wrong about the manifest facts that upon reflection it was clearly unreasonable to trust them. Our authorities are not like that. The science and mathematics you learn in your courses really is almost universally accepted, and it really does agree quite brilliantly in most cases with experience. But suppose that for whatever reason you have come to doubt these authorities. This means that you have come to doubt in a very general way your own received opinions, the battery of assumptions about how the world works that you have been indoctrinated to accept over the years. You want to know what the world is like. But in order even to begin this project you must first clear away the accumulated rubbish. It is not enough to say that you have doubts about what you have been taught. Unless you can genuinely purge yourself of these false and dubious opinions, they are likely to insinuate themselves into your thinking with your knowing it. So you need some device of mental discipline to prevent this from happening.

The First Meditation describes just such a device. It is the so-called "Method of Doubt". Descartes proposes to run through his received opinions, to ask which of them are dubious, and to suspend judgment actively whenever a reasonable doubt is possible. Of course he cannot run through his opinions one by one: that would take too long. So he proposes to take advantage of the fact that our opinions are structured like a building: some "rest upon" others, and some are so basic -- so foundational -- that nearly everything depends on them. If he can succeed in doubting one of these foundational opinions, he will thereby come to doubt everything that depends upon it. If, in the course of considering these foundational opinions, he comes upon something he cannot doubt, then he will have succeeded in finding at least one proposition to serve as the foundation for a genuinely rational system of scientific knowledge.

2. "All that up to the present time I have accepted as most certain I have learned ... from the senses"

Descartes observes that nearly everything we think we know depends on the assumption that our senses are reliable sources of information about the world. Whenever you come to believe that there is an apple in front of you simply because you seem to see an apple before you, you make this assumption. But equally, when you come to believe something on the basis of testimony, you assume that your hearing is a reliable source of information about what your informant is saying; and when you come to believe something because you have read it in a book or seen it in a film, you are assuming that vision is a reliable source of information about the words on the page or the images on the screen. Upon reflection, it is hard to think of any substantive belief about the world we inhabit that has not been acquired "through the senses". And this is just to say: it is hard to think of any opinion that does not depend for its justification upon the assumption that the senses are generally reliable.

Let us try to make this opinion more precise. Our sensory experiences have content. They represent the world to us as being a certain way. The best way to understand this notion of the content of experience is to imagine yourself in a situation in which the senses are likely to be unreliable. You are in a funhouse, surrounded by distorting mirrors. You seem to see a deformed person with an enormous head and enormous feet standing directly in front of you, and at first you are inclined to believe what you see. But then your remember where you are and you resist this inclination. You do not form the belief that there is a deformed person in front of you. And yet your sensory experience does not change. It continues to "tell you" this. It's just that you stop listening. We might say that your visual experience is an experience as of a deformed person; or we might say that it visually seems to you that there is a deformed person before you. It does not much matter which idiom we choose. The important point is that the senses are constantly "making claims" about how things stand in our environment, and that in the ordinary course of events, where we have no reason to believe that anything fishy is going on, we generally believe what our senses have to say.

3. The Cartesian Metaphysics of Experience.

Now Descartes himself seems to endorse a potentially controversial theory of what is involved in having experiences with a certain representational content. According to Descartes, all experience is a matter of being directly presented with ideas. In the cases of perception, imagination, hallucination, and the rest, these ideas are rather like pictures or images: they consist in a mosaic of colored shapes. But they are not ordinary pictures. Indeed, they are not physical entities at all. Descartes knew perfectly well that if we were to examine your brain while you were gazing at a blue flower, or dreaming of a blue elephant, we would not find any bluish flower-shaped or elephant-shaped expanse. Rather the ideas of perception and imagination are mental images: they exist "in the mind", but not in the brain or anywhere else in the physical world. The Cartesian picture of experience is therefore roughly as follows.

Figure 1: The external object (the tree) causes changes in the eye and brain which somehow cause a mental image, which the soul directly "perceives".


The corresponding picture of dream experience would then look something like this:

Figure 2: Processes in the brain somehow cause a mental image, which the soul directly "perceives"

In a dream, the mind is presented with ideas that are very much like the ideas it confronts in waking experience. The only difference is that in this case the ideas are not caused by the interaction between sense organs and the environment, but rather by some process in the brain of which we know very little.

Now this Cartesian theory of experience is useful for our purposes, since it permits a particularly sharp formulation of the skeptical problem. But it is important to realize just how bizarre it is. These "ideas" do not exist in the material world, as we have stressed. Descartes rather locates them "in the mind", which he regards as an immaterial spiritual substance that exists in time but not in space, and which is somehow "associated with" the living human body without being in any way identical with it or even spatially contained within it. Descartes offers arguments for all of these claims; and if we had more time we might consider them. Suffice it to say, however, that they are not clearly correct. It would therefore be useful to have a formulation of the skeptical problem that does not depend on this dubious metaphysics. And that is why I have employed the slightly peculiar idiom mentioned above. Rather than say: I am aware of a tree-idea -- i.e., a greenish, tree-shaped mental image -- we should say that I have an experience as-of a tree, or perhaps: it visually seems to me that there is a tree before me. Descartes' theory of ideas is generated by the assumption that whenever it visually seems to me that there is something green before me, there must really be something green before me: if not a physical object, then a a mental object tinted with greenish mental paint. But this is a resistible assumption, so let us resist it. Descartes' skeptical problem does not depend on the theory of ideas.

4. Skeptical Hypotheses

Our senses represent the world as being a certain way -- as containing rocks and trees and other people existing at some distance from our bodies in a public, physical space. We normally believe what our senses "tell us". That is, we normally reason in the following way:

I have a visual experience as of a world in which ....
My visual experience is presently reliable.
Therefore, I inhabit a world in which ...

Descartes seeks to cast doubt on such inferences by casting doubt on the second premise. He begins by noting that our visual experience is in fact notoriously unreliable about certain matters. When I see the mountains in the distance, I have a visual experience as of something purple. But closer inspections reveal that the mountains are really some mottled collection of brown and gray and green --- anything but purple. When I place a straight stick in a glass of water and look at it from the side, I have a visual experience as of a bent or broken stick, when in fact the stick is neither bent nor broken. This "Argument from Illusion" reminds us that the senses are not always reliable. But still, this observation has no tendency to undermine our confidence in our vivid sensory experience of nearby, medium sized objects in good light. Sensory illusions are cases in which some isolated subset of our sensory experiences disagrees with the rest. Our visual experience of the mountains from a distance is in conflict with our experience of the mountains from close up; our tactile experience of the stick is inconsistent with our visual experience when it is submerged, but not when it is removed. In these cases, we can use sensory experience to correct itself. But this procedure presupposes the general reliability of sense experience. And in any case, these illusions seem restricted to a narrow range of conditions. As Descartes says: it is one thing to take seriously the hypothesis that I might be under an illusion when it comes to the visual presentation of something very far away, or something very small or in bad lighting conditions. But it is another thing to suppose that I might be under an illusion to the effect that I am sitting in my dressing gown by the fire. This is not the stuff of "illusions", and any one who imagines that this might by an optical illusion of some sort is no longer a reasonable investigator but some sort of lunatic.

Descartes is much more impressed -- and rightly so -- by the thought that any given visual experience might be a dream. In a dream, your visual experience is massively and systematically unreliable even about the most obvious matters. As Descartes writes:

How often has it happened to me that in the I night I dreamt that I found myself in this particular place, that I was dressed and seated near the fire, whilst in reality I was lying undressed in bed. At this moment it does indeed seem to me that it is with eyes awake that I am looking at this paper; that this head which I move is not asleep .... What happens in sleep does not appear so clear or so distinct as all this. But in thinking over this I remind myself that on many occasions I have in sleep been deceived by similar illusions, and in dwelling carefully on this reflection I see so manifestly that there are no certain indications by which we may clearly distinguish wakefulness from sleep that I am lost in astonishment. (First Meditation)

The argument which this passage begins to develop might be presented as follows:

(1) Right now it seems to me that I am seated by the fire in my dressing gown, etc.
(2) So either I am awake and perceiving that this is so, or I am asleep and dreaming that this is so.
(3) I am justified in believing on the basis of my present experience that I am seated by the fire, etc., only if I can rule out the hypothesis that I am now dreaming.
(4) But there are no internal indications by means of which I can distinguish waking from dreaming.
(5) So I cannot rule out on the basis of my present experience the hypothesis that I am now dreaming.
(6) Therefore I am not justified in believing on the basis of my present experience that I am seated by the fire, etc.

But of course there is nothing special about the experience of being seated by the fire. I can rehearse an argument of this sort whenever I am inclined to trust my sensory experience as a source of information about the world around me, and it will always convince that I am not justified in doing so. So we have a general conclusion:


(7) Therefore, I am not justified in believing anything about the external world on the basis of my sensory experiences.


In order to resist this argument you must either reject one of the premises or somehow resist the reasoning. You should think about (4) in particular, just to see how strong Descartes's case really is. Our attention will be focused mainly on the transition from (4) to (5). It is one thing to say that I cannot be certain that I am not now dreaming, quite another to say that I am not somehow justified on the basis of my experience in believing this. Descartes is mainly interested in certainty. And it must be confessed that so long as this is the topic he is on very strong ground. But I have suggested that we should be more interested in what it is reasonable to believe than in what we can be certain of. And it might be, as we shall see, that sometimes the reasonable thing to believe is that one is awake, even when one's experience is strictly compatible with the hypothesis that one is dreaming.

The skeptical phase of Descartes' Meditations concludes with the invocation of the most radical skeptical hypothesis ever invented. The argument from dreams has certain built in limitations. If I dream that I am dressed in a purple robe, then the belief I form as a result of this experience may well be false. But if I dream that I have a brain in my head, or that I inhabit a world full of material objects, then even if my dream is wrong about some other matters, it will not be wrong about these particulars. So one might think that some propositions can be known on the basis of sense experience even if we cannot rule out that we are dreaming, namely those propositions about the world that would be true whether or not we were dreaming. One might then hope that these propositions could somehow serve as the foundation for a science that was generally immune from doubt. The point of Descartes' invocation of the famous Evil Demon hypothesis is to show that these slim hopes are illusory. My experience now is consistent with the hypothesis that I am awake, and also with the hypothesis that I am asleep in my bed. But it is also consistent with the bizarre hypothesis that there is no material world at all, and that my experience is being directly produced in me by the activity of an immaterial spirit, a demon "no less powerful than he is malicious". On this hypothesis, there are only two things in the universe: the demon and me. Both of us are immaterial spirits or souls, and all of my passive "sensory" states are caused in me directly by his activity. Everything we have said about the dream hypothesis applies here as well. The difference is that the Evil Demon hypothesis is so radical that it calls into question the existence of the material world itself. It shows -- as the Dream hypothesis does not -- that our experience is consistent with there being no material world at all. So if we are to know that there is a material world on the basis of our experience, we are going to have to "rule out" the Evil demon hypothesis.

Good luck.