Monday 1 August 2016

Denying the Conditional Premise: An Underrated Response to Skeptical Arguments

(This is a more focused re-write of a post from a couple of years ago, with better coverage of the literature.)

1. Introduction

Much ink has been spilled on skeptical arguments like the following:
  1. If you're a brain in a vat then you don't have hands
  2. You don't know that you're not a brain in a vat
  3. Therefore you don't know that you have hands
There are many variations on this sort of argument, and many issues have been raised about it, for example the issues of the closure of knowledge under implication, and the closure of knowledge under known implication.1

Here I will focus on the conditional premise part of this sort of argument. That is, on the premise which links a hypothesis according to which you are living in a simulation (e.g. the hypothesis that you are a brain in a vat (BIV)), to a seemingly-crazy hypothesis which flies directly in the face of things we normally think we know (e.g. the hypothesis that you don't have hands). Perhaps surprisingly, the truth of these premises is typically – we will consider a couple of exceptions below – taken for granted in contemporary discussions of such skeptical arguments.

For instance, Steup (2006, section 1) writes:

According to the BIV Hypothesis, you are a mere BIV without a normal body. This of course means, among other things, that you don't have hands.

Steup then goes on to consider five known kinds of responses to skeptical arguments. None of them, however, involves questioning (1), or questioning Steup's claim that '[t]his of course means, among other things, that you don't have hands'.

In this essay I want to argue against these conditional premises. I will try to be fairly non-technical and to avoid bringing in any very particular theoretical framework. I will focus on (1) in particular for the sake of definiteness, but the considerations will generalize in an obvious way to many similar conditionals.

In arguing that (1) and the like are not true, I am furnishing a way of avoiding the repugnant conclusions of skeptical arguments like the above without having to show that we do somehow know that we are not BIVs. We should instead, I say, just deny their conditional premises. This seems pre-theoretically attractive to me – it does intuitively seem to me that I do know that I have hands and that I do not know that I'm not a BIV. Or more carefully perhaps, and from a broadly contextualist perspective on knowledge-ascriptions2, it seems to me that there are contexts in which 'I know I have hands' comes out true while 'I know I'm not a BIV' comes out false.

Note that (1) and the like are generally supposed to be accepted readily, as though they were obvious (recall the Steup quote above). They just appear in skeptical arguments as premises which we're meant to accept without argument. Once you start scrutinizing (1) and the like as I propose, this can begin to seem rather odd. It can become hard to imagine what (1) ever had going for it. So, before arguing against it, let us first ask the question: why might (1) seem true?

Silly as it sounds – silly as it is – the answer appears to be something like this: when (1) strikes us as true, we are as it were picturing a brain sitting in a vat, and observing that there are no hands in that picture. Or we are picturing a brain sitting in a vat, and a mad scientist tending it, and noticing a striking contrast between the two figures – the scientist-figure has a body and hands, whereas the other figure is just an organ (in a vat). Something along those lines.

In this essay, I will argue that, by contrast, the denial of (1) has a good deal going for it. After first, in section 2, criticising and setting aside a superficial objection to (1), I will offer a volley of better arguments against it, beginning in section 3. In section 3 I will object that the question of whether a BIV has hands or not is better thought of as marking a different contrast from that between the BIV and the scientist above – a contrast such that some (possible) BIVs have hands and others do not. A possible doubt about this objection, which I will not try to allay in this essay, will lead us in section 4 to another objection based on it, which avoids the doubt. This objection, however, requires that when a BIV says 'I am a BIV' they are saying something true (however unknowable it might be to them). As natural as it is to accept this, it is contradicted by a famous argument of Putnam's, an argument designed to show that we are not BIVs. Accordingly, in this section I also examine and criticize Putnam's semantic ideas about 'I am a BIV' as said by a BIV, defending my objection against the threat from Putnam's argument. Then, beginning in section 5, I turn to a series of more intuitive arguments, designed to change the attitude of someone who is inclined to think (1) is true. In section 5 I make a case against (1) involving consideration of what is, and more importantly what isn't, implied by ordinary statements referring to hands. The idea here is to snap out of a false way of looking at such statements which we fall into when philosophizing about skepticism. In section 6, I contrast “normal” BIV scenarios with ones in which we are worse off epistemically, in order to motivate a view of “normal” BIV scenarios according to which their obtaining does not preclude our having hands. In section 7, I will suggest a reframing of the BIV hypothesis as a scientific one, and argue that from this point of view, the natural thing to do is to reject (1). This last suggestion will be seen to be essentially the position of Chalmers (2005), and therefore further supported by his arguments.3 I conclude briefly in section 8.

2. A Superficial Objection to (1) Rejected

Here I want to consider and put aside one particular line of attack on (1), to be found in Roush (2010). It is an almost comically literal-minded objection, which I found when I was searching the literature for previous attempts at calling propositions like (1) into question. Roush (2010) argues that it is not true that if you're a brain in a vat then you don't have hands, on the grounds that you might be a brain in a vat with hands just stuck on (!) – that is, where there are attached hands in the environment which contains the brain and the vat (as opposed to the environment simulated for the brain). Maybe the hands are just stuck on with glue and dangle there, or maybe they are delicately connected up with the brain, making for a queer straddling of two “worlds” (or environments, or levels of reality) on the part of the BIV.

There is something very frustrating about this objection. It is frustrating, I think, because if you just accepted this objection, deciding on its basis that (1) is false, and then walked away, you would have lost a valuable philosophical opportunity to understand (1) better. Furthermore, even if Roush's objection shows (1) to be false, the skeptical argument can easily be patched up by replacing (1) with 'If you're a brain in a vat without appendages as envisaged in Roush (2010), then you don't have hands'. Thus if we want to find objections to (1) in order to disarm skeptical arguments which threaten our putative knowledge that we have hands, we had better keep looking. We want an objection that is not so easily met with a patched conditional premise. (This is not to concede that the objection does show (1) to be false, by the way. For instance, we may ask whether a BIV with the envisaged appendages really counts as 'having hands', or whether this is really the best candidate meaning for 'having hands' in connection with a BIV.)

3. A Better Deserver of 'Having Hands' vs. 'Not Having Hands'?

We picture a BIV, and there are no hands in the picture. Then we picture a scientist tending the BIV and see a contrast between the figure of the scientist and the BIV-figure. And with this in mind, we might be tempted to say 'The BIV doesn't have hands and the scientist does'.

OK. But consider a different situation, in which we have two BIVs. It doesn't matter whether or not they are plugged into the same simulation. What does matter is that, in their lives in their simulation(s), one of them is an anatomically normal human, while the other has been in an accident and lost their hands. Mightn't we, if this was the first case we had considered, have been tempted to say 'One BIV has hands, the other does not'? And if we would be right in so saying, then we would be wrong to say (without shifting the meanings of relevant terms) that if you're a brain in a vat, you don't have hands; the first BIV would then be a counterexample to (1).

If we allow that we would be correct in saying, of the envisaged scenario, 'One BIV has hands, the other does not', this gives us an objection to (1). On this objection, 'has hands' vs. 'does not have hands', when applied to BIVs, is correctly used to mark this contrast, between BIVs like the first one and BIVs like the second one. Not the contrast between a BIV and a scientist tending it.

Here it might be objected that it would not be correct to say, unqualifiedly, 'One BIV has hands, the other does not' – rather, one would, to be both right and completely explicit, have to say something like 'One BIV has hands in its simulated environment, the other does not'.

In the present essay, I want to go along with – without endorsing – this objection to the present objection to (1), and use the present objection to (1) as inspiration for another objection, an objection which focuses on the truth-values of propositions uttered by a BIV in their simulated environment rather than on propositions we, standing outside a hypothetical scenario, might formulate about it. That is not to say there are no prospects for sticking with the present objection to (1) and arguing that the objection to it which we have just considered is mistaken – only that we will not pursue this here.

4. The BIV-or-Parity Argument

It may be that, of our first and second BIV above, we can't say with literal correctness that the first has hands and the second does not. But we can still ask about things the BIVs may say, in their simulations. And we could reason as follows: If the first BIV says, in the simulation, 'I have hands', they are, in the simulation, saying something true. And surely if they say, in the simulation, 'I am a BIV', they are, in the simulation, saying something true (even if they could never know it to be true). And thus, if they said 'If you're a brain in a vat then you don't have hands', they would be saying something false – something to which their very case is a counterexample. And if that's right, how could (1) fail to be false? How could our situation differ from the situation of the BIV in question in such a way that (1) for us is true, whereas their utterance – in the simulation – of 'If you're a brain in a vat then you don't have hands' – is not? I can see no way.4 So, I suggest the following overall argument. We are either BIVs or we are not BIVs. If we are BIVs, the above considerations about the truth-values of utterances made by an anatomically-normal BIV apply directly to us. If we are not BIVs, parity suggests that (1) for us has the same truth-value as the same sentence uttered by a BIV. Either way, (1) is false.

Now, the above reasoning seems natural, but of course it could be challenged. One way it could be challenged would be to follow Putnam's notorious (1981) in saying that, when the BIV says, in their simulation, 'I am a BIV', they are saying, in their simulation, something false, contra the above reasoning.

I do not have space here to lay out Putnam's arguments at length, but I think it is incumbent upon me to argue that Putnam is wrong on this point. I will now try to do this briefly and non-technically, but in a way that cuts quite deeply, perhaps more so than technical criticisms focusing on what might be non-essential features of Putnam's argument or reconstructions thereof. (For an overview of the literatue on Putnam's argument cf. Brueckner (2016).)

Putnam begins with a causal theory of reference, according to which what you're talking about when you say something is what stands in an appropriate causal relation with your utterance. He argues, from the causal theory, that since a BIV could have no causal contact with the brain they are, and the vat they are in, they could not be talking about that when they say 'I am a brain in a vat' – rather, their utterance is, according to Putnam, about 'vats-in-the-image', 'or something related (electronic impulses or program features)'5. And since they, the utterers, are not vats-in-the-image, i.e. not vats belonging to their simulation, nor the relevant 'related' things, what they thus say comes out false. Putnam goes on to argue, on this basis, that we are not BIVs, but we will stop here and just criticize this lemma – that a BIV saying 'I am a BIV' says something false – since the denial of that is all we need for our argument.

There are lots of things about this we could argue with – the idea that the BIV's talk might literally refer to electronic impulses or program features may seem crude and objectionable, for instance – but I will confine myself to a couple of key points.

Firstly, note that singular reference – reference to particular objects – isn't what is in question here. We are dealing with general terms like 'brain' and 'vat'. And what Putnam seems to be insisting on, in light of his causal theory of reference, is that, in order to really be about brains or vats, my talk needs to have an appropriate causal connection with some brains, or vats. So, going along with this way of thinking about what is required for talking about a kind of thing, in order for a BIV to think or say they are a BIV, it would seem that their thought or talk doesn't have to be causally connected with the particular brain they are or the particular vat they are in. It just has to be connected with some brains and some vats.

Now, why can't there be a general category marked with the word 'vat' which includes as members both “vats-in-the-image” - vats in simulations – and vats outside simulations? (And likewise for 'brain'.) I think we should say that there can be such a category. Consider things like happiness and intelligence: a BIV with a rich life is surely acquainted with these things, and causally connected with exemplars of them – and so they can have a category, for example marked 'expressions of happiness', and this category would include both things in their simulated environment and any appropriate things outside the simulation. And so Putnam's argument falls down here, by implicitly holding that the relevant reference classes – the 'brain' class and the 'vat' class – can only include things in the utterer's environment (or “level of reality”). Once we see this is not so, we can go along with Putnam's basic causal-theoretic starting point, but maintain that there is nothing stopping BIVs thinking they are BIVs, because they can form categories – by means of causal connections to brains and vats in their environment – which manage to include the brain they are and the vat they are in, despite those particular instances not being in their environment.

Now let us try to get a more positive sense of what is the matter with Putnam's argument, rather than just poking a hole in his reasoning. I want you to step back a bit, and just note how implausible and crude Putnam's interpretation of the BIV's utterance 'I am a BIV' is; it's supposed by Putnam to assert something which to the BIV would be obviously wrong – namely, something like: that they are brains in their environment in vats in their environment. And yet a reflective BIV might not find their utterance of it obviously wrong at all – on the contrary, it would be natural for them to regard it as something which it would be difficult or impossible for them to know the truth about. This suggests that something has gone awry. At a very general level, we may say that Putnam's problem is that he has inappropriately treated the language-game of talking about being a BIV as being just like an ordinary one about things in our environment. But it is plainly not that. Language, we might say, is playing a new trick here.6 We may not be able to come up with a theoretical understanding of this trick which would satisfy Putnam, but that does not mean he gets to falsify it.

So, I suggest that Putnam has not refuted the natural view that when a BIV says 'I am a BIV' they say something true, albeit something they may never be in a position to know. And thus the BIV-or-parity argument does not fail for that reason.

You may still be unconvinced, though, about the initial premise, i.e. that when an anatomically normal BIV – I mean their anatomy in the simulation, of course – says 'I have hands', they are saying something true. Accordingly, I will present further objections to (1) and the like which are supposed to convince you of this too, rather than just taking it as a premise. The general tendency of these arguments is to appeal to our common sense, as against the philosophical mood we are liable to be put into by skeptical arguments. (That this is a distinct mood is not something I am supposing to be obvious in advance, but if we are responsive to the appeal to common sense, I think we can see, in retrospect, that it was a distinct mood.)

5. Everyday Statements About Hands, And What They Do and Don't Imply

Consider statements made in ordinary, everyday conversation, statements which intuitively seem to imply that the utterer has hands. I want to urge that, intuitively, these seem not to imply that the utterer is not living in a simulation. That is, I think that is how it appears if we just regard these utterances with a normal, common sense attitude, instead of letting the skeptical argument put us in a philosophical mood in which our attitude changes and 'language goes on holiday'7. I suggest we accept these appearances, and conclude that (1) and the like are false.

For example, suppose someone asks me to help them with something and I say 'OK, one second - I'm just washing my hands'. This statement – that I'm washing my hands – surely implies that I have hands. Furthermore, I find it very intuitive – regarding the case from a common sense point of view – that it does not imply that I'm not living in a simulation, or that I'm not a BIV; that simply isn't at issue at all. That seems, I want to urge, quite independent of – extraneous to – the truth of what I said.

Going along with this: having hands is compatible with it not being the case that I'm not a BIV. And so, having hands is compatible with my being a BIV. And so it can't be true that if you're a BIV then you don't have hands.

6. That's Not A Nightmare. This Is a Nightmare!

The key intuition appealed to in the previous section – that my ordinary statement does not imply that I'm not living in a simulation – can, I suggest, be bolstered by thinking a bit about the space of different scenarios in which I am living in a simulation. Then we begin to see that it is possible to take an attitude to many of these scenarios which is quite unlike regarding them as epistemic nightmares, i.e. situations in which we're in really bad shape epistemically – where much of what we ordinarily think we know fails to be true. We can bring this out by contrasting “normal” scenarios in which we are in a simulation to what, by comparison, are the really nightmarish scenarios in which we are in a simulation. (Compare section 3, where we contrasted anatomically normal (in their simulation) BIVs with those who have been in accidents (in their simulation).)

Certainly we can imagine simulation-scenarios which are epistemic nightmares. We may be BIVs whose tending scientists are engaging in all kinds of foul play, planting false memories and moving things around on us. Also diabolical would be if some or all of the apparent agents we are interacting with are not sentient, or not as fully sentient as we think. I don't so much mean that they may not be constituted the way we are, or the way we think they are – after all, multiple realizability might be the case – but rather that maybe all there is to these agents is what's required to generate our interactions with them. Corners may be cut, so to speak – when we think they're off by themselves having a rich mental life, perhaps often nothing of the sort is true. But nightmarish scenarios like this are clearly a special subset of all simulation scenarios; in many of the latter, by contrast, I suggest that we may not be wrong about much of anything (or rather, not about anything to which the unknown fact that we are in a simulation would be relevant). It just might be the case that, unbeknownst to us, there is a higher level of reality “hosting” the one we inhabit, and this level may involve brains in vats.

Now, this of course raises the question: well, what about skeptical arguments which appeal to those really bad scenarios? Can't the skeptical arguments I am talking about just be patched up in light of this, thus making your criticisms no better than that of Roush, whom you criticised in section 2 for giving a superficial, easily avoided objection?

To this, I reply that, no, there is an important difference here. In the present case the 'patching up' is more complicated, and different BIV scenarios threaten different bits of what we normally think we know. It may be fairly easy to specify a scenario where certain supposed historical events in the simulation did not happen in the simulation, but the traces of them were put into the simulation in some other way. It may be fairly easy, but maybe a bit harder, to specify a BIV scenario where a BIV who hasn't been to France (in their simulation) is deceived about there being an Eiffel Tower (in their simulation). Harder still, but maybe doable, would it be to specify a scenario in which what a BIV takes to be other persons whom they know well are not actually fully persons, or do not always exist. As to what a BIV scenario where you are BIV who falsely thinks they have hands might really look like, I am not sure. I do not want to claim that no such scenario exists. The point is rather that, on the point of view I am advocating here, this would have to be quite a specific scenario – it merely being a scenario where you are a BIV doesn't automatically do the trick. This is, I think, a better, more nuanced way of thinking about BIV scenarios in relation to knowledge.

In further support of this viewpoint, I want to give one more suggestion, in the next section: that we reframe the BIV hypothesis as a broadly scientific one, rather than thinking of it as something like a threatening, sinister element in a story.

7. Reframing the BIV Hypothesis as a Scientific One

There is no need to respond to the news that you're a brain in a vat by revising your belief that you have hands. Why not treat the news instead as telling you, among other things, something new about your hands (and everything else in your environment), namely that they are “hosted” at a higher level of reality, or speaking crudely, are constituted by electrical impulses or program features. I say 'crudely' because the relation in question is obviously not the normal one of constitution from normal physical inquiry. Physics can be done in a simulation, too, and facts about the simulation being a simulation need not be regarded as belonging to that discipline. Nevertheless, I think it is quite natural to follow this suggestion and regard the BIV hypothesis as, in a broad sense, a scientific one. This seems to be a legitimate point of view. And once we take this point of view, it becomes natural to think of the hypothesis as one about the nature – in some sense – of your hands (and everything else in your environment), and thus not as a hypothesis which rules out your having hands. Compared with looking at these scenarios as something like sinister, threatening elements in a story, and then in that mood jumping to the conclusion that we don't have hands if those scenarios obtain, the point of view I am advocating seems the more clear-headed.

This last suggestion, that we regard BIV scenarios as scientific hypotheses, is substantially the same as Chalmers' suggestion in his (2005) that the hypothesis that we are in the same sort of situation as the main character Neo in the movie The Matrix is a metaphysical, rather than a skeptical hypothesis.8 (I prefer to use 'scientific' as the replacement category for 'skeptical', since I think we have a clearer idea of what science is, and of its successes, than we have of metaphysics. But presumably metaphysics is a kind of science, in a broad sense of 'science', so this is not a major disagreement.) Chalmers writes:

I think the Matrix Hypothesis should be regarded as a metaphysical hypothesis [...]. It makes a claim about the reality underlying physics, about the nature of our minds, and about the creation of the world. (Chalmers (2005), section 3.)

His argumentative strategy is (i) to formulate a 'metaphysical hypothesis' with three components, teased out in the way suggested by the last sentence quoted above, (ii) to argue that none of these components threatens to radically undermine what we think we know, and (iii) to argue that the metaphysical hypothesis is equivalent to the Matrix Hypothesis, in which case the latter does not radically undermine what we think we know either. Along the way, he responds to several possible objections.

In this section, I have simply outlined the idea of reframing the BIV hypothesis as a scientific one, and argued that it seems legitimate and more clear-headed than thinking of it as a threatening, sinister element in a story, i.e. the viewpoint we have when we think of it as a 'skeptical' scenario. Chalmers, modulo the minor differences between his discussion and mine outlined above, goes further on this front. Conversely, his discussion does not contain the earlier considerations of the present essay, such as the 'other contrast' argument of section 3, the 'BIV-or-parity' argument of section 4, the appeal in section 5 to common sense regarding everyday statements about hands, or the argument from contrast with really bad epistemic situations of section 69. Thus Chalmers' discussion and mine here are mutually supportive.

8. Conclusion

I have tried in this essay to make a good, multi-pronged case against (1) and similar conditionals, which feature as premises in much-discussed skeptical arguments. I hope I have convinced you that (1) and the like are false. Failing that, I hope I have at least convinced you that they are seriously open to question, not something which we should say 'of course' or 'obviously' about, as we saw Steup (2006) do.


Bouwsma, O. K. (1949). Descartes' evil genius. Philosophical Review 58 (2):141-151.

Brueckner, Tony. (2016). Skepticism and Content Externalism. The Stanford Encyclopedia of Philosophy (Summer 2016 Edition), Edward N. Zalta (ed.), URL = <>.

Chalmers, David J. (2005). The matrix as metaphysics. In Christopher Grau (ed.), Philosophers Explore the Matrix. Oxford: Oxford University Press.

Lewis, D.K. (1996). Elusive knowledge. Australasian Journal of Philosophy 74 (4):549 – 567.

Luper, Steven. (2016). Epistemic Closure. The Stanford Encyclopedia of Philosophy (Spring 2016 Edition), Edward N. Zalta (ed.), URL = <>.

Putnam, H. (1981). Brains in a Vat, in Reason, Truth, and History. Cambridge University Press.

Roush, S. (2010). Closure On Skepticism. Journal of Philosophy 107 (5):243-256.

Schaffer, J. (2015) Lewis on Knowledge Ascriptions, in A Companion to David Lewis (eds B. Loewer and J. Schaffer), John Wiley & Sons, Ltd, Oxford, UK.

Steup, Matthias. (2006). Knowledge and Skepticism. Supplement to 'The Analysis of Knowledge'. The Stanford Encyclopedia of Philosophy (Spring 2006 Edition), Edward N. Zalta (ed.), URL = <>.

Wittgenstein, L. (1953). Philosophical Investigations. Oxford: Blackwell.
Wittgenstein, Ludwig (1966). Lectures & Conversations on Aesthetics, Psychology and Religious Belief. Oxford: Blackwell.

1Cf. Luper (2016, section 5) for an overview of closure in relation to skepticism.

2. Cf. Lewis (1996). For an attempt to elaborate Lewis's core ideas about knowledge ascriptions more thoroughly and rigorously, cf. Schaffer (2015).

3. An important precursor to the present discussion and that of Chalmers (2005) is Bouwsma (1949). Bouwsma discusses, not a scenario in which we are living in a high-tech simulation, but an older kind of scenario in which an evil demon with supernatural powers is trying to deceive us. I have not tried to engage directly with Bouwsma's discussion here, since the evil demon scenario seems to me importantly different in a number of ways from a BIV scenario (and such differences aren't my focus here). Also, Bouwsma's piece is written in a literary, exploratory style, so that explicit commitments and arguments are not to the fore. Nonetheless, it is important to note that his piece exhibits, more than half a century earlier, the same overall tendency as the present discussion and Chalmers', i.e. to suggest that what may seem like epistemologically very threatening scenarios at first glance and in a certain philosophical mood, are more properly regarded as not so threatening.

4. At least, I can see no way which doesn't turn on us not being BIVs, not living in a simulation – and in that case, we wouldn't be able to know (1) without first knowing that we are not BIVs, in which case the skeptical argument could no longer be run.

5. (Putnam (1981), p. 15).

6. The phrase comes from Wittgenstein (1966), p. 1.

7. Wittgenstein (1953), section 118.

8. It is easy to miss Chalmers' piece when searching the literature for objections to (1), since Chalmers discusses, not the BIV scenario famous in recent philosophy, but the similar scenario depicted in the movie The Matrix (substantially the same as a BIV hypothesis, except Neo's brain outside the simulation is lodged in a body), and he doesn't explicitly frame his essay as an attack on the conditional premises of skeptical arguments, even though that is what it amounts to. (All this is in keeping with his piece having been written for a popular audience.)

9. Chalmers does touch on (Matrix-style counterparts of) what I in section 6 called 'really nightmarish scenarios in which I am in a simulation'. (In his section 5 he writes that '[t]here may be some respects in which the beings in a matrix are deceived. It may be that the creators of the matrix control and interfere with much of what happens in the simulated world.') But Chalmers does not touch on this in the service of a positive argument, by way of contrast, against (1) and the like. Rather, he discusses it defensively, as a possible threat to his claim that the Matrix Hypothesis is not a skeptical hypothesis; his point is just that this is orthogonal to the issue of whether you're in a matrix – being in a matrix doesn't by itself mean you're radically deceived, and the sort of 'interference' in question could also happen in life outside of a simulation.


  1. Gotta think about this. But I know one thing: If I were a brain in a vat, I'd want my name to be Roy G.

  2. Hi Tristan

    I find your main argument - i.e., your argument against premise (1) - persuasive. I previously didn't have a stance on the matter, but now I'm convinced it's false.

    Side issue: I'm not sure we don't know we're not BIV, even if we don't know we're not in a simulation.
    On that note, in the case of more specific scenarios (i.e., the ones that actually would be epistemic nightmares), I get the impression your reply takes into consideration that given the specific details, we know (or at least, we may well know) that we're not in one such scenario, because it is or it may well be proper to assign extremely low probability give the specific details, or something along those lines?
    If so, it seems to me a reply like that also might work against the BIV scenario, which it's far more detailed than the generic simulation scenario ("BIV" is very short to write, but it seems to me it contains a lot of very specific information). I'm undecided as to whether we know we're not BIV; I'm inclined to think we probably do know, though I think we probably don't know whether we're in a simulation.

    1. Hi Angra,

      Thanks for reading and being convinced, and for your characteristically thoughtful reply!

      That's an interesting idea, but I'm dubious that you could make a convincing argument here that we can know we're not BIVs. I'm open to it though. A few worries about the idea:

      - How do you assess the probability of any such scenarios? Where do you even begin?
      - Although one single very specific BIV scenario may have very low probability, don't forget there are probably very many different possible ones.
      - There's something odd about reasoning: this probably won't happen, therefore we know it won't happen. The first part of the reasoning seems to create a context where you are considering the scenario in question as a relevant possibility, and then to say you know it doesn't obtain, in that context, sounds fishy to me. On the other hand, you might think it's quite natural for a ticket-holder in a large lottery to say 'I know I won't win' - but can that really be regarded as the literal truth? I'm not so sure.

  3. You're welcome, and no need to thank me for being convinced; thanks for posting.

    Regarding your questions:

    "How do you assess the probability of any such scenarios? Where do you even begin?"
    I was making the argument on the assumption you were (probably) also making a probabilistic assessment to rule out other, more specific scenarios.
    If not, then I'd like to ask how you block them. But that aside, I would make an intuitive probabilistic assessment as I usually make (and I think we all do, even if implicitly). It doesn't have to be based on specific numbers like a lottery case (but see below).

    "- Although one single very specific BIV scenario may have very low probability, don't forget there are probably very many different possible ones."
    I was suggesting that the BIV scenario is already very specific and (probably) has much lower probability than the simulation scenario.
    The BIV scenario can be subdivided in many (infinitely many, I'd say) more specific subscenarios, but that seems to be always the case (this also applies to the scenarios of the most complicated varieties you mentioned).

    "On the other hand, you might think it's quite natural for a ticket-holder in a large lottery to say 'I know I won't win' - but can that really be regarded as the literal truth? I'm not so sure."

    I don't know it's improbable enough. It depends on the lottery, whether there are many winners (lower prizes), etc.
    But that has to do with the specific case of the lottery ticket, not the sort of reasoning involved, as the following example (I think) illustrates:
    I'm about to move my hand towards a door, and I do know the hand will not quantum tunnel through it (I just did it. It didn't quantum tunnel! I knew it!), even if I do think assign a nonzero probability to the hypothesis that QM is right about that being nomologically possible, and further, that if that is so, it might happen (this is regardless of whether the universe is deterministic).
    A simpler example:
    Justin Welby knows his biological father is Montague Browne. But he knows that on the basis of a probabilistic assessment, based on DNA evidence. There is a slight chance that it's not true, in which case he doesn't know it. But I'm assigning an extremely low probability to that.

    That said, in the DNA case, like the quantum tunnel case and also the lottery case (in the lottery case, depending on the lottery) the probabilistic assessment is (mostly) based on computations one could (or a scientist could, depending on the case) specify, whereas the one I suggested is not (it's just an ordinary intuitive probabilistic assessment).
    So, even if the reasoning in the lottery (if there are enough numbers), QM and DNA cases are not odd, you might still argue it's odd. I don't think so, but before I try to make an argument, I'd like to ask how you would reply to something like the more specific skeptic scenarios you suggested.

    Btw, here's a variant of the skeptic argument:

    1'. If you're a lone Boltzmann brain (definition: ), either you never had any family or friends, or they never loved you.
    2'. You don't know that you're not a lone Boltzmann brain.
    3'. You don't know that you ever had a family or friends who loved you.

    I don't think 1' is assailable, because even if one counts the memories of the lone brain as family or friends, they never loved you (and I do think our ordinary claims about feelings like love rule out zombies).

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