Are you sure you have hands?

leonardo_aby Massimo Pigliucci

Skepticism is a venerable word with a panoply of meanings. When I refer to myself as “a skeptic,” I mean someone inspired by David Hume’s famous dictum: “In our reasonings concerning matter of fact, there are all imaginable degrees of assurance, from the highest certainty to the lowest species of moral evidence. A wise man, therefore, proportions his belief to the evidence” [1]. Or, as Carl Sagan famously phrased it, “Extraordinary claims require extraordinary evidence” [2]. Oh, and if there is one thing I resent it is being mislabelled as a “cynic,” meaning a naysayer with no sense of humor…

But skepticism (and cynicism, for that matter!) in philosophy is much, much older than that, and has at the least a couple of additional meanings [3]. According to so-called (by Sextus Empiricus, second or third century CE, [4]) “academic skeptics” (because they belonged to Plato’s academy, post-Plato), such as Carneades (214-129 BCE) [5], we cannot have any epistemically interesting knowledge. A different type of skeptic, the Pyrrhonian (named after Pyrrho, 365–ca 275 BCE) denied even that we can deny the possibility of knowledge, a meta-skepticism, if you will. Few modern philosophers are interested in Pyrrhonism, while academic skepticism has a long and venerable tradition, including perhaps most famously Descartes’ “radical doubt” thought experiment, in which he imagined a Machiavellian demon determined to trick him about what he thought he knew. Descartes then asked whether it would be possible, under those circumstances, to actually know anything at all. His answer, of course, was in the affirmative, and took the form of his famous cogito, ergo sum (I think, therefore I am) [6].

There is, of course, a much more fun way to think about the problem of skepticism in epistemology, and that is by using the 1999 scifi move The Matrix as a philosophical thought experiment [7]. The movie famously begins with our hero, Neo, played by Keanu Reeves, living what he thinks is a perfectly normal life, which soon reveals itself to be anything but. Neo, turns out, is much closer to the famous “brain-in-the-vat” (BIV) scenario of modern philosophy of mind (to be precise, he is a body-in-the-vat), with all his “experiences” actually being fed to him via artificial stimulation for the purposes of an evil post-technological civilization of machines that have enslaved humanity.

There is a crucial scene in the movie [8] where Neo’s mysterious mentor, Morpheus (played by Laurence Fishburne) poses the question to Neo of whether he wants to keep living in the “reality” he knows, or if he has the guts to see “how deep the rabbit hole goes.” As we know, Neo chooses the red pill that characterizes the second choice and the movie unfolds from there.

Neo, of course, is initially (properly) skeptical (in the Humean sense) of what Morpheus is trying to convey. The latter might as well have asked his question along the lines of: “how do you know you are not a brain in a vat?” How would you answer that sort of question? Which is another way of asking: have we made any progress against (academic) skepticism?

My discussion here tracks the one put forth in Steup’s broader treatment of epistemology in the Stanford Encyclopedia of Philosophy [9]. We begin with a quick look at the minimal version of the BIV argument:

(1)  I don’t know that I’m not a BIV.

(2)  If I don’t know that I’m not a BIV, then I don’t know that I have hands.


(3) I don’t know that I have hands.

This is a formally valid argument, i.e. its structure is logically correct, so any viable response needs to challenge one of its premises — that is, question what in logic is called its soundness. Before proceeding, though, we must note (as Steup does) that premise (2) is tightly linked to (indeed, it is the negative version of) the so-called Closure Principle: “If I know that p, and I know that p entails q, then I know that q” — a principle that is definitely eminently reasonable, at first sight. The application to our case looks like this: If I know that I have hands, and I know that having hands entails not being a BIV, then I know that I’m not a BIV. But — says the skeptics — the consequent of this “BIV closure” is false, hence its antecedent must be false too: you just don’t know whether you are a BIV or not!

There are, of course, several responses to the skeptic’s so-called “closure denial.” Steup examines a whopping five of them: relevant alternatives, the Moorean response, the contextualist response, the ambiguity response, and what one might call the knowledge-that response. Let’s take a quick look.

A first attack against the BIV argument is to claim that being a BIV is not a relevant alternative to having hands; a relevant alternative would be, for instance, having had one’s hand amputated to overcome the effects of disease or accident. This sounds promising, but the skeptic can very well demand a principled account of what does and does not count as a relevant alternative. Such an account could perhaps deploy a type of approach naturally enough called relevance logic [10], but that would get pretty technical, so I’ll leave it for another time.

Second attack: G.E. Moore’s (in)famous “I know that I have hands” response. This is essentially an argument from plausibility: the BIV goes through if and only if its premises (I don’t know whether I’m a BIV, so I don’t know whether I have hands) are more plausible than its conclusion (I don’t actually know whether I have hands). Which, of course, Moore famously denied — by raising one of his hands and declaring “here is one hand.” But why, asks (reasonably, if irritatingly) the skeptic? To make a long story short, Moore’s counter to the BIV argument essentially reduces to simply asserting knowledge that one is not a BIV. Which, ahem, pretty much begs the question against the skeptic [11].

Third possible anti-skeptic maneuver: the contextualist response. The basic intuition here is that what we mean by “know” (as in “I know that I have hands,” or “I don’t know that I’m not a BIV”) varies with the context, in the sense that the standards of evidence for claiming knowledge depend on the circumstances. This leads contextualists to distinguish between “low” and “high” standards situations. Most discussions of having or not having hands are low standards situations, where the hypothesis of a BIV does not need to be considered. It is only in high standards situations that the skeptical hypothesis becomes salient, and in those cases we truly do not know whether we have hands (because we do not know whether we are BIVs). This actually sounds plausible to me, though I would also like to see a principled account of what distinguishes low and high standard situations (unless the latter are, rather ad hoc, limited only to the skeptical scenario). Perhaps things are a bit more complicated, and there actually is a continuum of standards, and therefore a continuum of meanings of the word “know”? [12]

Fourth: the ambiguity response. Here the strategy is to ask whether the skeptic, when he uses the word “know” is referring to fallible or infallible knowledge. (This is actually rather similar to the contextualist response, it seems to me, though the argument takes off from a slightly different perspective, and I think is a bit more subtle and satisfying.) Once we make this distinction, it turns out that there are three versions of the BIV argument: the “mixed” one (“know” refers to infallible knowledge in the premises but to fallible knowledge in the conclusion), “high standards” (infallible knowledge is implied in both premises and conclusion), and “low standards” (fallible knowledge assumed in both instances). Once this unpacking is done, we quickly reach the conclusion that the mixed version is actually an instance of invalid reasoning, since it is based on an equivocation; the high-standards version is indeed sound, but pretty uninteresting (okay, we don’t have infallible knowledge concerning our hands, so what?); and the low-standards version is interesting but unsound (because we would have to admit to the bizarre situation of not having even fallible knowledge of our hands!).

Finally: the knowledge-that response, which is a type of evidentialist approach. The idea is to point out to the skeptic that the BIV argument is based on a number of highly questionable unstated premises, such as that it is possible to build a BIV, and that someone has actually developed the technology to do so, for instance. But we can deny these premises on grounds of implausibility, just like we would deny, say, the claim that someone has traveled through time via a wormhole on the ground that we don’t have sufficient reasons to entertain the notions that time travel is possible and that someone has been able to implement it technologically. Yes, the skeptics can deny the analogy, but the burden of proof seems to have shifted to the skeptic, who needs to explain why this is indeed a disanalogy. Can someone please get me a red pill?

Now, why on earth did we engage in this, ahem, academic discussion? Because I wanted to give you a flavor of how philosophy makes progress, and why it isn’t particularly fruitful to compare it with progress in the natural sciences (did you see any systematic observation or experiment peeking through the above?). Indeed, I am writing a whole book on this topic, which I will hopefully deliver to Chicago Press by the end of the summer. No, make that I will definitely deliver by the end of summer…

The idea is that philosophy is concerned with exploring conceptual, as distinct from empirical, spaces, which is precisely what we have done above. Indeed, you could go through it again and try to build a concept map [13] to see whether you followed the discussion correctly and to visualize its unfolding. The five responses presented by Steup can be thought of as five peaks in the conceptual space defined by the BIV problem, with other possible responses having been examined and discarded during the long history of the debate (those would be conceptual valleys, to continue the metaphor). Not all peaks are necessarily of the same height — where the height roughly measures how good a given response is, and even the precise position and shape of the peaks may vary over time, as philosophers keep refining them in response to counterarguments from the skeptics.

Moreover, the metaphor should make clear that even to ask the question of what is the true answer to the BIV problem is, in a fundamental way, to misunderstand the whole process. If the BIV question were an empirical one — like “how many planets are there in the solar system” — then it would have one definite answer [14], and a bunch of bad ones. But in conceptual space there often are several reasonable ways of looking at a particular problem (“answers”), and it will not be possible to pare them down to just one. (Another way to put this is to say that conceptual space is wider than, and underdetermined by, empirical space.)

So, what should we then make of (academic) skepticism and its critics? I think the value of the skeptical position is that it fosters epistemic humility: we are really not as smart as we often think we are, and in fact we don’t even have unequivocal answers to very basic questions about knowledge. As for the responses to the BIV problem, the five sketched above represent peaks of different heights in the proper conceptual landscape, and in my mind the ambiguity and the knowledge-that peaks are significantly higher than the rest, the Moore response is the lowest, and the relevant alternative and contextualist options are somewhere in the middle. But I’m sure we can have a discussion about that.


Massimo Pigliucci is a biologist and philosopher at the City University of New York. His main interests are in the philosophy of science and pseudoscience. He is the editor-in-chief of Scientia Salon, and his latest book (co-edited with Maarten Boudry) is Philosophy of Pseudoscience: Reconsidering the Demarcation Problem (Chicago Press).

[1] In An Enquiry Concerning Human Understanding, published in 1748.

[2] A dictum, by the way, which can easily and rigorously be formalized in Bayesian terms.

[3] Here is the obligatory SEP entry, by Peter Klein.

[4] On Sextus Empiricus.

[5] Here is my take on Carneades.

[6] On Descartes’ epistemology.

[7] You can do this and much more by engaging the fun essays in the collection put together by Susan Schneider, Science Fiction and Philosophy: From Time Travel to Superintelligence, Wiley-Blackwell, 2009.

[8] Here it is, for your viewing pleasure.

[9] Epistemology, by Matthias Steup, SEP, 2005.

[10] Relevance logic.

[11] On George Edward Moore.

[12] I know, I know, this is beginning to sound rather Clintonesque. Then again, the former President of the United States did study philosophy at Oxford as a Rhodes Scholar…

[13] On concept mapping.

[14] Well, kinda. The famous “demotion” of Pluto to “dwarf planet” hinges on the rather arbitrary — and in some sense philosophical — question of what counts as a planet and why. Incidentally, and contra popular perception, it was Caltech astronomer Mike Brown who was chiefly responsible for killing Pluto, not my friend Neil deGrasse Tyson. Neil did, however, write a popular book about the story.


273 thoughts on “Are you sure you have hands?

  1. DM: “. . .so given *the evidence of our sensory experiences* we have to choose whether to interpret them as senses reflecting an external world or as some kind of delusion which does not reflect an external world” (emphasis added).

    You can probably anticipate what I might question here. Let me rewrite as follows: “so given senses” the rest–“evidence,” “experiences,” choice, and external worlds–are seemingly gratuitous as you’ve written it. I realize that “sensory experience” is commonly used, but would pause to ask whether this expression is helpful without elaboration. In other words, as expressed, it opens the door to a distinction that is implicit, i.e., experience that is not sensory or perhaps that which is sensed is in turn filtered through some other aspect. That, in turn, raises the question whether to sense is to experience–perhaps an equivalence that is not warranted without engaging what it means to experience. Having said this, I’ll concede that positing the sensory without at the same time positing a purpose for the senses creates a nightmare. Still, from the skeptics vantage, it seems to me you have to overcome not the possibility of sensory delusion or hallucination, but rather the possibility that your delusion or hallucination is that you in fact have senses–obviously radical. This is not an approach that I would take. I believe that if we agree we have senses, we can assert that they convey information that is external to us, not only about us.


  2. Asher,
    No, it is not absolute proof, but then it is simply a peripheral quote and not the basis of my argument.


  3. I just discovered this blog through a BLOGGINGHEADS interview Massimo did a few weeks ago, and love it. I use the MATRIX example in teaching my physics and other science-related classes because it is a good example of a non-falsifiable hypothesis (and thus NOT a theory in science). It thus equates with creationism in its various forms. I am, obviously, a good Popperian. I think students in science courses need to be educated more deeply in the philosophical basis of science, and of the limits of science. I’ll look forward to more blog posts.


  4. Hi Aravis,

    1. Agreed, but as noted I’m taking reason as an axiom. It’s the rest of the argument that bothers me.
    2. I think this is where our intuitions differ. When it comes to parsimony, I think very much along Coel’s lines. The external world hypothesis is relatively parsimonious because it only assumes that there is a system of physical laws by which beings like me can evolve, etc. The solipsistic world hypothesis is very arbitrary by comparison – it assumes both that my mind exists in all its detail but also that there is som systematic way of generating the delusion of a world that appears to be surprisingly consistent, and which includes or implies the system of laws that the external world hypothesis relies on. That the world (especially other people) can surprise me, that I can learn of my mistakes and so on is hard to reconcile with a solipsistic world. I suppose I’m not that interested in getting into a debate on this point, because it’s not the one that interests me right now. The point which confuses me is the one you address next.
    3. Yes, we only have *sense* experience if we have sense *organs*. I agree, and I thought I was clear on this point. So, we only know we have experience which could be an illusion, or it could be sensory. So I conclude that it is sensory (and that I have sense organs) because I think this is more parsimonious. Again, the fact that the experience is sensory is a conclusion, not an assumption, therefore there is nothing circular to the argument.

    I’m on board with everything else, especially because of the skepticism of reason itself. I just don’t think that it is true to say that belief in an external world is unjustified by reason (given faith in reason), while admitting that this belief could be false.


  5. @Aravis:

    We can have *experience* without sense organs, of course, as we do in our dreams, when our sense organs are “turned off.”

    This is really misleading. First of all, our sense organs are not turned off. Muted, perhaps, but not turned off. Second – and more to the point – our sensory apparatus is a necessary condition for the experiences we have in dreams, even if it is not being engaged specifically in the same way. The ability to have sensory experiences in dreams is predicated upon having had sensory experiences.

    Indeed, I would argue that this is *more* parsimonious than the external world hypothesis

    That’s only true if you multiply entities by insisting that the self is somehow separate from the “external” world. I agree that parsimony doesn’t favor either side, but in each case there’s just one entity – either the ur-self or the universe.


  6. “I am slowly coming to the conclusion that the real problem is our imperfect grip on reality. We simply cannot see all there is to be seen…”

    I don’t see this is a problem, but as the expected state of affairs. Why would we ever think we would have anything but an imperfect grip on reality.

    What I take to be the heart of the issue is that people misunderstand the relationship between reasons and beliefs. It seems to me that many think that every belief should have a reason, and that in the absence of one, the belief is “unwarranted” or “irrational.” What they do not realize is that a certain reason, R, for a belief, B, *can only function as a reason* IF some other thing, A is already assumed. So, for example, people think that there is an unproblematic relationship between the claim that I am currently seeing rain and the belief that it is raining, even though, the former only *counts* as evidence for the latter, if one assumes that one’s senses are working correctly and one is not dreaming. But then, the question arises, “What is the reason for believing *those* things?” and for obvious reasons, the answer cannot lie in further appeals to the senses.

    Once one realizes that this *defines* the relationship between reasons and beliefs, one quickly comes to understand that it is *structurally* impossible that every belief should have a reason. Even better, one may come to realize that the offering of reasons is itself an expression of certain beliefs, for which no reasons can be given. And one is really catching on, if one realizes that this implies that a number of long standing assumptions that mainline philosophy has made about human nature are simply categorically false.


  7. Asher Kay wrote:

    “our sensory apparatus is a necessary condition for the experiences we have in dreams, even if it is not being engaged specifically in the same way. The ability to have sensory experiences in dreams is predicated upon having had sensory experiences.”


    This is all based on what we know from empirical science and thus, begs the question against the Cartesian arguments.


  8. @Aravis:

    This is all based on what we know from empirical science and thus, begs the question against the Cartesian arguments.

    Actually, I don’t think it is. I’m only arguing that you’re wrongly drawing a line between the “external” world and the experiential self at “sense organs”. I’m saying that this arbitrary idea is unsupported by the notion of having sensory experiences in dreams.

    There’s nothing to do with scientific knowledge here – only phenomenological knowledge. If we “see” in a dream, we must have “seen” in some other sense for us to call it “seeing” in the dream.


  9. @Aravis

    Actually, I’m going to try to do a little better, because what I’m trying for here is not to dispute what you’re saying. And it’s definitely not to agree with DM, because I think that he is completely wrong. What I’m shooting for is to see if I can convey to someone with an obviously insightful philosophical mind: A) how your view at least partially rests upon the assertion of a separating line between the self and the external world, as well as an assumption about where to draw that line; and B) how not drawing a separating line between the self and the external world results in a more coherent, if not more parsimonious, view.

    So I’d ask you these questions:

    1. What work do you believe that sense “organs” are doing in your argument? Why is this your line between the self and the “external” world?
    2. If the self-only, solipsistic world is coherent to you, is a physical-only world in which what you’re calling the “external” world actually includes the self any less coherent? From my perspective there’s not much difference between the two apart from what’s generating them.

    I do understand that what I’m talking about here doesn’t have any relevance to whether we can know whether we inhabit a solipsistic world or not. But once one has accepted that we can’t know that with certainty, then one has to decide what to believe, if anything. And that involves deciding what we will count as justification.


  10. There ;has been a lot of discussion about the first part of the blog and not so much about the second – how progress is made in philosophy.

    This is something which has always fascinated me.

    Say we have a statement which is not a scientific nor a mathematical fact, nor is a statement about a person’s subjective experience – like “my head hurts”.

    A person might make this statement and then supply an argument to demonstrate that it is true.

    If a second person disagrees that the argument demonstrates the truth of the statement then what method do we have of adjudicating who is right and who is wrong?

    We might scan the argument for logical fallacies – but an argument might be completely free from logical fallacies and still not demonstrate the truth of the conclusion.

    We could ask the person to formulate the argument into formal logic – but this almost never happens and even when it does, formal logical treatments of plain language claims are fraught with pitfalls.

    So how do we tell who is right or wrong in this kind of an argument?

    Which makes me wonder, do philosophers seek to demonstrate the truth of what they say?

    Or rather do they only seek to persuade others of the truth of what they say without actually demonstrating that truth?


  11. Aravis,
    I don’t see this is a problem, but as the expected state of affairs. Why would we ever think we would have anything but an imperfect grip on reality.

    Yes, I agree it is the expected state of affairs(for the reasons you give) but qualify that by saying it is expected by an intelligent, informed, philosophic observer, such as yourself.

    For the rest of us, Hamlet uttered the sotto voce belief that most people hold ‘and thus too much thought doth make cowards of us all‘. In other words we are too consumed by the practical realities of life to spend much time questioning it.

    I don’t see this is a problem

    The problem is that the majority are possessed by an unshakeable conviction in the correctness of their knowledge and beliefs, reinforced by the certainty that the other person’s beliefs are false or incomplete. I needn’t enumerate the problems that causes. One need only mention Republican/Democratic politics to see those unshakeable convictions evidenced in full fury by people who claim to be skeptics, unaware of the irony in the way they betray the term ‘skeptic’.

    Why would we ever think we would have anything but an imperfect grip on reality.

    It is an impossibility for the reasons you give. This is also the point of Massimo’s post and the reason that he urges epistemic humility. That point has largely been lost in the debate. He said he was miffed that the debate centred mostly on attempts at rebuttal of the BIV but in fact that verifies his point, that there really is a need for epistemic humility. I don’t think the message got through.

    We could have had an interesting debate about what epistemic humility really is and how we should practice it in the way we form beliefs and conduct disputes.


  12. Sorry – I’m commenting during processing runs ;).

    I want to try to clarify how I think not separating the self from an “external” world makes the problem easier to think about. I thought I had made a good point above, and, though I certainly don’t feel owed a response, I’m curious whether people didn’t respond to it specifically because it sounds like the sort of “crank” ideas that sometimes get posted here.

    What I said was:

    But from a perspective in which the senses and sense organs *are* the “external” world – in which the separation is essentially arbitrary – the problem doesn’t arise. If my belief in my senses is justified, my belief in the world is automatically justified as well.

    What I was trying to say here is that in the solipsistic scenario, the seeming world is just one thing (our “self”). And in the monistic physicalist scenario, the seeming world is just one thing (the world) that we (including our experiences, thoughts, etc) are inseparably a part of.

    Our beliefs concerning *existence* in both scenarios are essentially identical — we believe that something exists that gives rise to our experiences. Having experiences at all seems to justify that, and I take that to be the spirit of the Cogito.

    If “only I” exist, then I am the world. If “only the world” exists, then I am the world.

    But now the question isn’t about existence, per se, but rather the nature of the world. This is where, for me, the sort of justification that we want to require for ontological questions isn’t necessary. Our justification becomes something more pragmatic — we are looking for something that “works”, i.e. allows us to create conceptual models that are consistent with how the world behaves.

    If this sounds like a crank idea, I really don’t mind people saying so, and pointing out why it is crazy-seeming.


  13. Okay – last one. So my question is this: Is the solipsist open to the same skeptical attack as the person who asserts the existence of an external world? My intuition tells me that she is not, because she doesn’t “believe that she has hands”, to put it in Massimo’s original terms. It’s not the belief in the nature of the universe that the skeptic is attacking – as you seem to have been at pains to point out – but rather the belief in the existence of the world at all. If this is so – and maybe I’m just confused – then there is an asymmetry between the solipsistic position and the position of belief in an external world. It’s this asymmetry that I think the position of monistic physicalism fixes.


  14. Coel,
    Let me remind you of the original definition of the principal of parsimony, given by Blessed John Duns Scotus, Tractatus de Primo Principio, 2.45 (which he in turn attributed to Aristotle)

    (Fifteenth conclusion)Plurality must never be assumed without necessity

    The operative word here is ‘necessity’. We may multiply entities where there is a clear necessity. But how do we know when we should multiply entities if we have no knowledge of that necessity? In the presence of ignorance of necessity we make false appeals to the principle of parsimony because we have no knowledge of the necessity to multiply entities.

    Consider two possibilities:

    1) You are a BIV.
    The BIV has no evidence of the larger world so it cannot conceive of a necessity to multiply entities. Using the the principle of parsimony it claims it is not a BIV(falsely, in this case).

    2) You are not a BIV
    You see no evidence of a larger world and using the principle of parsimony, you claim(rightly, in this case) there is no larger world.

    So, if you appeal to the principle of parsimony you will always claim you are not a BIV, regardless of the truth. In other words, the principle of parsimony has no power to discriminate between true and false statements in the presence of ignorance of necessity.

    For a very good discussion of Blessed John Duns Scotus’ thinking see ‘The Philosophical Vision of John Duns Scotus’ by Mary Beth Ingham. (I know, more book recommendations you will never read).

    Let me give you a real life example to bring home the point. In the local East Cape mountains that I love to roam are a great many caves. Most of them contain Bushmen paintings ( They speak to a simple hunter/gatherer people that roamed these remote mountains until about 150 years ago(don’t ask what happened to them). Imagine for a moment that you have climbed up into these mountains 200 years ago and spent the evening in a cave with one of these groups, munching on a Kudu haunch and sipping fermented honey. In the course of the conversation you asked them about their conception of the world. They would have described the peaks, ridges, ravines, plants and game that circumscribed their world. There were some remote groups of Bushmen but mostly they kept their distance unless it was for the exchange of brides. They knew of nothing else, no European powers with their interminable wars, no science, no literature, no arts, no industry, no commerce. The horizon was blank, limited by other rugged mountains.

    You questioned them closely. Was this the whole world? Yes they said, we have never seen anything else. You continue insistently, but how do you know there is nothing else? The philosopher among the Bushmen gazes at you, puzzled by your stupidity. It is simple to us Bushmen, he said, the principle of parsimony requires that we should never assume plurality without necessity. Look around you, there is no evidence of necessity for plurality. This is all there is.


  15. Hmm,

    I still think it is feasible to take reason and logic as your foundation, and all other beliefs should follow from these and sensory experience. So, as long as we assume our thinking apparatus is correct, then I think everything else does have a reason.


  16. Hi Asher,

    in each case there’s just one entity – either the ur-self or the universe.

    I don’t think that’s a particularly useful way of counting entities, or else we would have no way of comparing hypotheses since all hypotheses ultimately assume one reality (even multiverse hypotheses, I would say).

    I think the more parsimonious explanation is the one that takes less information to specify completely, to the point where a computer simulation ought to be possible in principle. I can see approximately what kind of information would be needed to simulate the external world (namely an algorithm to simulate the laws of physics), but if what you are is a disembodied mind then we would need an algorithm to simulate that mind, as well as an algorithm to simulate the physics of an external world. That’s much more complex in my view.


  17. Asher,
    My argument is that an absolute state has no information, because it has to distinctions, or actions, meanwhile information is all about relations and connections between differences, so absolute knowledge is a contradiction.
    Now an absolute state is equilibrium and the universe consists of positive and negative curvature, which cancel out to overall flat space, to if you were to ‘compute the universe,’ the ‘answer’ would be zero.


  18. To further that point, there is no reductionist model of the universe which would explain all details, so even if it adds up to zero, the knowledge of the universe can’t be reverse engineered from that final, absolute answer.


  19. I don’t agree that we should be so relaxed about our “imperfect grip on reality”. The phrase sums up the skeptical predicament quite well. Except that “imperfect” is perhaps too mild a word. If I don’t even know if I have hands then I don’t really know much.

    It’s not just that there are lots of facts we don’t know about the world. We can feel confident in our track record as a species and as a society when it come to accumulating new facts as and when we need them. And it’s not comparable to our worries about theories of particle physics that suggest the world is radically strange at a microscopic level. That doesn’t affect my beliefs about the medium-sized objects I encounter in my everyday life.

    Cartesian skepticism is more fundamental and really does challenge my “grip” on even everyday reality. If we draw the conclusion that the world we experience may be radically different to the true reality which is hidden behind it, a reality that we can know nothing about, then this is an intellectual tragedy.

    It’s worse than anti-realism. Anti-realists tell us that the question of how things really are is meaningless. As they see it they would liberate us from such metaphysical concerns. But the skeptic does not let us off so easily. The truth really is out there but it will always be beyond our reach.

    Knowing that you can often be wrong about various things may foster intellectual humility. Believing that we fundamentally know nothing is intellectually corrosive.


  20. Aravis,

    I fully agree with you that we perceive the action of gravity with our senses, it is obvious. But, in my opinion, our senses developed as an adaptive strategy to deal with an environment ruled by the gravity force. From the physical point of view we are compelled to build up a sensory system to deal with gravity. That said, the yogi´s paradox aims to reveal that is possible to quit a skeptical argument on empirical bases. I also know that my argument is realistic and empiricist, is not my intention to detract the BIV argument or any other skeptic approach from empiricism, it is just another sight on this issue.


  21. A central point in this discussion is how to decide between alternative view points, which labnut summarized as;
    “Consider the problem that several, well informed and intelligent people can examine the same political or moral problem and reach very different conclusions. I know all the usual explanations but I can’t get past this point, if I were able to enter the other person’s mind and perfectly apprehend his point of view it would make perfect sense to me. But we can’t enter each other’s minds and we always reach different conclusions (DM, I am looking at you).”
    I have been arguing that knowledge is inherently subjective, ie. absolute knowledge is an oxymoron and I would like to examine the problem in that context;
    Now consider the idea of space as three dimensional; Effectively it uses three coordinates to define space, yet unless we define a specific set of coordinates, it is just an abstract statement. For instance, your set of coordinates could be at a different angle, or moving relative to mine and while each would be applicable to each of us, either would be an external frame to the other. In fact, on the basis of the different frames, each encounters either different circumstances, or the same from different angles. Thus they also create different narrative timelines, ie, different time vectors/dimensions. Yet both can still exist within the same general space. Consider how this applies to conflict, for example, Israelis and Palestinians. Essentially they use different coordinates and different narratives to describe the same space.
    Now combine this with the neurological function of distilling a narrative sequence out of a thermodynamic environment, where much of the actual signal/noise is being lost/discarded and it is easy to understand how different people can arrive at entirely different viewpoints and yet both be valid from their point of view.
    This then goes back to my original point about how knowledge is inherently subjective and not just an artifact of our own subjectively, for which no one has actually refuted, only said was beside the point of the discussion, or my anecdotes don’t prove the point. Now I’ve been musing over this observation for a couple of decades and so would find some pleasure in agreement, but I’m not immune to it being refuted either, since my livelihood isn’t dependent on it being right. So if anyone can come up with counter arguments, I’m open to them.


  22. @DM

    I don’t think that’s a particularly useful way of counting entities, or else we would have no way of comparing hypotheses since all hypotheses ultimately assume one reality

    It’s a particularly useful way of counting entities when it is precisely those entities that are in question. And there’s nothing in what I’m saying that implies that one would have to count entities the same way for all hypotheses.

    With the assumption of the self and the “external” world, you’ve got two things. With the assumption of the self alone, you have one. As I said above, this creates an asymmetry wrt the skeptical attack, because you have to have one thing believing in something separate from itself to say that certainty of existence isn’t possible (assuming you accept the Cartesian approach to this).

    So again, as I said above, I think this way of counting is very useful when you are looking at certainty about existence. When you are attempting to justify one theory about the *nature* of reality over another, perhaps a different way of counting would be more effective.


  23. Disagreeable Me: “The external world hypothesis is relatively parsimonious because it only assumes that there is a system of physical laws by which beings like me can evolve, etc.”

    Aravis Tarkheena: “solipsistic one; i.e. ‘I am the only thing that exists, and what I am is a disembodied consciousness, whose experiences are all self-generated, as dreams are.’”

    Solipsism has, at least, two points:
    One, only one’s own mind is sure to exist.
    Two, knowledge of anything outside one’s own mind is unsure; the external world and other minds cannot be known, and might not exist outside the mind.

    Then, as a metaphysical position, solipsism goes further to the conclusion that the world and other minds do not exist.

    The key issue here is about the definitions of a few key words (exist, mind, knowledge and outside).
    a. Does the ‘body’ of my ‘mind’ exist?
    b. While the ‘other’ minds cannot be known, do the other minds exist?
    c. While the other minds can be parts of my mind, are there bodies (houses for the other minds) outside of my mind as real objects?

    All these questions are positively answered when one solipsist ‘seeks’ the help from a doctor for his illness.
    1. In addition to his mind, he does have a body (got sick).
    2. In addition to his existent, there is an object (the doctor) outside of his existent.
    3. In addition to his ‘knowledge’, he is asking the help from a ‘knowledge’ outside of his own. And, this outside ‘knowledge’ knows about ‘his’ body and mind while his own knowledge on his own body and mind is useless at this occasion.

    DM tried to counter the solipsism with an ‘external world hypothesis’, and that hypothesis can of course be rejected by the solipsists. But, can solipsists reject their own framework? Does the above description show that solipsism is not defendable? Is there any chance of convincing a solipsist that solipsism is not defendable? I don’t think so because there is a type of argument that is able to swallow all these questions. It is not really an argument but is a meta-argument (the chicken/duck logic).

    Chicken says: gaga
    Duck says: yaya
    Then, there are two possible outcomes for this chicken/duck argument.

    (1) For every chicken (gaga), there is always a response of duck (yaya). Therefore, Gaga = yaya
    (2) For every chicken (gaga), I always have a duck (yaya). Therefore, your gaga is wrong.

    For solipsism, it has many gaga.
    Gaga 1: my mind exist, and no one cannot deny that.
    Gaga 2: at least one part of my mind cannot be known by anything but my mind, and no one can deny this.
    Gaga 3: ‘you’ (everyone) can be a part of my mind, and I know this for sure in my mind.
    Gaga … n: …

    As all my gaga (1, 2, … n, …) are solidly true, I have a solid ground to stand on, that is, I can deny all your yaya (whatever they are) without in any kind of danger.

    Indeed, solipsism cannot be defeated with any ‘external’ doctrine but it cannot survive its own bullets as this chicken/duck logic is not a genuine argument. Of course, it can still be held as a personal ‘position’ and belief as an ‘allowance’, allowed to be in the category of ‘wrong’.


  24. Massimo,

    Thanks for the response.

    “That, seems to me, would lead straight to a type of pragmatic anti-realism about scientific theories, which is a perfectly reasonable position to hold (though not one that I endorse).”

    I haven’t really thought much about realism or anti-realism, but here’s a stab at it.

    I’m not so sure I have to be entirely anti-realist here. Hypothesis testing is a kind of interaction with the world that involves anticipation (if I do X, then I can do Y). It also involves presuppositions about the external world; namely, that the conditions under which doing X is appropriate are in fact present, making it possible to do Y. For instance, suppose I have a theory that predicts a particular object should be visible at a given coordinate in the sky if I have a telescope of specified power. To point the telescope anticipates seeing the hypothetical object, and it involves numerous presuppositions about that location in space and the proposed object—that the location indeed has that object, that the object reflects or emits light, that it has certain relations to other objects which affect its position in space, etc. The success or failure of hypothesis testing provides a criterion for judging the truth of its presuppositions. If the hypothesis test fails, then its presuppositions are in error; the conditions under which the test would be successful are not present. Such failure would indicate the necessity for a change in presuppositions (theory change). Success, on the other hand, would warrant further testing of the same theoretical presuppositions (theory selection).

    This whole process relies on the existence of a reality independent of us. The very possibility of unanticipated error indicates that such an independent reality must exist. After all, if you construct your own reality, then you also construct the conditions under which any interaction in that reality is successful. You could hardly be surprised, then, when your actions fail to attain their goal.

    More concisely, to test theoretical predictions, and thus theoretical presuppositions, and then to let the results of that test guide theory selection and theory change is to allow an independent reality to constrain the construction of scientific knowledge. Such a process implicitly (if not explicitly) presupposes at least the realism of an independent reality.

    I think the only reason why recognizing the validity of the radical skeptical argument would lead to anti-realism is if you conflate epistemic contact with epistemic content. Most theories of representation treat content as the state of being in correspondence with the environment, which is caused by contact with the environment. But contact in this perspective is just a matter of creating the correspondence. The radical skeptical argument is premised on that very conflation, and as you said, it leads straightforwardly to anti-realism (idealism in particular). If you cannot go outside of yourself to check whether or not the hypothetical correspondence holds, then what reason do you have to think that any correspondence exists at all? But the model I am discussing here makes no such conflation. Content is constituted as indications for further interaction, but such indications are constrained by (not caused by) contact with the real world. Rather than taking hypothesis tests to confirm correspondences between theoretical objects and their referents in nature, I take them as indications of the success of theoretical presuppositions. This allows us to be realist about presuppositions in addition to being realist about the existence of an independent reality.


  25. Hi labnut,

    But how do we know when we should multiply entities if we have no knowledge of that necessity?

    You are misunderstanding the phrase “… must never be assumed without necessity”. “Necessity” here means required by empirical evidence. Thus parsimony is saying don’t add information to the model unless we have empirical evidence that demands it. Thus there isn’t “necessity” that we don’t have any knowledge of.

    (At least, that is how parsimony is used in science today, whatever Dun Scotus meant by it.)


  26. Hi labnut,

    My answer to questions 1, 2 & 6 is given here, justifying parsimony on probabilistic likelihood grounds (see also Solomonoff’s theory of inductive inference).

    The answer to 3, 4 & 5 is that parsimony is a probabilistic likelihood argument. It is a reliable and compelling argument, but being probabilistic it is not exception-free.

    My answer to 7 is “of course” and my answer to “apply skepticism to your own statements and beliefs” is that I do.


  27. Hi Aravis,
    I’ve mostly been away, so am just reading the thread. In common with DM I don’t see how your response negates the sort of argument in my first comment on this thread.

    I don’t see how the external world hypothesis is more parsimonious then the solipsistic one; i.e. “I am the only thing that exists, and what I am is a disembodied consciousness, whose experiences are all self-generated, as dreams are.” Indeed, I would argue that this is *more* parsimonious than the external world hypothesis.

    I totally disagree. Parsimony should be approached from the point of view of the information content of the model. If the model is that “all experiences are self-generated dreams” then to fully model all those experiences and the capacity to dream in a purely ad hoc way you would need a vast amount of information. In contrast, the “real world” model requires only a limited amount of basic physics to set it up, and that is it.

    Further, the “all experiences are self-generated dreams” is pretty hopeless at predictive power (by which I mean predictions of future experiences) unless one subsumes a whole “real world” model into the “dreams” model, which then makes the “dreams” model the much less parsimonious and indeed superfluous.

    We only have *sense* experience, if we have sense *organs*. We only have sense organs if the external world exists. We can have *experience* without sense organs, of course, as we do in our dreams, when our sense organs are “turned off.”

    Fine, so call it “experiences”, rather than “sense experiences”, and proceed from there. The real-world model is still vastly prefered in terms of parsimony and explanatory and predictive power.

    … the realization that belief is prior to reason (that is, one must believe things, first, before one can justify anything).

    Yes, in the sense that we have “working beliefs” that we use to analyse a situation, yet one can then re-think and re-consider those beliefs. They are not primary. In the end “beliefs” are the product of considering experiential data (whether “dreams” data or “sense” data or whatever).


  28. As I said to some of the others, if you think you’ve successfully overcome the skeptical challenge, you should publish it. You’d be more famous than Hume and Kant combined.

    I don’t know how else to explain the point. The arguments you make above all beg the question against Descartes, insofar as the skeptical arguments raised doubts about *both* the reliability of sense experience and reason (including logical deduction).


  29. Coel,
    if I may interject here.
    Just imagine, in turn, for a moment the following exclusive and exhaustive scenarios:

    1) You are not a BIV.
    2) You are a BIV.
    The important thing to note, is that from your perspective, as a putative BIV, you possess identical information in both cases and so you must reach the same conclusion in both cases.

    Now let us examine what conclusions you would arrive at in these two scenarios, using the principle of parsimony.

    1) You are not a BIV.
    according to your arguments, the principle of parsimony leads you to conclude you are not a BIV. That seems perfectly reasonable. Why imagine a greater and more complicated world when there is no necessity(Blessed John Duns Scotus). This is a truthful conclusion.

    2) You are a BIV.
    You have identically the same information as in case (1) above and so you are compelled to reach the same conclusion, from parsimony. You have to conclude you are not a BIV. This also seems reasonable. Why imagine a greater and more complicated world when there is no evidence of necessity(Blessed John Duns Scotus) and the BIV has no evidence of that necessity. You have reached a false conclusion by appealing to the principle of parsimony.

    Therefore the principle of parsimony(in the case of the BIV hypothesis) can equally result in a false conclusion as well as a true conclusion. In other words, the principle of parsimony has, in the BIV hypothesis, no discriminatory power whatsoever. But surely, that is not surprising, since in both scenarios, you have identical information!!

    If the principle of parsimony has, in the case of the BIV, no discriminatory power, you cannot possibly appeal to it to decide the issue.

    The principle of parsimony is totally useless in adjudicating the issue of the BIV.

    Finally, as Aravis said, ‘if you think you’ve successfully overcome the skeptical challenge, you should publish it. You’d be more famous than Hume and Kant combined.

    Good luck with that.


  30. Coel,
    I am trying to understand what it is that underlies your failure to grasp what is obvious to professional philosophers.
    What I think it comes down to is an obsessive concern with data. You are married to your observations and these represent the entire world to you. As far as you are concerned, nothing else exists and so you see no need to consider alternative scenarios. Seeing no need to consider alternative scenarios, you do not seriously examine the possibility that a BIV would reach the same conclusion as a non-BIV. Since you won’t consider that scenario, there is only one possibility and that is you are not a BIV.

    This is interesting to me because it illustrates, in a very fundamental way, how scientsimists will always have a partial understanding of reality. They cannot imagine more than their data.


  31. Hi labnut,

    So you’re saying that parsimony cannot choose between BIV and non-BIV? OK, how about we take two scenarios: (1) real world, (2) real world plus Invisible Pink Unicorns who can make no difference at all to our experiences.

    By your reasoning, if (2) were the case then, by adopting parsimony and rejecting the IPUs, we go wrong. This is entirely correct (and as I’ve said all along, parsimony is a probabilistic argument not an infallible one). But, there is a literally infinite number of other “real world plus …” scenarios (real world plus Russell’s teapot is another, but there are infinitely more).

    So, what do we do? Well, “real world plus IPUs” could be true. But so could any of the others. Since we have (ex hypothesi) zero data to guide us, the chance of accidentally hitting on the right “plus” is literally infinitesimal. Thus the sensible thing is to reject the IPU-model as having only an infinitesimal chance of being right, and thus stick with the bare “real world”. The same applies to any other possible “plus”.

    Now, the BIV argument is in effect a “real-world plus” argument, with the “plus” being the “vat” and everything about the vat. This is because any BIV model still contains the “real world” stream of experiences and has to account for it. Of course it would then be a “simulated world” rather than a “real world” but the difference between “simulated” and “real” is just a label, and one can readily slap the label “real” on the stream-of-experiences. So the BIV model, in terms of information content, is simply “real world plus vat”.

    The above argument again applies: yes the “plus vat” *could* be correct, just as “plus IPUs” could be. And if any of these were actually the case then we’d go wrong by excising them using parsimony. But, as above, the chances of us correctly postulating any of these by chance is infinitesimal.

    The same applies to every model that essentially takes the real-world model and applies a wrapper to it (e.g. that our whole universe is a simulation on a kid’s computer from some hyper-advanced parallel universe).

    So, I stand by my claim that parsimony is a robust and compelling argument. It is, though, a probabilistic argument and thus is not infallible.


  32. Hi Aravis,
    Reason is validated by the fact that it works when applied to the stream-of-experiences. Thus reason is ultimately empirical (taking empiricism to refer to the stream-of-experiences). Of course the entire kaboodle (stream of experiences plus reasoning about it) could be a “simulation” created by some meta-reality, but if that is true then the only sensible thing to do is to apply the label “real” to that stream-of-experiences. After all, it is the thing that is most “real” to us.

    By taking the stream-of-experiences is primary and as what we declare to be “real”, you can then validate reason, logic and maths as being regularities in that stream-of-experiences, and from there proceed to full-blown science. I don’t see any question-begging in that.


  33. Coel,
    stop, this has nothing to do with pink unicorns and flying teapots.
    So let’s talk about BIVs only.
    I have made a clear and simple argument.
    Please address and answer the actual points I have made in my argument.


  34. Like I said. Publish it, in a peer-reviewed journal. You’ll be the most famous philosopher, since Kant.

    In all seriousness, I don’t even understand the first paragraph of your post. What is clear, though, is that you are engaging an issue of your own, not the issue that historically has been a concern for philosophy, since the Ancient Greek skeptics. That, however, is the issue that Massimo’s initial post is about.

    Have a good day.


  35. labnut,

    Seeing no need to consider alternative scenarios, you do not seriously examine the possibility that a BIV would reach the same conclusion as a non-BIV. Since you won’t consider that scenario, …

    That’s a rather bizarre thing to say considering that I have not only examined the possibility that a BIV would reach the same conclusion as a non-BIV, but I have explicitly stated my agreement that that is exactly the case.

    This is interesting to me because it illustrates, …

    Whereas you are interesting to me as an example of someone who routinely ignores what replies to you actually say!


  36. Hi Labnut,

    Coel is entirely right, in my view, and I’m sure there are plenty of phiosophers who would agree with him (even though I may not be familiar enough with the literature to identify them). I think it would be a mistake to take Massimo and Aravis as representative of all professional philosophers, although I’m not sure even they would endorse your apparent comfort with radical skepticism. Massimo’s post establishes that it is possible that we are brains in vats. It does not at all suggest that it is reasonable to think that this is anything like a probable scenario.

    Coel does consider the possibility that the BIV scenario would be true and that the BIV Coel would reach the same conclusion and be wrong. You think that is fatal to his reasoning but it is not, and because it is not he persists in believing his argument works even though he is fully aware of this possibility.

    Playing the lottery is probably not a good way to make a fortune, but there are some people who succeed at doing just that. It doesn’t change the fact that it is probably not a good idea. The people who made their money in this way were not wise investors, but lucky fools.

    (OK, fool is a bit harsh. Some people enjoy the fantasy and so get something out of simply entering the draw. But it would be foolish to buy a lottery ticket if your only goal is to maximise your return.)

    The same is true, in Coel’s and my view, of anyone who seriously thinks he or she may be a BIV. Even if this happens to be the case, that person is a fool. So if Coel denies that he is a BIV, it is possible (though vanishingly improbable) that he is wrong. But if he is wrong, he’s still no fool.


  37. Coel,
    wonderful, we are making some progress.
    Just to confirm, you agree that
    1) the BIV and non-BIV have identically the same information?
    2) the principle of parsimony reaches the same conclusion in both cases? In other words no BIV?
    3) the non-BIV reaches a truthful conclusion?
    4) the BIV reaches a false conclusion?

    Do you agree with these statements?


  38. I would side with Coel on this.
    To quote a certain Sec of Defence; There are the known knowns, the known unknowns and the unknown unknowns.”
    One of the many aspects of rationality is being able to distinguish between the three.


  39. Coel, Aravis,

    We should not be that far apart. Let me state at least my position clearly.I suspect Coel would agree with it.

    1. It is possible that we cannot reason, and only think we can, because reason cannot be justified except with reason.

    2. If we start by assuming we can reason, which we must if we are to engage in philosophical discussion, we can account for why we have reason with evolution, and we can validate that reason seems to work by logic/empirical investigation. So while we cannot fully justify reason, reason is at least not particularly mysterious and we have no cause to doubt it.

    3. It is possible that we are brains in vats. The skeptical argument therefore illustrates that even assuming reason, there is very little of which we can be absolutely certain.

    4. It is reasonable to adopt the hypothesis that we are not in fact brains in vats because this is the more parsimonious explanation. This is Coel’s main point, and I don’t think this is anything revolutionary, certainly not enough to make him eclipse Hume or Kant. This is, I think, a somewhat standard view, or at least I would be very surprised if it were not. In fact I believe it to be too obvious to publish. Aravis seems to be interpreting Coel as claiming that he has utterly defeated the skeptical argument by showing that we are certainly not brains in vats. Coel has never made this claim.

    Coel’s point is I think offered as a counterpoint to some misguided interpretations of the skeptical argument that have been posted to this comment thread. It does not at all undermine or address the fundamental point of the skeptical argument, which stands. The skeptical argument should be interpreted only as establishing possibility, not plausibility, whereas Coel is only addressing plausibility.

    There ought to be no disagreement between you as far as I can see, apart from perhaps disagreeing on whether the BIV hypothesis is implausible because of parsimony (which you seem to interpret differently) or some other criterion.

    For me, the skeptical argument is why “justification” in the “Justified True Belief” definition of knowledge should never be mistaken for certainty. This is a very valuable contribution to epistemology, and I think we are in danger of missing the point if we start to forget that there are still reasons to find some possibilities more plausible than others.


  40. Hi Aravis,

    I don’t even understand the first paragraph of your post. What is clear, though, is that you are engaging an issue of your own, not the issue that historically has been a concern for philosophy, since the Ancient Greek skeptics.

    Having that long a history of pondering an issue without arriving at answers can be a sign that the issue is not being thought about in the correct way.


  41. Hi labnut,

    I have made a clear and simple argument. Please address and answer the actual points I have made in my argument.

    That is exactly what my comment did.


  42. “Extraordinary claims require extraordinary evidence.”

    Do they, really? Let’s examine an extraordinary claim, and see if this dictum holds up. I think it’s on shaky ground.

    Extraordinary claim: A human being can fly (without the aid of a mechanical device).

    What evidence would be required to defend such a claim? I say it would be extraordinarily mundane: all one would need to do is take flight in view of the skeptics. Such a feat would *seem* extraordinary, because we do not usually see a human being do such a thing. One might say it is by definition *extraordinary* because no human has done such a thing in plain view.

    But that’s really besides the point (epistemologically speaking). After all, non-human animals fly all the time: insects, avians, reptiles, fish, other mammals, perhaps even, in the past, certain primates. We intuitively think the proof of the claim is extraordinary, but we are conflating the claim with the evidence, which in this case would not be extraordinary at all.

    I think I can guess what everyone who reads this post is thinking to herself: but even if I *saw* a human being take flight, I would insist on checking that I was not duped somehow — Are there wires? Mirrors? A blue screen projecting CGI? Was I drugged? And if you were satisfied that those special effects had not caused you to see a human being fly, would you continue to insist on *extraordinary* evidence? What might that evidence be, if your own eyes can’t be trusted?

    The point isn’t whether “I can fly” isn’t an extraordinary and ridiculous claim. Of course it is. The point is whether that particular claim requires some sort of special, extraordinary evidence to demonstrate it is — or is not — true. It does not (necessarily).

    As much as I admire Carl Sagan, I think he got that one wrong.


  43. Hi DM,
    I mostly agree with your summary, however:

    1. It is possible that we cannot reason, and only think we can, because reason cannot be justified except with reason.

    The wiki page on “reason” starts: “Reason is the capacity for consciously making sense of things …”. In this context the “things” are the “stream of experiences”. It is then true that we do see regularities in the stream of experiences, and thus we can indeed “make sense” of that stream of experiences. That is our justification for reasoning; it works.

    Of course this argument only holds about that stream-of-experiences (whatever that is, either real-world sense data or BIV simulation), and it may be that there is some meta-reality and meta-reasoning that we are oblivious to but which is actually more “real” and more “correct” than “our reasoning” about the stream-of-experiences.

    But, even if that is the case, the thing that is “real” to us is the stream of experiences, and our reasoning is indeed valid about that. So we simply use the labels “real”, “valid” and “true” about that stream of experiences and about our reasoning about that stream.

    At which point we have justified our reasoning; it works.


  44. Hi labnut,

    Do you agree with these statements?

    Yes, I agree with all four statements (as I indeed said in my IPU comment above). Note that the equivalent of all four statements would also be true about the hypothesis of “real world plus Invisible Pink Unicorns which have no possible impact on sense data”.

    Further, the fact that your four statements hold does not invalidate the use of parsimony, which remains a robust and compelling argument (though, being probabilistic, not an infallible one). For an explanation of why parsimony is still a compelling argument see the “IPU” comment just above which you ignored first time.


  45. Hi miked,
    I think you’re misinterpreting what “extraordinary” evidence is, it just means extraordinarily robust evidence that survives detailed scrutiny. As you say: “I would insist on checking that I was not duped somehow — Are there wires? Mirrors? A blue screen projecting CGI? Was I drugged?”.


  46. You’re misunderstanding Sagan, I think.

    even if I *saw* a human being take flight, I would insist on checking that I was not duped somehow — Are there wires? Mirrors? A blue screen projecting CGI? Was I drugged? And if you were satisfied that those special effects had not caused you to see a human being fly, would you continue to insist on *extraordinary* evidence?

    This is the whole point. It’s an extraordinary claim, so any evidence that claims to support it should survive extraordinarily robust levels of scrutiny, ideally being replicated in a lab under controlled conditions.

    The kinds of considerations you suggest here are exactly the kinds of things Sagan would insist that we consider. If the claim is mundane, such as that I am a male human being, then you would not require any such evidence in order to take it at face value.

    So, you agree with Sagan completely.


  47. Hi Coel,

    It is then true that we do see regularities in the stream of experiences, and thus we can indeed “make sense” of that stream of experiences. That is our justification for reasoning; it works.

    Yes, it works, but only if we assume that we have the capacity to interpret and think about this evidence correctly.

    It is possible that both of us are irredeemably insane and we only think we are rational because this is part of our delusion. The apparent ability we have to make sense of our experiences could be entirely illusory.

    As I argue, this is not a possibility that I think should be taken too seriously, but I think it is correct that it should be acknowledged.


  48. Hi DM,

    Yes, it works, but only if we assume that we have the capacity to interpret and think about this evidence correctly.

    What do you mean by “correctly” in that sentence? We can test whether our reasoning does match the stream-of-experiences. That is what verifying predictions does. Are you suggesting that even if our reasoning does match the S-of-E then it could still be “incorrect” as judged by some meta-standard? If so I simply define “correct” to mean “whatever matches the S-of-E”.

    It is possible that both of us are irredeemably insane and we only think we are rational because this is part of our delusion.

    What do you mean by “insane” and “delusion” in that sentence? I’d regard “sane” and “non-delusion” as meaning a correct match to the S-of-E. If you are suggesting that there could be some meta-standard by which those things are “delusional”, for example that everything we experience is a simulation in some meta-kid’s computer, then I just shrug and stick with my definitions of “sane” and “non-delusion” as meaning a correct match to our S-of-E.

    The apparent ability we have to make sense of our experiences could be entirely illusory.

    What do you mean by “illusory” as used in that sentence? I simply define our S-of-E to be “reality” and thus the “apparent ability we have to make sense of our experiences” is by definition non-illusory, since I define non-illusory to mean a correct match to the S-of-E.

    If you’re suggesting that there is some meta-standard that is different then again I just shrug and stick with my definitions. After all, the S-of-E is what is real to us, and these possible meta-standards are, by definition, not something we can ever know about and thus not “real” to us.


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