Chapter 1: External world skepticism

The problem of external world skepticism

Matthew Van Cleave

In the Wachowskis’ 1999 film, The Matrix, Morpheus asks Neo:

“Have you ever had a dream, Neo, that you were so sure was real? What if you were unable to wake from that dream? How would you know the difference between the dream world and the real world?”

Philosophers would recognize that this question is not a new one. In fact, René Descartes (1596-1650, pronounced day-CART) asked this exact question 350 years before the Wachowskis in his Meditations on First Philosophy, published in 1641. The question, it turns out, is a philosophically deep one and the problem is that it seems that there is no good way to tell whether one is in fact dreaming or not.

You might think that the answer to the question of whether I am dreaming right now is an easy one: of course I’m not dreaming because I remember waking up this morning and eating breakfast! Or: I know I’m not dreaming because I can pinch myself and it hurts! Or: I am reading this philosophy textbook right now and that is not something that I would ever do in my dreams! However, all of these responses seem to miss the deeper challenge of the I-am-dreaming-right- now scenario. For all you know, your whole life has been one long, vivid dream that you have never awoken from. Sure, you have gone to sleep and woken up and even had dreams when you’ve “slept,” but this has all happened within one long meta-dream, which has been your life to date. All kinds of questions could be raised about such a possibility, such as: if I have been dreaming my whole life, how could I eat? And if I haven’t eaten how could I survive to continue dreaming?

These are good questions, but the modern skeptic has an easy solution to them, since there are many other skeptical scenarios that don’t raise these questions.

The scenario presented in The Matrix is a good example, so let’s switch to it. Imagine that your whole life has been a kind of computer simulation—your conscious mind interacts not with the “real world” but with a computer simulation. Within the computer simulation are all kinds of objects (trees, houses, cars, computers, other people, etc.) that appear to you exactly as they would in the “real world”—i.e., in a non-simulation world—the kind of world that

we think that we do live in. I can imagine someone asking: wouldn’t I be able to tell that a simulated coffee cup wasn’t a real coffee cup? Wouldn’t I be able to tell that it is a simulation? Consider what Morpheus says to Neo in The Matrix: “What is real? How do you define ‘real’? If you’re talking about what you can feel, what you can smell, what you can taste and see, then ‘real’ is simply electrical signals interpreted by your brain.” The point is that the object itself is not actually necessary in order for us to have a perception of it. If the coffee cup is just a pattern of brain activity, then we should be able to stimulate the brain in that particular way and thus produce a perception of a coffee cup—even if there is no coffee cup. Hallucinations and illusions are a kind of everyday case in point. External world skepticism is the claim that I cannot know whether I live in a simulation or a non-simulation because from inside my own conscious experience I would not be able to distinguish between the two different scenarios. After all, I would have exactly the same experience in the simulation world as I would in the non-simulation world. If this is so, then it seems that I cannot know if there exists anything outside of my own experience and perceptions. If I am really just in a simulation (i.e., a kind of dream world) then it seems that the objects with which I am interacting are not real objects—the dog bite in my dream is not the same as the real life dog bite. Of course, the objects in the simulation would seem real to me but in some deep sense they wouldn’t be real. Simulated dog bites are not real dog bites.

For example, if I had a dream that I was drinking coffee out of my favorite coffee mug and if I said to myself in that dream, “I am drinking coffee right now,” then what I have said would seem to be false (assuming I didn’t know that I was dreaming). Likewise, if in my dream my friend Bob kicked me and I formed the belief, “Bob kicked me” I would seem to have a false belief. Bob didn’t really kick me; I just dreamed he did. And I am not really drinking coffee; I just dreamed I did. In general, dreaming that x happened is not the same as x actually happening. So if in my vivid dream I come to believe any of these things, then those beliefs are false. In fact, all of my beliefs would seem to be false since all of the things I’m dreaming about are not real. The same would seem to be true if I am living in a simulation since the objects in that simulation are not “real” in the way we think that non-simulation objects are real.

Another way of laying out the problem of external world skepticism is to note that our experience does not allow us to distinguish the nature of things outside of us (since there’s no way for us to, so to speak, get outside of our own minds or experience) and that, since there are multiple ways that our experiences could

be caused (dreams, hallucinations, computer simulations, etc.), we cannot know which of those things is the cause of our experiences. But since this is so, it poses a deep problem for our knowledge. After all, everyone thinks that they can know that we live in a world of normal, physical objects and that these objects are what we are experiencing—i.e., they are what are causing our perceptions. But as soon as we become aware of the radically different scenarios that could equally explain our experience—what we can call skeptical scenarios—we must acknowledge that we don’t know which of these scenarios we are actually in. If I cannot know whether I am a person with a body and who interacts with real physical objects or a brain in a vat that is hooked up to a supercomputer which stimulates my brain to produce perceptions of what I think is the real physical world (but actually isn’t), then I cannot know whether there is an external world or not. That is the problem of external world skepticism.

We can put the problem of external world skepticism into a reconstructed argument like this:

  1. I know there is an external world only if my experience allows me to distinguish between the real world and skeptical scenarios.
  2. My experience does not allow me to distinguish between the real world and skeptical scenarios.
  3. Therefore, I cannot know for certain that the external world exists.

The logic of this argument is airtight. That is, if we accept the truth of the premises, then the conclusion has to be true. The above argument has the following sort of logical form:

  1. If A is true then B must be true
  2. B is not true
  3. Therefore, A is not true

As you should be able to see, there’s nothing wrong with the logic of this argument. But that leaves open whether or not the premises are true. So are the premises true? Premise 1 seems to be true. After all, if I am merely dreaming I am in my friend Kali’s living room right now, then I am not really in her living room and thus can’t know that I am.[1] Premise 2 also seems to be true: if my experience is confined to my own head, it seems that I am not able to see what is “really” outside of me. One might say at this point that I can compare my experience to the experience of others and in this way I could determine whether or not there are objects outside of my experience. Certainly, if I am experiencing a pink unicorn and I ask everyone else around me whether they see the pink unicorn and everyone looks at me like I’m crazy, then I have good reason to doubt whether the pink unicorn is real. Probably it is an illusion or hallucination. But this response neglects the fact that if I am in a dream/simulation, then my experience of you is itself part of the dream/simulation. Since you are part of the dream/simulation, our agreement concerning our perceptions doesn’t prove that the objects we seem to be perceiving are real, since we would both be under the same illusion.

Notice what the problem of external world skepticism is not. The external world skeptic is not claiming that we actually are living in a computer simulation, that we actually are dreaming, or that we actually are a brain in a vat. Rather, they are claiming that we cannot know that we aren’t in such a skeptical scenario.

And if we cannot know such a fundamental thing as this, then we cannot really have any knowledge about the world outside of us. None of this is to deny that we have experiences. The problem of external world skepticism does not question our experiences themselves. Rather, it questions whether we can know what those are experiences of—whether we are experiencing what we think we are (a normal, physical world). The skeptic’s claim isn’t that we are in a skeptical scenario but that we cannot know we’re not. And that is supposed to undermine all (or almost all) of our knowledge about the world.

Study questions

  1. True or false: The external world skeptic claims that we are probably living in a computer simulation or dreaming.
  2. True or false: External world skepticism claims that there is no external world; the only reality is our own perceptions.
  3. True or false: External world skepticism claims that we cannot know whether there is an external world (similar to what we perceive) that exists outside our perceptions.
  4. True of false: According to external world skepticism, if our experience does not allow us to distinguish skeptical scenarios from (what we think of as) the real world, then we cannot know there is an external world.
  5. True or false: It is possible that the argument for external world skepticism has flawed logic.

For deeper thought

  1. Suppose that you were dreaming that you were eating ramen noodles; would it follow that you were not really eating ramen noodles? Why or why not?
  2. If you were dreaming (and believed in your dream) that you were in Pensacola, FL and it turned out that you really were in Pensacola, would that mean you knew (in your dream) that you were in Pensacola? Why or why not?
  3. How would you respond to the external world skeptic? Can we in fact show that there exists outside of our minds a world that is roughly similar to what our perceptions tell us? Why or why not?

Responses to external world skepticism

It seems difficult to refute the external world skeptic. That is, it seems difficult to prove that there are objects that exist independent of our perception since we cannot get “outside” our perception to observe them. Nonetheless, philosophers over the centuries have attempted to refute the external world skeptic. We will consider three different answers to the skeptic: one from G.E. Moore in the early 20th century, one from Ludwig Wittgenstein, also from the early 20th century, and one from David Chalmers in 1999.

Before we look that those responses, let’s consider one famous argument that there are some things we can know even if we are in a skeptical scenario. René Descartes has a famous five-word that you may be familiar with: I think, therefore I am.[2] Here is Descartes’s point: even if I am dreaming, and thus wrong that there is a physical world outside of me, I cannot be wrong that I exist as a thinking thing. Why not? Well assume that I am dreaming. In that case, there must be something that does the dreaming since if there are dreams then there must be a dreamer. Descartes’s point is that I cannot be mistaken about the fact that I have perception and thoughts when I am having those perceptions and thoughts. Although I may be mistaken (if I’m dreaming or in some skeptical scenario) about all kinds of things outside of my experience, as long as I’m focusing on the experiences themselves, I cannot be mistaken that I am having them. If something is having those experiences, then that thing—my mind— most certainly exists. My mind may not have an attached body and may not look anything like what I think “I” look like, but I cannot be wrong that there is a thing that has experiences.

Consider the difference between the following two statements:

  • I am eating a juicy steak
  • I seem to be eating a juicy steak

Descartes would say that although you cannot be certain that the first one is true (since you could be merely dreaming that you are, or in a simulation, etc.), you can be certain that the second is true since it is about your experience rather than the objects outside of you. Even if you are merely dreaming that you are eating a steak, your statement that you seem to be doing so is true, whether you are dreaming or not. The point Descartes is trying to make is that I can be

certain that my mind exists even if I cannot be certain that anything exists outside of my mind. What I am unsure about is not whether my mind exists, but whether there is an external world. There cannot be any doubt, Descartes thinks, that there is an internal world—i.e., a mind that is doing the thinking and perceiving. This is the meaning of the famous dictum, I think, therefore I am.

Descartes’s argument that I cannot be mistaken that I exist (as a thinking thing) doesn’t provide a solution to external world skepticism, it only reinforces the problem. The external world skeptic is not skeptical about the existence of your thoughts and perceptions. Rather, she is skeptical about whether the external world exists. The skeptic’s point is that you cannot know that there is such a world outside your mind.

Moore’s response to external world skepticism

G.E. Moore was a 20th century British philosopher who thought he could prove that there was an external world. His argument is simple, but it will take a little bit of unpacking. Here is Moore’s argument (you have to imagine Moore demonstrating it to you with gestures as he says it):

  1. Here is a hand [Moore holds up one of his hands]
  2. Here is another hand [Moore holds up his other hand]
  3. Therefore, two human hands exist at this moment (from 1-2)
  4. This same argument could be made for any object I could hold in front of you.
  5. Therefore, external objects exist (from 3-4)

If you’ve followed the problem of external world skepticism, there’s something that seems fishy about Moore’s argument. Moore’s argument seems to commit the fallacy of begging the question. The problem is that premises 1 and 2 already seems to assume that the objects he/we are perceiving exist outside our minds in roughly the way we perceive them (what else could justify the inference to line 3)? Alternately, if premises 1 and 2 are simply referring to our perceptions, then line 3 doesn’t seem to follow since “hands” are the kinds of things which we conceptualize as existing independently of our perceptions of them (i.e., my hands still exist even if I’m sleeping or not currently perceiving them). But the skeptic would protest that Moore is not entitled to make any of these assertions because these assumptions are precisely what the skeptic is trying to put into question! This is the informal logical fallacy of begging the question—to assume as one of your premises, the conclusion that your argument attempts to establish.

Although Moore doesn’t think he is begging the question, it is important to understand the general strategy that Moore uses to defend his response to external world skepticism. That response turns on a comparison of our level of subjective certainty about certain propositions rather than trying to show that we aren’t in a skeptical scenario. In fact, Moore accepts that we cannot prove that we aren’t in a skeptical scenario, but he doesn’t think that we need to prove that in order to know that there is an external world. Which of the following are you more certain of, Moore asks:

  • Here (holding up your own hand) is one of my hands
  • It is possible that I am actually in a computer simulation right now
  • If I am in a computer simulation, then I do not know that I have hands

Moore’s point is that he is more certain of the truth of the first proposition than he is of the second or third (both of which are premises in the argument for external world skepticism). Moore thinks that because he is more certain of the premises of his argument than he is of the premises of the skeptic’s argument, his argument is therefore a better argument. Moore’s response to external world skepticism is thus what I would call an indirect response because it doesn’t attempt to refute the skeptic’s argument but, rather, attempts to put a competing argument forward. The conclusion of Moore’s argument is that we can know that there is an external world; the conclusion of the argument for external world skepticism is that we can’t know that there is an external world.

Moore doesn’t think that we can prove that the skeptic’s argument is wrong; he just thinks that his argument is better. We might wish that we could do better than this against the external world skeptic. We might wish that instead of merely asserting a different argument, we could show what is wrong with the skeptic’s argument. Moore doesn’t try to do this (and doesn’t think it can be done), but David Chalmers thinks we can show that there’s something wrong the skeptic’s argument. In particular, Chalmers will claim that the first premise of the skeptic’s argument is false. Before turning to Chalmers’s argument, however, we will take a look at a response to Moore’s argument from one of the most famous philosophers of the 20th century—and, according to some, one of the most brilliant philosophers of all time, Ludwig Wittgenstein (1889-1951, pronounced VIT-gen-stein).

Wittgenstein’s response to Moore

In On Certainty (which is really just a collection of his notes to himself), Wittgenstein reflects in on the concept of knowledge and how Moore uses (or misuses) it. Wittgenstein thinks that the fundamental flaw in Moore’s response to the skeptic is the mistaken conception of knowledge on which his response depends. The basic problem with Moore’s response, Wittgenstein thinks, is that he equates subjective certainty with knowledge. Just because a person is certain of something does not automatically mean that they know it. Sometimes we can be certain of things that turn out to be false (and thus we didn’t really know them). There are plenty of counterexamples of people being very sure of things that aren’t true (and thus they don’t really know). For example, someone in the grips of a schizophrenic delusion might be very, very sure that they are Jesus Christ reincarnated—or the creatress of the universe who is temporarily a nid. There are lots of things about which an individual could feel very certain and yet not really know them. So the fact that Moore is very certain about the truth of the proposition “here is a hand” does not mean that he knows it. For Wittgenstein, one’s level of subjective certainty about x doesn’t establish that one knows x.

To whom does anyone say that he knows something? To himself, or to someone else. If he says it to himself, how is it distinguished from the assertion that he is sure that things are like that? There is no subjective sureness that I know something. The certainty is subjective, but not the knowledge. So if I say “I know that I have two hands”, and that is not supposed to express just my subjective certainty, I must be able to satisfy myself that I am right. But I can’t do that, for my having two hands is not less certain before I have looked at them than afterwards. But I could say: “That I have two hands is an irreversible belief.” That would express the fact that I am not ready to let anything count as a disproof of this proposition (On Certainty, 245).

So what is knowledge, if not subjective certainty? For Wittgenstein, to know something is to be able to rule out doubts concerning that thing. For example, if Ana said that Sara was at Dagwood’s yesterday and I said, “How do you know that?” then whether or not Ana really knew that Sara was at Dagwood’s yesterday would depend on her ability to rule out specific doubts that I raised. For example, Ana could say, “Because I was at Dagwood’s yesterday too and I talked to her there.” But, Wittgenstein asks, can Moore rule out doubts raised against the proposition “this is my hand”? In everyday discourse no one would raise such doubts, but in the context of philosophy the skeptic is raising exactly that kind of doubt—perhaps you are dreaming (or in a simulation). But can this doubt be ruled out? Wittgenstein thinks not and Moore actually agrees with him on this. Their dispute is whether or not my claim to know something depends on my ability to answer specific doubts regarding that thing.

Wittgenstein thinks that Moore’s mistake is making knowledge into a private, subjective kind of thing, when in fact, according to Wittgenstein, knowledge is a very public, objective kind of thing. You cannot assure yourself that you know something just by believing it really hard (On Certainty, 245, 550). Rather, knowledge requires a kind of back and forth discourse between two different parties; it is what Wittgenstein famously called a language game. The language game involving the word “knowledge” involves being able to answer specific doubts raised about a claim. When those doubts have been answered, we call this “knowledge”; when they have not, then it is mere belief (even if it is a strongly held belief).

So how would Wittgenstein answer the external world skeptic? In contrast to Moore, Wittgenstein doesn’t think that our most basic beliefs (like “this is my hand”) can be justified and, thus, they do not count as knowledge. Rather, such beliefs are simply our starting points, our axioms. In mathematics, axioms cannot themselves be justified; rather, they are what the whole system depends on. Likewise, Wittgenstein thinks that our language games depend on certain propositions that cannot themselves be justified. This is the biggest contrast between Moore and Descartes, on the one hand, and Wittgenstein, on the other: Moore wants there to be a rational foundation for our knowledge about the world. He thinks that the most basic propositions of our language are also pieces of knowledge and that because of this the whole system of knowledge is a rational system. Moore’s “here is a hand” argument is supposed to be the kind of thing that justifies the most basic propositions of our language—such as that there is an external world of physical objects. Wittgenstein, in contrast, does not think that the most basic assumptions of our language can be justified. Rather, they are just things we do or assume but that cannot themselves be given a rational justification. Here is a very telling passage from Wittgenstein’s On Certainty where he makes this point:

If I say “Of course I know that that’s a towel” I am making an utterance. I have no thought of a verification. For me it is an immediate utterance. I don’t think of past or future. (And of course it’s the same for Moore, too.) It is just like directly taking hold of something, as I take hold of my towel without having doubts. And yet this direct taking-hold corresponds to a sureness, not to a knowing (On Certainty, 510-511).

For Wittgenstein, the most fundamental, basic propositions of our language— the ones that the external world skeptic is trying to cast doubt on—cannot properly be said to be “known.” That is, they aren’t pieces of knowledge.

Rather, they are just something we assume within a system—they are just something we do. Like taking hold of a towel that is before me, I don’t even think about whether this is a towel or how I know it is, I just assume it is—or act as if it is. So Wittgenstein does not think that propositions like “there is an external world that exists outside of my perceptions” is something that can be known or proven. But the fact that we cannot justify the most basic propositions of our language isn’t a defect in our language (or our knowledge); it’s just the way things are and must be based on how language works. Just as it isn’t a defect in a mathematical system (e.g., geometry) that the axioms themselves cannot be justified, so too it isn’t a defect of our language that the axioms (such as that there is an external world) cannot be justified. Moore wants to be able to prove those axioms; Wittgenstein thinks that they can’t be proven.

So is Wittgenstein an external world skeptic? Not really. Rather, he thinks that both the skeptic and Moore are making the same kind of mistake regarding what knowledge is. Both Moore and the skeptic accept the claim that if we cannot justify our most basic, fundamental claims, then all of our knowledge about the external world is thrown into question.[3] Wittgenstein, in contrast, rejects this proposition. To have knowledge is to be able to answer specific doubts raised about the claims one is making and one doesn’t have to answer the external world skeptic in responding to specific doubts. For example, in my earlier example Ana didn’t have to consider possible responses to Descartes dream argument in order to satisfy my doubts regarding Sara’s whereabouts.

Likewise, I don’t have to answer the external world skeptic in order to know that it was a brick that broke my window, for example. Moore and Descartes think that we need an answer to the skeptic in order to have knowledge about the world; Wittgenstein rejects this assumption. Wittgenstein’s position is sometimes called anti-foundationalism because he rejects the foundationalist assumption (shared by Moore and Descartes) that for something to be known it must be justified by some other piece of knowledge. Wittgenstein thinks that the most basic assumptions of our discourse cannot themselves be justified and are not properly called knowledge. This might seem like a kind of skepticism, but if it is, it is very different from external world skepticism. The external world skeptic takes the fact that we cannot justify the most basic claims (such as that I am not dreaming or am not living in a computer simulation) as undermining all the rest of our knowledge of the external world. Wittgenstein agrees that the most basic claims cannot be justified but disagrees that this undermines the rest of our knowledge. As we will see, David Chalmers’s view has some affinities with Wittgenstein’s on this point.

Chalmers’s response to external world skepticism

Invoking the Wachowskis’ movie, The Matrix, the contemporary philosopher David Chalmers asserts that “even if we are in a matrix, our world is no less real than we thought it was. It just has a surprising fundamental nature.”[4] This, in a nutshell, is Chalmers’s response to external world skepticism. Let’s unpack it to see what he’s getting at.

Recall that one of the skeptical scenarios we’ve been contemplating (and that is an oft-used example in contemporary philosophy) is that of a computer simulation—for all you know, your whole life has been lived within a computer simulation. The skeptic’s point is that if that were true, then there wouldn’t be an external world of the sort you think (one of physical objects in time and space). The skeptic also thinks that you can’t know that you aren’t in a computer simulation. Chalmers does not try to dispute the claim that we can’t know that we aren’t in a simulation. Rather, he disputes the first claim—that if we were, then all our knowledge claims (that implicitly assume an external world) would be mistaken. Chalmers thinks that this doesn’t follow. Here’s why.

Consider the example of Eddington’s two tables. Arthur Eddington was a famous physicist who in a 1927 lecture at University of Edinburgh gave the following example. Consider a kithen table as revealed to us by science: it is composed of mostly empty space (since an atom is mostly empty space), is in constant flux (since atoms are), has no colors (since atoms are not colored), etc. This is in stark contrast to the kitchen table as revealed by our senses which is solid (thus not empty space), stable (thus not in constant flux), and has a specific color (in my case, cream-colored). But does the existence of the “scientific table” undermine our “common sense” table? It would be absurd to think that it does.

Rather, what science reveals as true about the table (and all other objects, too) is just something surprising about the table’s underlying nature. The table is still the common sense table; it is just revealed to have a surprising underlying nature. Thus, the table’s surprising underlying nature doesn’t make the table not a table anymore. It’s still a table and we can still properly describe it as being solid, stable, and cream-colored.[5]

Now consider what would have to be the case if the reality in which we were currently living was in fact a computer simulation. That reality would still have a physics, a chemistry, a biology, etc.—in short, it would have all of the levels of reality that we think it would. But it would also have one further level of reality that we don’t think it has: it would have a level of reality lower than physics—the computational level of the computer program. Physics (think electrons, quarks, etc.) is typically thought of as the lowest level at which we can study reality.

Physics applies to everything that exists (well, assuming only physical things exist), whereas biology applies only to some things. That is, the laws of physics apply to everything whereas the laws of biology apply only to some things. For example, a frog is a biological and physical thing, whereas a rock is a physical thing and hence biological rules do not apply to it. If we were in a simulation right now, there would still be frogs and there would still be biological organisms and physical realities, it’s just that those physical things would also have a further, surprising underlying nature: they would be also be, at root, computational entities—that is, they would really be 1s and 0s of a computer program. But does that mean they wouldn’t be physical objects? No, it doesn’t. Rather, the frog in a computer simulation would still be a biological and physical organism, it would just also be a computational object. This would be surprising (if we didn’t think we were living in a simulation), but it wouldn’t make the frog not a frog.

So Chalmers’s response is specifically about matrix-like scenarios—i.e., scenarios in which we are living in a computer simulation. His argument contrasts with what the external world skeptic would have us believe, the fact that we were living in a computer simulation does not mean that there isn’t an external world filled with objects that have roughly the kinds of properties that we perceive them to have. Thus, Chalmers takes himself to have shown that there is a certain class of skeptical scenarios that actually don’t do what the skeptic thinks they do. Sure, there are some beliefs that would be false if we were, in fact, living in a simulation. For example, the belief that physics is the most basic level of reality would be false in a simulation (since in a simulation it would be the computational level that would be the most fundamental). But these beliefs are more abstract philosophical/scientific beliefs rather than every day beliefs. Since the skeptic takes the skeptical scenarios to undermine all of our knowledge about the external world, the skeptic is mistaken.

Chalmers’s response relies on a particular account of how our words apply to things, what is called the causal theory of reference. The basic idea is that the way our words attach to (refer to) things in the world is by means of our causal histories with those objects; we don’t have to understand the complex natures of those objects (e.g., that water is H2O) as long as those are the objects we have always used our words to refer to. So, for example, if I have always lived in a computer simulation and in that simulation I have always used the word “frog” to refer to a particular kind of object in that simulation (i.e., the frog simulations) then that is what the term “frog” means for me. On the other hand, if I am not living in a simulation then the term “frog” refers to objects whose fundamental nature is physical. But here is where a little wrinkle arises in Chalmers’s neat argument: suppose that I have learned the term “frog” in a non-simulation world but then one night in my sleep I am kidnapped and put into a simulation that is identical (from the point of view of my experience) to the world as I’ve always known it. In that case, the skeptical problem would return in full force because now none of my words (which are anchored to the non-simulated reality in which I learned the terms) refer to anything in my new simulated reality and thus almost all of my beliefs and assertions are now false. So when I now say “there is a frog” when the froggish thing hops by, that statement is false because the meaning of “frog” for me is tied to my former reality—the non-simulated reality that I falsely think I’m still in.[6] In light of this, Chalmers’s victory against the external world skeptic is a limited one since it applies only to certain skeptical scenarios but not others. If I am and have always been in a simulation, then Chalmers’s response to the skeptic is successful. But as we have just seen there are other kinds of skeptical scenarios where Chalmers’s response is not successful and the external world skeptic wins. So Chalmers’s response is not an unmitigated defeat of external world skepticism.

Study questions

  1. True or false: Descartes does not think that he can be certain of anything.
  2. True or false: Moore thinks that he can prove that he isn’t dreaming (or in a skeptical scenario).
  3. True or false: Moore’s response to external world skepticism is direct rather than indirect.
  4. True or false: Wittgenstein thinks that Moore is correct to say the he knows that “this is a hand.”
  5. True or false: Foundationalists claim that in order for some piece of knowledge to be justified, it must be justified by some other piece of knowledge
  6. True or false: Wittgenstein and Moore agree that we can know basic propositions like “this is my hand.”
  7. True or false: Wittgenstein thinks that as long as one is subjectively certain about something, one knows it.
  8. True or false: Chalmers thinks that if we were living in a simulation, then most of our beliefs about the external world would be false.
  9. True or false: Eddington’s two tables is supposed to show that physics proves that there aren’t really tables.
  10. True or false: Chalmers thinks that his defeat of external world skepticism applies equally well to any skeptical scenario.

  1. Note that that fact that I’m dreaming does not imply that I’m not in Kali’s living room. For example, someone might have kidnapped me while I was sleeping (and having this dream) and put me in Kali’s living room. It would still seem to follow, however, that I didn’t really know I was in Kali’s room, since my true belief would have been a kind of fortuitous accident.
  2. Actually, it is only a three-word argument in Latin (the language in which Descartes—and all medieval philosophers before him—wrote): cogito ergo sum. Descartes’ argument is sometimes referred to the “cogito argument,” “cogito” being Latin for “I think.”
  3. Where Moore and the skeptic part ways is that Moore rejects the consequent of the conditional whereas the skeptic embraces the antecedent of the conditional.
  4. David Chalmers, The Matrix as Metaphysics in Philosophers Explore the Matrix, Oxford University Press, 2005.
  5. There is actually a deep philosophical (metaphysical) question about the relationship between the different levels of description which we won’t get into here. But this metaphysical question doesn’t seem to undermine the claim that we can properly describe the table as solid, etc.
  6. Note that the same would be the case if I had always lived in the simulated reality and then were somehow moved to the non-simulated reality.


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