LARRY: Okay, I got one. Brain in a vat walks into a bar. Bartender says, ‘Can I get you a drink?’ Brain in a vat says, ‘No thanks, I already am one.’

STAN: Real funny.

L: You didn't like it? Okay, how ‘bout this? Brain in a vat walks into a bar. Some drunk sidles up and says, ‘Hey, can I smell your hands?’ Brain in a vat says, ‘Absolutely not. I don't even have hands.’ Drunk says, ‘Well, then it must be your feet!’

S: Dumb and derivative.

L: Oh, so you want original? You want clever? Brain in a vat runs into a bar, stops abruptly at the counter, and says, ‘Hey barkeep, gimme a drink.’ Bartender shakes his head and says, ‘No way. You're sloshed.’

S: Ouch. And how does a brain in a vat run into anywhere? Or walk.

L: Dude. What's your problem? I'm trying to lighten things up around here. I mean, yeah, we're both pulling eight bucks an hour in a crappy nightwatchman job at some community college science lab making sure somebody doesn't break in and steal a bunch of brains in vats. But that doesn't mean we can't have some fun to pass the time. Geez. Now on that note, any cold ones left in the cooler? I'm dry.

S: Cooler's cashed. But I stashed a sixer on the floor behind the cryogenic freeze chamber. You know, where they keep the zebra? I opened the release valve a little to blow cold air on ‘em.

L: Sweet. Why would anybody ice a zebra anyway?

S: No idea. I hope we don't end up thawing the thing out.

L: Meh. We'll just drop it off at the zoo. They'll never know. More important to keep our stuff cold. And well hidden. I'll grab you one too.

S: Don't bother. I'm not in the mood.

L: Listen. I'm getting real sick of your mopey attitude. Either you tell me what's up, or I'm throwing you in there with the zebra.

S: All right here's the thing: You ever wonder what it's like?

L: What what's like?

S: To be brain a brain in a vat. You know, there you are confined to a tub of 7-Up your whole life just to serve the perverted interests of a bunch of stupid scientists. And then, as a further insult, some know nothing rent-a-cops make lame jokes and bounce quarters at you all night.

L: Well, it's not like they feel anything. I mean, they're brains in vats!

S: What are you talking about? Each one of those brains is alive and physiologically the same as a normal one. And they are being stimulated in exactly the same ways. It's just that the stimulation comes from the computer over there by the copier instead of sensory organs. In a normal human being, experiences, beliefs, emotions, and everything else involved in consciousness is just a product of brain stimulation. Doesn't matter the source. Eyes, ears, nose, computer in a lab, doesn't matter.

L: So what are you saying? They have lives just like ours?

S: Seems that way to them. In fact, I overheard one of the whitecoats—you know, the bald one, always has the clipboard?—saying it's rigged up so these brains have the same sorts of perceptual sensations a normal person would. One of those brains over there could be having the same experiences as you right now.

L: You mean dying of thirst while having to listen to this tripe? Well, I do feel sorry for them then.

S: I'm serious.

L: Well, then wait a minute. If their experiences are the same as ours, it's not like they know they are brains in vats.

S: Exactly. They don't know. But I think they have a right to know. I don't think these scientists can deceive them like that for the sake of some pointless experiment. These are human brains we're talking about here.

L: So what are you going to do? Tell them they're brains in vats?

S: I was thinking about it. You see that microphone on the counter there? Next to the computer? I overheard the shrinks saying it's a brand new brain-in-a-vat voice interface system. You talk into the mic, and the computer processes it and then sends the signal over to the brains, and then they believe what you say. I was thinking I should tell them what's really going on around this place. You know, blow the whistle.

L: So if you talk into the mic, they hear you?

S: Not really. The earliest voice interface systems worked that way. But the problem was the brains usually wouldn't believe what they were told. Brains can be real stubborn. So the new system bypasses the hearing mechanisms and puts the information right into the brain.

L: I don't know, man. They told us not to mess with any of the equipment. Could screw up the experiment. Then what? This job stinks and everything. But after that night with the volleyball team, the college won't let me drive the parking garage shuttle anymore. So I lose this gig, I'm done. Besides, wouldn't that make it worse for them? Wouldn't they get lonely just sitting there knowing they're brains in vats?

S: Lonely? No. They've got it wired up so they can communicate with each other. You see those headphones? That's how the lab techs listen in on what the brains in vats say to each other.

L: Say to each other?

S: Yeah, I know, they don't have tongues. Or ears. But tongue movement is just a product of stuff going on in the brain. They got it rigged so the brain activity that normally causes talking sends a signal from one brain to the other brains. The others receive the signal as the experience of hearing a sound—the same sound that would have been made if the brain was hooked up to a tongue. And you can hear it all with the headphones.

L: But what's their life gonna be like once you liberate them? Aren't they just gonna see a bunch of bubbly fluid and the inside of a glass container? Aren't they worse off that way?

S: I don't know how to control what they see or hear or smell or anything like that. That's done with the computer software, and you need a password to get in. I tried ‘password’ and some dirty stuff, but none of that worked. Scientists are smart.

L: So you're going to tell them that they are all a bunch of brains in vats but their experiences will stay more or less the same? So they'll believe that they see stuff in front of them, but they also believe it's not there?

S: No, here's the thing. You see the chrome box next to the microphone? With the red button on it? That's a reboot switch. I heard the scientists telling one of those tour bus groups the other day about how there was this big problem with the phase 2 voice interface prototypes when they first figured out how to put information right onto the brain. They would speak into the mics, and about half the time, the brains would explode. Huge friggin’ mess. And these brains ain't cheap. Turns out there was some theory where you can't have inconsistent beliefs or something. And then there's some other theory where a brain has so many beliefs there's no way to know whether a new one will generate a contradiction.Footnote ¹ Sounds like a bunch of baloney to me. People contradict themselves all the time. But what do I know? Anyway, that's why they added the red button. You push that, and the brains reboot around what you just said.

L: Reboot?

S: Yeah. Each brain believes what you say, and if you reboot, it maintains anything it believed before—provided it preserves consistency. Stuff that's inconsistent with what you said gets tossed. Some new stuff gets filled in so it all hangs together. And it's done seamlessly.

L: Seamlessly.

S: Yeah, in other words, the brain doesn't notice the reboot. It's as if what you told it was something it believed all along. The reboot reboots the brain's memory too, I guess.

L: Whoa.

S: Yeah. The system's brand new, so they haven't installed it into the computer yet. You don't need the password to use it. But it won't be like that for long. So, if I'm gonna do the right thing, I need to move quick.

L: Whatever. Long as I got nothing to do with it.

S: Yeah, okay. Why don't you go grab us a couple of those zebrews?

L: Way ahead of you.

SUBJECT 948: Your body illusion is looking good, Subject 237. Have the mad scientists who control all of our consciousness been regularly subjecting your brain to workout experience simulations?

SUBJECT 237: Indeed. My pleasant hallucinations of eating and drinking were becoming closely correlated with very unpleasant body illusions. Luckily there's a new program running. Thanks for noticing, 948. What have you been merely appearing to do for fun lately?

948: I've been enjoying a computer-generated sequence of illusory fishing experiences.

237: Followed by fish illusions?

948: No. But I guess that's why we call them fishing hallucinations and not fish hallucinations!

237: You know it.

Subject 43: Hey, gang, what's up?

948: Oh hey, Subject 43. We were just talking about 237’s body illusion and how tight it merely appears to be lately. I mean just take a second yourself and enjoy those bicep-shaped sense-data he's got going on.

43: That's interesting because I've been thinking a lot about our various hallucinations of physical objects lately.

237: Oh yeah?

43: Yeah. Lemme ask you guys this: What if we actually have bodies? Instead of everything we've ever thought about the universe, what if all of our experiences are not illusory but veridical? What if—contrary to what we all think—experience accurately represents real things in an external world? What if, instead of this being an illusion of a hand, it's a real hand?

948: How is that even possible?

43: Well, imagine all your experiences are caused by sensory organs responding to external objects in a brain-independent spatiotemporal world. Now, I don't mean the mere illusions of sense organs that we receive from the computer when we merely appear to look into a mirror. But real eyes, ears, and noses. In other words, what if we're not brains in vats but brains in skulls, which are inside bodies, and our sensory experiences are accurate, and we just think we are brains in vats who hallucinate all the time?

237: Oh boy. Here we go, Mr. Philosopher!

948: Yeah, really dude. Brains in skulls? C'mon! Everybody knows we're brains in vats. It's common sense!

43: But think about it. We believe there's a computer causing us to have illusory experiences, but how would it look if our experiences were veridical? It would look just like this! So how can we know we don't have hands? We all believe that. But how can we know? How do you know you're not just a brain in a skull?

237: Like he said, common sense.

43: It's important to preserve common sense only if we start from the assumption that common sense is mainly correct. But that's exactly what I'm challenging. I'm asking for some sort of evidence for these commonsense beliefs we have.

237: Okay, I'll play. Let's grant there's a ‘possible world’ where brains like us inhabit skulls instead of vats. And in that alternate reality, those poor saps are totally wrong.

948: Hang on. Shouldn't the brains in skulls think they are looking at hands?

43: Maybe you could have a case like that. But I'm imagining a situation where a brain in a skull has all the same experiences, reasoning processes, and beliefs that you do. If you like, you can imagine an exact duplicate of yourself inside a skull, which is inside a body, which inhabits a world where all this stuff we hallucinate is real. And that stuff gets accurately represented to the brain through sense organs instead of through a computer in a mad scientist's lab. Now, since that brain is identical to you, it too will believe that the world it experiences is an illusion, that it doesn't have hands, and all the rest. So my question is, how do you know you're not that brain in a skull?

237: Yeah, yeah, I get it. They think they don't have hands and that their experiences are mere hallucinations just like we do. But they're totally wrong. I'll grant you that's a possibility. The mistake happens when—

948: Wait. Should we really grant that's a real possibility? I mean, c'mon! How did the brains get in the skulls in the first place? And if there are no mad scientists running the lab, who keeps them alive? Who hooked up their comwires? How do they talk to each other? You're gonna have to spell out some details here before you can get me to buy the idea that brains in skulls are possible. Otherwise you're just leaving too many unanswered questions.Footnote ²

43: Okay. Well, what if, instead of a scientist putting it there, the brain is somehow born into a skull and then—

237: Look, it really doesn't matter how you fill in the details. Here's why: knowledge is just true belief formed by a reliable process.Footnote ³ My belief that I do not have hands was formed by the process of counterperception—a process we employ all the time, every day. I am confronted with a perceptual image such as this hand-shaped sense datum. And this causes me to believe that I do not have a hand. Given the relevant sensory inputs, the same sort of process generates all sorts of true beliefs about what the world isn't like. This foot illusion, for example, generates the belief that I do not have a foot. Since I am a brain in a vat who is being fed these hallucinations by a team of mad scientists, this familiar, commonsense, and completely natural process of counterperception will produce true beliefs most of the time.

43: But then what about the brain in a skull who forms that same belief by the same process but ends up always wrong?

237: You said it. He's wrong, so he doesn't know. And notice his counterperceptual belief-forming processes are totally unreliable. But that's his problem. As long as our counterperceptual processes are reliable, we know we don't have hands.

43: Okay, provided counterperception is reliable, you know you don't have hands. But how do you know counterperception is reliable?

237: Don't confuse knowing with knowing that you know. To know that P I need to form a true belief that P via a reliable process, but I do not need to know that the process is reliable. You say knowledge is impossible. So all I need to do is lay out the conditions under which it is possible for me to know that I do not have hands. And I did. It is possible that my belief that I do not have hands is true and produced by a reliable process.

948: I thought 43 was out of his vat there at first, but now I'm thinking he might be on to something. I want a reason to think I know that I do not have hands. You've given me a reason to think it's possible that I know. But I want a reason to think knowledge is actual. I don't think you can give me that unless you can show that 43’s brain in a skull hypothesis is actually false. Not possibly false, but actually false.

237: Oh, that's all you want? A reason to think I'm not a brain in a skull looking at real hands? Well that's easy. This is a hallucination of a hand. So is this. Therefore, I'm not a brain in a skull looking at real hands.Footnote ⁴ QED, BIV.

43: That's begging the question!

237: Not at all. The fallacy of begging the question, that is, ‘circular reasoning’, occurs when a subject's belief in a premise of an argument is epistemically dependent on prior acceptance of the conclusion of that argument. Suppose S offers P as a premise in support of Q. But then when you ask about P, it turns out that S’s support for that is his belief that Q. When that happens, S hasn't really offered a reason for Q.

43: Uh huh. And that's exactly what you just did.

237: No, I didn't. Look at it this way. I hadn't even thought about the possibility that this might be a real hand because I am a brain in a skull until you showed up and started going on about it. Ergo, my belief that this is a brain in a vat's hallucination of a hand is certainly not grounded in a prior acceptance of the proposition that I am not a brain in a skull. So there's no fallacy of circular reasoning going on here.

948: Well, I'm glad you brought that up because this has been bothering me all along. From the fact that these are hand hallucinations, why does it follow that I don't have a hand? Couldn't a brain with hands still hallucinate hands?

237: How's that?

948: Well, in 43’s science fiction story it's the eyes rather than a computer that causes the brain in a skull's visual sensations. But what if something gets messed up along the way? Suppose the eyes are closed, but somehow the brain still produces a hand image. This would mean that the brain in a skull hallucinates a hand and, at the same time, has hands. Hallucinations can be veridical. From the fact that this is a hallucination of a hand it doesn't logically follow that I don't have hands.

237: Two things. First, the argument was this: These are hallucinations of hands; therefore, I am not a brain in a skull looking at real hands. That follows. Second, your understanding of ‘hallucination’ is confused. Maybe you can introduce a sense of ‘hallucinating that p’ where it is compatible with p’s being true. And if you are unfortunate enough to be a brain in a skull, I guess what you described might happen. But that's not how we use the phrase. Under the common everyday conception of it, hallucinating is antifactive. In other words, hallucinating that P entails not-P; ‘hallucinates that’ is an antifactive mental state operator or AMFSO.Footnote ⁵ That's just how we use the term in our language.

43: It is?

237: Yeah, it is. Somebody looks up and says he's hallucinating a hawk circling above him. And then you convince him there really is a hawk where he's looking with exactly the features he's experiencing. Well, he'd surely retract his claim that he was hallucinating it. ‘Hallucinate’ is a failure word.Footnote ⁶ But if you want to insist that you can hallucinate that P when P is true, then just take my use of the words ‘hallucination’, ‘illusion’, ‘image’, and the like to mean mere hallucination, mere illusion, and mere image. If S merely hallucinates that P, then not-P. And since I'm merely hallucinating that I have hands, it follows that I don't.Footnote ⁷

43: But that's the whole point! The brain in a skull will say the same thing. But he's got a hand! So how do you know it's a mere hallucination rather than a veridical experience?

237: If it appears to walk like a mere duck-shaped sense-datum, appears to quack like a mere duck-shaped sense-datum, then, well, you know the rest.

948: I want to be on your side 237, but I think you're ignoring 43’s point.

237: No, I'm not. Just attend to the features of your experience. Isn't this is exactly what a computer-generated hallucination of a hand would be like? It has visual aspects that appear three dimensional, it generates olfactory sensations. You can even superimpose your tongue-shaped sense-datum over it and generate taste sensations.

948: That's kinda gross.

237: Not for me. What sense data have your hand illusions been overlapping lately? 43 is demanding we produce evidence that this is a hand illusion rather than a real, brain-independent hand. What more evidence could you possibly want than the very features of that experience? Anyone who pays attention to those features and applies our normal epistemic procedures will get the result that this is a hallucination of a hand.

948: But he's raising a challenge to our normal epistemic procedures.

237: Well, if the only acceptable response is to step outside my vat and directly compare my brain's hallucinations against objective reality, then the ‘challenge’ is nonsensical. This passes every conceivable test we have for being a computer-generated hand illusion. To continue to suggest that it might not be a hand illusion makes no sense.Footnote ⁸

948: I want to agree with you, but that still doesn't seem like an adequate response.

237: You should note something else very peculiar about 43’s ‘challenge’. He's demanding that we somehow disprove his hypothesis that these are real hands because we really are brains in skulls. But he's given us no reason for thinking his far-out science fiction hypothesis is true. He's not provided any reason to think we are brains in skulls. He is just—in true philosopher's fashion—floating it as a possibility. He hasn't rationally motivated the idea that I am a brain in a skull, he's merely raised it.Footnote ⁹ Now maybe if he were to provide some sort of positive support for his strange hypothesis, a simple appeal to experience along with our ordinary everyday epistemic procedures wouldn't do. But as of right now, I don't see why I owe 43’s screwball idea any more attention.

948: But he's trying to defend a skeptical position here—we don't know one way or the other. You can't expect him to give evidence that you are a brain in a skull.

237: Exactly. Not only has he not given us reason to think we are brains in skulls, on pain of contradiction, he can't give us that. But, as I've already noted, our perceptual experience provides tons of evidence for thinking we are hallucinating. So, in the absence of any defeaters, we are justified in believing that we do not have hands.

948: I agree this thing looks just like a hand illusion. Sure. But doesn't it also look just like a hand? So, isn't our evidence just as good either way?

43: That's what I've been saying all along. If we were brains in skulls, our basis for thinking that we don't have hands would be exactly the same.

237: There's another place where we disagree. Suppose you have two cases. In one case, the one commonsense tells us is real, we are brains in vats whose perceptual experiences are all hallucinations generated by a computer that is controlled by a bunch of mad scientists. Call that the Good Case. In the other case, we merely think we are brains in vats whose experiences are illusory but really we are brains in skulls accurately representing a brain-independent external reality. Call that the Bad Case.Footnote ¹⁰

43: Okay. Good Case: brain in a vat, hallucinating mere images of hands; Bad Case: brain in a skull, experiencing real ones.

948: Sounds right. Got it.

237: Now, 43’s argument presupposes that the evidence in these two cases is the same. I disagree. The subject in the Bad Case has different evidence.

948: But the two are indistinguishable. So the evidence is the same.

237: Just because two things look the same doesn't mean they are the same.Footnote ¹¹ Skeptics can't just stipulate that the evidence is the same in both cases.Footnote ¹² Furthermore, in the Good Case, your evidence for thinking that you do not have hands is that you are hallucinating a hand. Ex hypothesi, the poor saps in the Bad Case are not hallucinating. They actually see hands. Ergo, they don't have the same evidence we do. This is the cure for the skeptical disease.

43: I feel a relapse coming. Even if we accept that seeing and hallucinating are different things and thus different types of evidence, the question of whether you know you don't have hands then just becomes a question of whether you know you are hallucinating rather than seeing. And how do you know that?

237: Again, don't confuse knowing with knowing that you know. Also, hallucinating that P is both antifactive and reflectively accessible. In other words, I can know via introspection that I am hallucinating that P.Footnote ¹³ Now before you say ‘But a brain in a skull will also think he is hallucinating and he's wrong’, don't take what I said to mean that I can discriminate my hallucinations from their corresponding veridical perceptions. I can't. But unless there is a defeater—and there isn't—knowledge doesn't require discriminating evidence; it only requires favoring evidence.Footnote ¹⁴ So yes, if I were in the Bad Case, I'd think I was hallucinating, and I'd be wrong. But since I know I'm hallucinating, I know I'm not in the Bad Case.

948: I think I've identified what bothers me about your response to 43’s skeptical challenge. Let's go back for a second to something I mentioned earlier. Let's imagine a case, kind of like the Bad Case, but where the subject correctly believes he is a brain in a skull. In other words, suppose you had a subject who believed his experiences accurately represented a real hand-filled world, and suppose that in fact they do. And he bases this belief on his veridical experiences of hands. Call this the Not So Bad Case. Seems like you should say that the subject in the Not So Bad case knows that he has hands. He sees rather than hallucinates his hands; he believes that he has hands, and his belief is based on the fact that he sees his hands.

237: Uh, okay. So what?

948: You think the Good Case and the Bad Case look the same from the inside, right?

237: They look the same from the inside, but evidentially speaking they are not the same. In the Good Case, the subject is hallucinating; in the Bad Case, he isn't.

948: Okay, so, aside from the fact that his beliefs differ, things will look the same in Not So Bad Case too. The Not So Bad Case will be one where the experiences are indistinguishable from the other two.

237: That's correct. And I suppose, if we set aside what they believe, the evidence in the Not So Bad Case and in the Bad Case will not just look the same but it will be the same—since they both see rather than hallucinate hands. But don't forget a subject in the Good Case—that is, I—will have different evidence.

948: Yes, yes, I get it. Now here's my problem. Given my current experiences, how do I decide what to believe? If I start with the assumption that I am a brain in a vat, I can say I am in the Good Case because I hallucinate hands. But if I begin with the assumption that I am a brain in a skull, I can just as easily say I am in the Not So Bad Case because I see hands. Why believe one over the other?

237: Well, this Not So Bad Case is another philosopher's invention. And we certainly have no reason to think it actually obtains. What kind of nut would believe he has hands? It's much more natural to think that we're all in the Good Case, and thus we are hallucinating hands. All you need to do there is appeal to the experience itself and follow our normal epistemic procedures.

948: I could imagine someone who thinks he is in the Not So Bad Case saying it's more natural to believe you have and see hands than to believe you lack and hallucinate them. I think what's going on here is that the arguments you are offering in response to 43’s challenge aren't really what's doing the work. What's really happening is that you are beginning with certain commonsense assumptions about the way the world is—the very assumptions 43 is challenging—and then you are building a philosophical theory on top of that. But this means all your philosophical theories never really address the challenge.

237: Well, let's make it real simple. 43 says I am not justified in believing that I do not have hands. But it seems like we are brains in vats hallucinating hands. Even 43 agrees with that, right? Now, if it seems to S as if P, then S thereby has at least a prima facie justification to believe that P.Footnote ¹⁵ And it seems to me that this is a computer-generated hand hallucination. Therefore, I am justified in believing that's what this is. And therefore skepticism is false.

43: So if something seems true, it is?

237: No. Pay attention. If it seems to S as if P, then S has a prima facie justification to believe that P; the fact that it seems like this is a brain in a vat's hand hallucination will justify me in believing that's what it is if there is no defeater. And again, you've presented no reason to think I am in fact a brain in skull. So, the way it seems stands.

948: But why does it seem that way? And why does it seeming that way justify you in believing anything?

237: The answer to your second question lies in the answer to the first. The basis of my perceptual seeming is the content of my perceptual experience. It seems to me that this is a brain in a vat's image of a hand in virtue of the content of my current perceptual experience. My experience has the content that this is a brain in a vat's image of a hand. This content is asserted by my experience.Footnote ¹⁶ So, naturally, that's how things seem to me. Now, to answer your second question, a visual experience with the content that P can provide a noninferential basis for belief that P. So my belief that this is a brain in a vat's image of a hand is noninferentially justified by my current perceptual experience.Footnote ¹⁷

948: Suppose we accept the theory that a perceptual experience with the content that P can immediately justify a belief with that same content. Why should we also accept your claim about the content of your experience? Why should we think your experience has the content you say it does?

237: You think I might be wrong about the content of my own experience?

948: Your claim about the content of your experience is introduced to answer a challenge to your commonsense picture of the world. But I'm worried that you've got things upside down. Is that commonsense picture of the world supported by this claim about the content of experience? Or is it the other way round? Do I believe I don't have hands because my experience tells me so, or do I think my experience tells me so because I believe I don't have hands?

237: I don't know what you mean, 948.

43: I do. You claim your present sensory experience has the content that this is a brain in a vat's hand image. But this is not some intrinsic feature of experience. You think your experience has this propositional content only because you are already indoctrinated into your commonsense ontology. Someone who did not already accept our commonsense account of the world would never claim that his experience has the content you say yours does. Someone who took himself to be in the Not So Bad case, for example, would say his experience has the content that this is a real hand. And someone with no prior assumptions about the nature of reality wouldn't be able to say anything one way or the other about the propositional content of his perceptual experience. His experience would be one great blooming buzzing confusion.Footnote ¹⁸ Your claim about the content of your experience is a product of your commonsense worldview. And thus it cannot be used to undermine a skeptical attack on that worldview.

948: The situation would be helped, 237, if you could come up with an independent argument to show that the content of your present experience is what you say it is.

237: The propositional content of an experience is present to the mind simply by virtue of having the experience, independently of any beliefs about what external states of affairs the experience is connected with. That is the model of experience I employ.Footnote ¹⁹ And, as I said, this experience quite obviously has the content that this is a brain in a vats’ image of a hand.

43: That's an argument?

237: The content of a visual experience is, roughly, the content of the belief that it tends to produce.Footnote ²⁰ Experiences like this tend to produce in me the belief that I don't have hands. Therefore, that's its content. And that's an argument.

948: But once again, couldn't someone who accepted the brain in a skull ontology just as easily say her experiences typically cau—

Subject 707: Hey! Any of you guys know if the mad scientists will be running the bank illusion tomorrow? I gotta deposit some bitcoin by Monday to cover my rent. I tell ya, a crystal vat's nice, but, man, is it pricey!

948: Oh, hey, 707. We're kinda in the middle of a conversation here.

707: Oh, yeah? Sweet. What's it about?

237: Oh, you'll love this. 43’s been wasting our time trying to convince us that, in spite of everything we've ever believed all of our lives, we might just be brains in skulls who experience some sort of brain-independent objective reality and as a result nobody knows anything. Can you believe this guy?

707: Well, nothing you can do to win that one.

948: Another skeptic?

707: Nope. I just said there's no way to win that argument.

948: And why's that?

707: Here's what happens when somebody like 43 shows up. What he does is he introduces some far-out possibility—such as the possibility that we are really just brains in skulls—and, in doing so, he shifts the conversational context. You see, in our ordinary conversations, we make knowledge claims all the time, and when we do, far-out possibilities like the one 43 is kicking around just aren't in play. For example, suppose the mad scientists are running an extended hallucination of a walk in the garden.

948: Is it one of the smooth and predictable group hallucinations like the scientists normally generate in us? The kind we both remember afterwards? Or is it one of those chaotic disorderly and scarily unpredictable individual programs the scientists feed us every 18 hours or so?

707: Let's suppose it's a group one. Like the one we're having now.

43: I've wondered about that distinction too. Here I am thinking I'm being fed a group hallucination same as you guys. But how do I know this isn't one of those solipsistic hallucinations?

237: The solipsistic ones are unpredictable. But we can all predict you'll keep making that same stupid argument.

707: ZING! Anyway, suppose we're enjoying an extended group hallucination of a walk in the garden, and I say ‘Hey, look, there's a goldfinch illusion nestled inside that bush-shaped sense-datum’. If we're being fed a typical hallucination of a walk in the garden, then you might ask, ‘How do you know it's not a woodpecker illusion?’ And in response I might point to specific features of the illusion—color, shape, size, you know, stuff like that—that indicate that it's a goldfinch illusion rather than a woodpecker illusion. And, given my expertise in bird hallucination identification, I can rule out the possibility that it's a woodpecker illusion. Footnote ²¹ And, so, in that context, I can truthfully say, ‘I know that's a goldfinch hallucination’. If asked to defend that, I'd point out those characteristic features, and my answer would be accepted, and that would be the end of it.

948: But then why did you say 43 always wins the argument?

707: In ordinary contexts, I know it's a goldfinch illusion because I can rule out the possibilities that are in play in that sort of conversational situation. For example, that it's a woodpecker illusion, a squirrel illusion, and so on. But what 43 does is, he introduces some far-out possibility, such as the possibility that I am a brain in a skull, and thus it isn't an illusion of any kind but a real goldfinch. And this is a possibility I cannot rule out. Because, as he keeps repeating, a real goldfinch will look just like one of your garden-variety goldfinch hallucinations.

43: Exactly, so you never know anything.

707: Not so fast. You see, normally these whacky possibilities are not on the table. So, ordinarily, it is true to say ‘I know that's a goldfinch illusion’ because, in ordinary conversational contexts, I don't need evidence that excludes far-out possibilities such as this distant possible world where I am a brain in a skull who sees a real goldfinch. All I need is evidence good enough to rule out woodpecker illusions and the like. But once you bring in this far-out sci-fi stuff—brain in skulls and whatnot—you create an extraordinary philosophical context where it is no longer true to say ‘I know that's a goldfinch illusion’ because the meaning of that utterance has shifted. By getting us to take these goofy possibilities seriously, 43 creates a situation where the standards of knowledge are so high, you cannot truthfully assert a knowledge claim any more. But, no need to fret; this is not the normal sense of ‘knowledge’. Ordinarily, the standards are much lower. In one sense, he wins; in another sense, everyday knowledge remains untouched. So don't let this guy bother you. If you say ‘I know I don't have hands’, you say something that is true most of the time.Footnote ²²

948: I'm having the same feeling I did right before you showed up, 707. You are beginning with the assumption that our commonsense story of being brains in vats is correct. And this allows you to call the kind of story that 43 proposes—where we are brains in skulls—a ‘far-out’ possibility. So the theory isn't really doing anything. The real meat of it is the framework of commonsense beliefs you are starting from—the philosophy is just so much word gravy. Your ‘solution’ in essence begs the question.

707: In saying that my ordinary claims to know are true, I betray my conviction that I am a brain in a vat and that I do not have hands. These are things 43 thinks I do not know. Indeed, I am ready to admit to 43 that if I am a brain in a skull, then under any standards for knowledge I do not know that I do not have hands. But, of course, as I firmly believe, I am not a brain in a skull. Is it legitimate for me to use this conviction in a debate against 43? Not if we're playing King of the Mountain. But if 43 is marshaling deeply felt intuitions of mine in an attempt to give me good reasons for accepting his skepticism, it is legitimate to point out that other of my beliefs militate against him and ask why I should give credence to just those that favor him.Footnote ²³

43: What the heck kind of a respon—

237: Well, now I see another thing rotten with 43’s position.

43: What's that?

237: The idea that the bulk of our commonsense beliefs might be false. The whole idea that 43’s brains in skulls would be ‘massively mistaken’ betrays a failure to understand semantics.

948: Please explain.

237: Well, you see, it is crucial to 43’s argument that the brain in a skull has all the same beliefs you do. This is why 43 thinks the brain in a skull is wrong when he has an experience like this and says ‘This is a hand hallucination’. But this assumes that the contents of our thoughts are completely determined by ‘internal’ factors. In other words, since your enskulled counterpart's brain is a molecular duplicate of your brain, he believes exactly what you believe when his brain is in the same state as your brain. But this is all confused because the contents of our thoughts are at least partially determined by external factors, that is, by the kinds of things that typically cause them.

43: I don't know that I agree with that. But even if it's true, why is that a problem?

237: Here's why: The brain in a skull's veridical experiences are caused by real hands, real trees, and the like. Our hallucinatory experiences are caused by computer circuits and other stuff controlled by mad scientists. And it is from their experiences that both types of brains learn to think and talk. So brains in skulls cannot refer to or even think about hand hallucinations—at least not in the way we do. When the brain in a skull says ‘This is a computer-generated hand hallucination’, she does not have the same thought you have when your brain uses those words. You learned to say, ‘This is a computer-generated hand hallucination’ in the presence of a computer-generated hand hallucination. Thus, in our language ‘computer-generated hand hallucination’ refers to a computer-generated hand hallucination. But in the brain in a skull, what typically causes the thought, ‘This is a computer-generated hand hallucination’ is a real hand.

43: Exactly! That's why the brain in a skull is wrong.

237: No. That's what I've been trying to tell you. You are assuming those words have the same content for us brains in vats as they do for those brains in skulls. But in Skull English, terms like ‘hand hallucination’ and the brain states associated with those terms do not refer to hands-in-the-image as they do for us. Given the brain in a skull's environment and given the way it learned its home language, thoughts involving terms like ‘hand hallucination’ will refer to their typical causal sources, namely, hands-in-the-flesh. And that is how the brain in a skull's utterances get their propositional content. So when a brain in a skull says, ‘This is a hand hallucination’ in the presence of a real hand, what he says is true. And the same holds for utterances like ‘This is a tree hallucination’, ‘This is a fish hallucination’, and so on. These too will have different contents in Skull English—contents that allow that the brain in a skull will be mostly correct in her commonsense beliefs.Footnote ²⁴ So the so-called Bad Case is not one that involves massive error. And once we realize that, your whole argument falls apart.

707: Well, as y'all know, I'm fully in favor of semantic solutions to epistemological problems but this one ain't gonna do it.

237: And why's that?

707: All 43 needs to do to block this move is introduce the case of the recently enskulled. Instead of imagining a brain identical to ours but who has always been inside a skull, imagine a brain that lived its whole life normally just like us in a vat, but this one was taken out of the vat and enskulled five minutes ago. And imagine it was done seamlessly so that the brain didn't notice it. That brain would be thinking in Vatglish but living in Skulltown. And so when it says ‘This is a hand hallucination’ in the presence of a real hand, it really is wrong. Your semantic theory might handle the skeptical hypothesis of the always enskulled, but it won't handle the recently enskulled.Footnote ²⁵

237: Well, that's real clever, but it doesn't matter. I still say it is incoherent to think we could be massively mistaken in the way 43 thinks we might be. And there's an even simpler way to prove it.

43: Oh, yeah?

237: Yeah. Suppose there is a being who is almost omniscient. He knows every fact there is to know about the world including all of the facts about my brain activity. The only thing this being does not know are the contents of my thoughts; he does not know how to assign meanings to my various neural firings and simulations of speech. Then one day this being sets out to do just that. The omniscient interpreter, like all interpreters, will have to employ the principle of charity. He will have to interpret my thought and speech simulations in such a way that they largely agree with his own point of view. And since he is omniscient, his point of view is also objectively correct. So the omniscient interpreter will ascribe beliefs to me in such a way that most of them are true. Therefore, most of my beliefs are true. And the same argument goes through for my enskulled counterpart. There could be an omniscient interpreter who employs the principle of charity to interpret the brain in skull's linguistic behavior. And since the interpreter is omniscient, he will ascribe mostly true beliefs to the brain in a skull. Therefore, most of the brain in a skull's beliefs are true. And therefore, even if I am a brain in a skull, most of my beliefs are true. The argument generalizes to any subject actual or possible. Massive error about the world is simply unintelligible.Footnote ²⁶ Belief is in its nature veridical.Footnote ²⁷

948: That's a good one. But I still think this is all show. Like I said before, if we start from the assumption that we are brains in skulls who do have hands, we could just as easily appeal to the omniscient interpret—

237: I'm really getting sick of this. Lemme make it even simpler. The best 43 has is a deductive argument with the conclusion that we don't know that we don't have hands. But it is obvious that we do know that we don't have hands. This is far more obvious than any philosophical premise 43 might appeal to in support of his position. So 43 hasn't provided any reason to doubt that we don't have hands. The most he's done is given us a reason to doubt whatever philosophical premises he used to reach his ridiculously implausible conclusion.Footnote ²⁸

707: Hey, look, there's 1781’s illusion going by. Just like clockwork. Whatcha up to 1781?

Subject 1781: Well, as you can hallucinate, the mad scientists are imposing my regular walk illusion upon our faculties.

43: You know, I wonder a lot about these mad scientists too. What's their world like? What are they like? Are they flesh and blood? Are they immaterial spirits? Is their knowledge unlimited? Are they omnipotent? Do they love us? Do they have free will? Do they live forever?

237: Borrrr-RING!

1781: Your questions are not answerable, 43. Even to attempt to answer them would be to engage in transcendental employment of the understanding to the unconditioned laboratoumenal reality that lies beyond all possible experience and is thus unknowable. The principles of understanding are a priori principles of the possibility of experience and all a priori synthetic propositions relate only to experience and wouldn't be possible otherwise.Footnote ²⁹

237: Is it just me or is does this guy never make any fu—

L: Gotcha! Woo! These things are cold. Good idea, stashing ‘em behind the cryogenic freeze chamber.

S: Larry, what took you so long? And who's the guy in the mesh t-shirt?

L: Smoke break. This is Rico. You don't know him? Dude rocks. He was running the floor buffer out in the hallway, and I asked if he wanted to help us soak up some suds.

RICO: Stan, the man. Good to meet you, dude. Cool earmuffs. What's the scuttlebutt over there in Seavattle, Illusionois? Any of those little guys seen The Matrix yet?

S: Seattle's in Washington.

R: Huh?

S: Interesting you should bring up The Matrix. Larry, do you remember how I said one of those brains could, at least from his own point of view, be exactly the same as you right now?

L: Did you find one like that? Awesome! Always wanted a son.

S: What I'm trying to say is, you're sitting here making fun of these brains in vats but how do you know you're not one of them? How do you know you're not just a brain in a vat?

L: You think I'm a brain in a vat? Well, watch me bust open this tallboy. Boom! There you go. I refute it thusly. BIV don't crack no PBR. That must be the worst part of it. Am I right, fellas?

S: But a brain in a vat could have an experience indistinguishable from your own experience of popping that Peeber. So how do you know you're not in the vat?

R: Oh, look at you, Stanley! You wanna throw down some epistemology? And here I was thinking you security guys just came in here and took naps on a pile of mail. Well, I'm game, my friend. It is on.

L: Yeah, Stan. Rico here's a certified philosophizer from NYU. Just like Patrick Swayze in Road House. Kicking ass, taking names, man's search for faith.

R: And polishing the floor.

S: Without anesthesia?

R: Pain don't hurt. Now I don't normally talk about this philosophy stuff to ordinary people—Epictetus advises against it, you know.Footnote ³⁰ But you guys seem alright, and I wouldn't want you to go getting all depressed over some skeptical pseudoproblem. So let me clear the dust here.

S: Okay, hit me, Dalton.

R: Well, here's the thing, provided our perceptual processes are reliable, any true beliefs produced by them will constitute knowledge. Now, I know some people whine about how reliabilism doesn't properly take evidence into account. Blah, blah, blah. Doesn't matter. Go ahead and bring evidence in all you want because, after all, the evidence a brain in a vat has is totally different. We see hands; they hallucinate them. What better evidence could you want than what your experience tells you? And experience is telling us that this right here is a hand. Now, if you still want to sit there and insist that I need to disprove some far-out brain in a vat story in order to know anything, all you're really doing is introducing some extraordinary sense of ‘knowledge’ with these superhigh standards where it becomes impossible to know anything in that sense. But in any ordinary everyday context, we don't worry about distant possible worlds like that. So our knowledge ascriptions will still be true most of the time. Furthermore, anybody who thinks we could be massively mistaken is woefully ignorant about some of the most important developments in semantics of the last few decades. But we're gonna need more than a sixer if you guys want me to break all that down for you.

S: Some of what you said there sounds familiar. But imagine I am fully convinced that I am in fact a brain in a vat. Instead of thinking that this is a hand, suppose it seems totally normal for me to think it is a mere image of hand generated by a computer controlled by a bunch of dorks in long white coats. And then suppose that some ‘skeptic’ comes along and suggests that maybe I'm not a brain in a vat. Maybe I'm a brain in a skull, and this a real hand! Couldn't I use everything you just said to support the claim that I know I don't have hands? For example, couldn't I say, ‘Well, gee, that can't be right. After all, brains in skulls see hands, and I'm hallucinating them’? Or if I found it natural to think I am a brain in a vat, wouldn't I say that the content of my current experience is that this is a mere image of a hand? And isn't that a problem? I mean doesn't that show that these theories of yours aren't really doing anything? Aren't you just assuming your commonsense worldview at the outset—which is the very thing in question—and then sticking some epistemological theory on top of it? Or to put it another way, what am I supposed to believe? Am I supposed to believe I am a brain in a skull and I have hands, or am I supposed to believe I am a brain in a vat and I don't have hands? Whichever one I choose, I can endorse any of those philosophical theories to justify it. So doesn't that mean the theories are just some empty foam on top of some ultimately unquestioned commonsense ontology?

R: Whoah! Flipping my arguments on me? That's some jiu-jitsu mind stuff there, Stannyboy. ‘Brains in skulls’ as a skeptical hypothesis! I never heard that one before. But the move does remind me a little of Law's 2010 paper on the argument from evil and of Steen's Reference Steen2011 parody of psychological egoism. And Borges's short story ‘Tlön, Uqbar, Orbis Tertius’ does something similar with Berkeley's idealism, of course. But being just a nightwatchman, you wouldn't know anything about that stuff. Anyways, look, this little trick of yours doesn't raise any problem for these solutions to skepticism. The most it would show is that if I am a brain in vat, I can know I don't have hands. But since I'm neither a brain in a vat nor a skeptic, that doesn't bother me. In fact, this is exactly what any nonskeptic should expect: brains in vats can know they don't have hands just as we know we do have hands. Sounds good to me. And now that I think about it, I'm not so sure all these responses to skepticism can be flipped so easily. Take contextualism, for instance. That's a theory about the semantics of ‘knows’ in our language. But brains in vats aren't part of our linguistic community. No telling what goes on over there. So maybe one of these theories works one way and not the other for some reason. Now if it does, that'd mean that theory is superior to the others in an important way. So I gotta admit: vatterizing solutions to skepticism is a novel way to think about these issues, and it does seem to present a new way to test them. It also seems to show that the classic skeptical paradoxes really leave us with two problems. First, the obvious: Do we know anything? But even if we answer that, there's a separate ontological problem: are we brains in vats or brains in skulls? An answer to the first I guess won't always answer the second. And hey, what you said about the theories ‘not really doing anything’ makes me wonder whether this a new way to think about what it is to beg the question. This move might even be some sort of operational test for whether a theory is guilty. You know, it's too bad you're not a real philosopher like me because you could write this thing up and publish it in the best jour—