How do I know I’m not a brain in a vat?

 T image of the brain in the vat-quite apart from what Putnam actually intended it to do- is used as simply another presentation of the foundational challenge of modern philosophy: how do you know that you know? How do you rule out that you are a brain in a vat? We can imagine this being the case, so it must at least be possible…right?

No! No! Ten thousand times no! Our imagination, whether taken narrowly or broadly, is neither the measure nor the cause of any real possibility. There is no relation between imagination and real possibility: some imagined things are possible, others impossible, others necessary; and the same is true for those things that cannot be imagined. As soon as we allow that our imagination indicates real possibility, we allow a parallel universe of possible beings that are evident to imagination alone, and whose real existence does not follow from actual external sensation.

This kind of argument is not philosophy, but precisely the sort of thing that philosophy seeks to avoid- namely spellcasting and shamanism. The power one feels in the brain in the vat argument is the exactly the same power that voodoo witch-doctors use to cast hexes, because whether one falls prey to a hex or the BIV argument, the mechanism is the same- we think some thing is a real possibility to be feared, fought against, or even accepted simply because we can imagine it being so. How can we rule it out? Maybe we aren’t wholly convinced by the argument/spell, but isn’t it better to assume it is true just to be safe? After all, the most reputable people in the village/campus think it’s so.  

How do I know I’m not a BIV? It’s not because “this is my right hand, this is my left”; or because realism is a more effective practical postulate; or because the BIV offends my intuitions; or because I ignore the question (and it is a real question that deserves a response. I call it shamanism in all respect. Shamanism has real power). I know I am not one because I have not set up imagination as indicating real possibility.


  1. Peter said,

    February 28, 2008 at 3:11 pm

    > I know I am not one because I have not set up imagination as indicating real possibility.

    That really is the crux of it.

    I can’t tell you how many times I have encountered people who think that because they can *imagine* some piece of counter-evidence (or scenario, like the BIV or “The Matrix Situation”) that that somehow undoes all the *actual* evidence to our senses that we work from.
    It is especially annoying when the appeal to the fantastical is used as an excuse to be skeptical about the evident and obvious.

    Not that thought-experiments are bad, but isn’t it interesting how frequently in contemporary debates one hears the phrase, “I can imagine. . . .” compared with Aquinas or Aristotle?

    On the other hand, we do use imagination for certain things pertaining to reason: such as constructing a theory to explain something, as a detective does in a crime scene. It would be cool if you did a follow-up post on the proper role of the imagination in reasoning.

    Great post!

  2. Jason Zarri said,

    February 28, 2008 at 4:12 pm

    Hi Thomist,

    I agree that imaginability and/or conceivability are probably not good guides to real possibility. Still, I’m not sure how this is supposed to ground our knowledge that we are not brains in vats. Sure, being able to imagine that we are actually brains in vats doesn’t prove that we really could be–but neither this nor anything else you’ve said shows that it is either impossible or even wildly improbable. It might very well be impossible that I am a brain in a vat– in a purely epistemic sense of “might”– but the question remains of how I am to know that it is impossible. You’ve acknowledged that neither imagining something nor failing to imagine it proves anything either way concerning its real possibility: Maybe it’s possible, and maybe it isn’t. So my question is, if imaginability (or lack thereof) does not justify our belief that we are not brains in vats, what does? Or are you saying that we do not need justification in order to know it?

  3. a thomist said,

    February 28, 2008 at 11:28 pm


    To be clear, I am NOT saying that our imagination fails to justify our belief p where p= we are not BIV’s.

    I say here that the BIV problem presupposes believing “if something is imaginable, it is therefore a real possibility”. Putnam’s whole argument (which is Descartes “evil demon” too) rests on this premise. It is also the premise advertisers presuppose when selling diet pills.

  4. a thomist said,

    February 28, 2008 at 11:45 pm


    I’ve dealt with all those Matrix people too (how important it is to have good art as the first teacher of right philosophy! Plato was so right in casting out the lying poets! The problem is eve bigger too, most people believe that there is no real difference between living things and machines because o Movies and television- more false imagination as the source of error.)

    Imagination is vital in knowledge, and in fact it provides us with the mathematicals, which are the great light and exemplar of all knowledge. It’s true that imagination has a role to play in forming hypotheses, and in such things- but insofar as art is concerned, experience is more important. The cop who uses hi imagination well to solve the crime is the one who has the most experience- and similarly with the scienctist forming hypotheses.

  5. February 29, 2008 at 1:15 am

    Hi Thomist,

    If you are talk about Putnam’s Brains in a Vat, it is an argument against the idea that we are brains in a vat.

    But, really I don’t see what is problematic with BIV or Matrix scenario. I surely don’t find anything implausible in it, given that every day we are getting better in creating virtual realities. And, as far the Putnam’s argument against BIV is concerned, it seems to me more of a reductio against the strict causal/historical account of language, than really an argument against possibility that we BIV.

  6. a thomist said,

    February 29, 2008 at 1:43 am


    Right, I mentioned one of Putnam’s own takes on his argument in the first paragraph, and then went on to speak of where the real power in the argument is. You are right that he initially framed it or another purpose than the one I speak of here. I could have spoke to this too, but I chose to speak about something else.

    Your second paragraph is the sort of argument I spoke to in the post.

  7. February 29, 2008 at 2:24 am

    Oh, sorry, I misread some parts of your post as saying that Putnam is accepting the BIV scenario.

    As I see it, the issue is not that people are thinking that the scenario is possible because they can imagine it. It is that in imagining it, people CAN’T see any inconsistency. That of course, doesn’t mean that there is no inconsistency in that idea. It might be just that we didn’t spot it.

    But I don’t think it is fair to say… “Even if we might imagine it without noticing any inconsistency, there might be some inconsistency. So, if you don’t prove that there is no inconsistency I won’t accept it as a possibility.”. Because most people CAN imagine it without inconsistencies, it is up to the philosopher who is against it to point to the inconsistencies.

    So, either one can contribute to figuring out some inconsistency in the scenario, or accept that *as far it is known* there is no inconsistency, and the scenario is really possible.

  8. a thomist said,

    February 29, 2008 at 4:29 am

    Good response, and I think it pushes the argument further. It is true that if we tried to find every possible inconsistency in an argument before we took it seriously, we would never take anything seriously. This is definitely to be avoided.

    But the BIV argument still falls prey to the same fallacy, even with this qualification. Regardless of whether one is speaking about inconsistencies or consistencies, neither follow from imaginability or the lack thereof. One cannot prove that something is really possible or impossible from either imaginable consistency, or the lack of it.

  9. the.pilgrim said,

    February 29, 2008 at 5:21 am

    The point of the BIV argument, as I see it, is not that we should be skeptics because we can’t be sure we aren’t in a vat. The point is that a person in such a vat would have the same phenomenal experience as us, but wouldn’t know anything. It invites the question, what’s different about us? Well, we are in a environment which is conducive to the proper function of our belief producing cognitive faculties aimed at the truth. The fellow in the vat isn’t in such an environment. So we are furnished with knowledge, while he isn’t.

    Of course we don’t have to know that we are in such an environment to have knowledge. We just have to be in that kind of environment.

  10. Peter said,

    February 29, 2008 at 5:28 am


    Not only is Descartes’ “evil demon” very similar to this, but also the “dream argument.”
    The aristotelian Peter Simpson wrote an interesting (and short) essay on it in which he draws similar points: “…for what matters here is not the degree of force the argument has but what its structure is meant to be. Descartes clearly intends its structure to be such that it establishes at least the *possibility* of the conclusion, and hence that it *allows one to assume them.*”

    It’s worth looking at:

  11. a thomist said,

    February 29, 2008 at 6:44 am


    I really like that reading! But again, the BIV argument admits of many different interpretations. The skeptical reading, however, is the first reading that people tend to have, and with good reason. Taken in this way, it is a serious argument that deserves a response, and I haven’t seen the response given here anywhere else. After we refute the skeptical take on the argument, we can still use it to show other things, as you do.

  12. the.pilgrim said,

    February 29, 2008 at 11:32 am

    As I see it, the skeptical reading turns on this: that unless you know you are not a BIV, then you can’t know anything via your perceptual faculties.

    Of course, I disagree. My response is that what counts is that you are in fact not a BIV, whether or not you know it. That is, I’m an externalist. You don’t have to know that you know in order to know.

  13. Kohl said,

    March 31, 2008 at 7:05 pm

    Please excuse the unacademic presentation of my thoughts on this, Im not very academic in philosophy. More of an art-guy really. And sorry but this goes on for a while.

    The only thing I can know to exist is my own mind.
    Regardless of the reality I am a part of, all realities necessarily require creatures to have their own private experience. Meaning, the only thing a creature can know to exist is its own mind, whichever postulated universe it is a part of. Therefore, this argument applies to this reality, and any other reality that may be over-arching us if we were BIVs.
    The natural way of things, indicates that all other living creatures definately have minds as real as mine. (for example, microcosm, macrocosm…as below, so above…gravity is the most powerful of all forces…)
    The BIV hypothesis takes the concept of hypothesised advanced technology, and states that it is ‘scientifically possible,’ while clumsily avoiding the conceptually larger or ‘more potent’ issues, that point to the fact that it isn’t at all possible.
    Although I believe that yes, it is, in a way scientifically possible (in a causally disconnected bubble – the realm of imagination that you mention) to take a brain and manipulate it in a vat to such subtlety and mastery that the world it is in is almost indistinguishable from reality, I would argue on the contrary in terms of this notions practicality, that it is not possible given all other humanistic issues. While a particular scientific theory argues that anything can happen within a long enough period of time, the short existence of the human race, and all sentient races, simply wouldn’t be sufficient to realise the BIV system. It would undo the natural law of things if it were true. What im saying, is that being a BIV its possible in its details, but we can realise (even from deductive, sceptical reasoning) that its not possible in the bigger picture. And what is the biggest picture? That everything is unfolding in the way that it should. Why else would you exist in the first place?
    The universe is inherently dotted with life, just as it is dotted with matter.
    Life conforms to the same principles of matter – paticularly gravity. (see Carl Jungs collective unconcious, it sort of fits in here)
    The BIV theory would separate life from matter.
    This is impossible.

    So bottom line is, claiming that one day science will be able to (or already has managed to) create a BIV system, is like saying ‘anything can happen in a long enough period of time…my lifetime is very long, so I must eventually be able to grow my own spiders from my nostrils and shoot them out at things!!

    Hope this has made sense

  14. Anonymous said,

    February 1, 2009 at 9:42 pm

    One can entertain BIV on a practical level, meaning, judging from our perception, it SEEMS possible because of the observations we’ve made (I.E. illusions, hullicination, etc.) however, on a purely logical level it can’t ever be true. The argument for BIV always collapses in on itself. BIV like Solipsism implies that one can never formulate a priori using perception. However, consequentially BIV is included in this, considering perception is used to formulate the possibility of BIV. Therefor according to BIV, one can never assume BIV. It’s self refuting. BIV is escentially saying “Perception cannot be used, therefor this theory cannot be used”.

    Another case against BIV is that not only is it self refuting, but it implies a ridiculous organization of the theory. Meaning, it’s the same thing as saying “It is uncertain that maybe we are brain’s in jars”. Since BIV is a “Maybe” scenerio in the first place, to say that the “maybe” scenerio is just another another “maybe” in the first place is absurd.

    Now, this is not to say that there can’t be “distortions” of reality. Obviously there can be distortions, senses can be tricked, one can think they’re seeing a monster when in reality it’s a tree blowing in the wind at night. However, it is reasonable to say that perception=reality. For all that exists, all senses, including cognition is reality. There’s nothing else besides the senses, and cognition. And reality IS all that is and ever will be.

    So rest assured there’s more than one way to refute BIV. Be that as it may that no theory is perfect, there’s more than just one hole in BIV, there are a number of holes.

  15. Anonymous said,

    February 1, 2009 at 9:56 pm

    One last thing, BIV is also guilty of special pleading. It assumes that even it’s conduit is un-reliable in making any true statements about itself, but that somehow BIV is exempt, and in a sense, superior to it’s conduit.

  16. February 2, 2009 at 9:23 am

    tithenai ta phainomena

    … to prove what is obvious by what is not is the mark of a man who is unable to distinguish what is self-evident from what is not.

    He says that it is ridiculous for anyone to attempt to demonstrate that nature exists. For it is manifest to the senses that many things are from nature, which have in themselves the principle of their own motion. To wish, moreover, to demonstrate the obvious by what is not obvious is the mark of a man who cannot judge what is known in itself and what is not known in itself. For when he wishes to demonstrate that which is known in itself, he uses that which is known in itself as if it were not known in itself. And it is clear that some people do this. A man who is born blind may sometimes reason about colours. But that which he uses as a principle is not known to him per se, because he has no understanding of the thing. Rather he only uses names. For our knowledge has its origin from the senses, and he who lacks one sense, lacks one science. Hence those who are born blind, and who never sense colour, cannot understand any thing about colour. And so they use the unknown as if it were known. And the converse applies to those who wish to demonstrate that nature exists. For they use the known as if it were not known. The existence of nature is known per se, insofar as natural things are manifest to the senses. But what the nature of each thing is, or what the principle of its motion is, is not manifest.

  17. Jake Hay said,

    June 5, 2009 at 12:31 pm

    Even Bertrand Russell admitted…that there’s a sense in which solipsism is
    irrefutable. If I remember right this is in his introduction to philosophy.
    “I’ll admit there is a sense in which…” is how I remember this. It’s been years.

    See Wittgenstein on solipsism. “My” world…and how this shrinks to an extensionless point and leaves everything just as it is. Wittgenstein’s remark
    went something like: What the solipsist says is incorrect…but what he means
    IS correct.

    If it’s really true that we only know our own states? Well. There seems to be a delicious little discovery lurking underneath this issue. BIV is one way of looking at this…”solipsism” another…but obviously…BIV is comic…and “solipsism” ? Even if there are “others” the solipsist can never know this and will simply admit it. Even if you exist, I cannot strictly know that because I only know my own states. You certainly seem to exist. Then again. I seem to exist too. “Seem” is accurate.
    “Know” is not. If you’re honest now. Whatch those opinions. They’re prickly
    and they ARE opinions. Well. Interesting stuff. There’s something underneath this. Let’s find a great vocabulary for this. Not BIV not solipsism…something free of connotations. JK

  18. Henway said,

    June 22, 2009 at 6:35 pm

    Yep, I agree with you. It’s not just imagination, it’s learning/conditioning too. Just because someone came up with the concept of life being a brain in a vat, or a dream, and you read about it doesn’t make it a real possibility. It’s the same with religion. Just because some members of the church came up with the idea of a God, hell/heaven, eternal damnation 1900 years ago doesn’t mean it’s likely to be true.

    The true nature of our reality, and afterlife probably is something none of us could have figured out.

  19. M said,

    August 20, 2009 at 12:16 pm

    I’m a layperson… but I’m wondering if the supercomputer in a BIV situation may be able to reveal to the brain that it’s in fact in a BIV situation? The absolute authority of reality seems to be in the hands of the perception-giver, and the perception-giver in that scenario is the supercomputer. When the BIV demands proof from the supercomputer, well, the supercomputer can very well just alter the perception of the brain to convince it.

    In our case, the perception-giver is the natural world we live in. It is vain to acknowledge God if God had not revealed Himself through the natural world, in which He had done through history. When humans demand proof from God, well, God can very well show that He is God through His truth and power… as He did by dividing the Red Sea, etc.

  20. Passa said,

    December 31, 2009 at 12:38 pm

    Keeping in mind the problems with solipsism, and its opposite, theorizing an objective world, it doesn’t seem as Descartes thought, that because the senses provide perceptions that are historic thereby causes one’s experience of the world to be temporally faulty, nullifying the objective world. Seems like that even though that could be the case, that what is perceived even at a later time, regardless of how short a time that might be, is thereby nullified. It seems that the sensations are of something that was real at the moment of the sensation. Why must it be concluded that they were not real? Is there a problem of thinking the past was not real at the time the past event occurred?

    I hope this is coherent.

  21. JC TOO said,

    December 8, 2015 at 6:42 pm

    It has nothing to do with “imagination.” One can just as well say that they are “imagining” that they are not a brain in a vat. The question rather is how to know what the outside world IS. And if it is a brain in a vat, would it not he perceptually identical? The trick is not to have a “default” position which the brain in the var scenario is a departure from.

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