The problem: "Since the brain in a vat gives and receives exactly the same impulses as it would if it were in a skull, and since these are its only way of interacting with its environment, then it is not possible to tell, from the perspective of that brain, whether it is in a skull or a vat"(from wikipedia)

So, to just use the example of an eye connected to a brain, the eyes receives light, which is then transmitted to the brain. The skeptic would say how do you know you are actually seeing things with your eye, and its not just transmissions directly to the brain.

My "solution" would be to imagine what type of machine could "fake" the transmissions to the brain without actually seeing. This machine must be cabale of causing the specific "transmissions" (whatever those are) that produce phenomenal visual experience to go to the brain. These transmissions cannot be uncaused, but in order for them to create the specific illusion of visual perception in the brain, the causes would have to be identical to those that are normally produced by the eyes photo receptors. Therefore, in order to create an illusion of reality, the machine would have to be identical in causal function to an actual eye, but it that case there would be no illusion, but just actually seeing (with some sort of cyborg eye machine). Does this make sense and is it a legitimate solution?

I have also tried to explain what I mean more clearly in comments.

  • But what you see might not be real. Think of watching StarTrek (not real) on TV(sort of cyborg eye machine), take a borg epsiode...
    – draks ...
    Apr 11, 2016 at 19:54
  • I always thought the simpler solution to all skeptical applications of illusion and hallucination is to hold that to identiy a thing as being an 'illusion' or 'hallucination' assumes knowledge about the way reality actually is, so that a global application of any such arguments overlook the means by which they are possible.
    – Ovid
    Apr 11, 2016 at 20:51

5 Answers 5


In the typical brain-in-a-vat scenario, the brain's sensations might be entirely simulated (computer generated, or manifested by a malign spirit), unconnected to any external reality at all.

Two general approaches to skepticism about reality are to argue:

A) that the scenario of a faked reality is needlessly more complex and contrived than to accept reality as it presents itself --basically a version of Occam's Razor

B) that it is meaningless to speak of a deeper inaccessible reality if it makes no perceptible difference in our experience (this is Hume's approach)

Your approach is similar to Hume's --essentially you're arguing that seeing through an organic eye and seeing through a robotic eye are the same thing (in the case that they are indistinguishable). How effective your approach is depends on the extent to which you're willing to agree with Hume that the appearance is the reality for all meaningful intents and purposes. If your argument requires that there is an external reality largely the same as presented to the brain, and that only the specific source of the sensations is in question, then you haven't responded to the full force of the "brain in a vat" scenario.

  • I wonder, under which approach would you classify Putnam's response?
    – E...
    Apr 11, 2016 at 20:54
  • I am not saying that the illusion and reality are the same for all intents and purposes. I am saying that illusion is impossible. since the skeptic still believes in a brain, something must be causing the phenomenon that the brain experiences. What am saying is that the only thing that can function in a causal process of producing a phenomenal visual experience in a brain is an eye. A vat machine or anything else could in no way cause a visual experience that could be considered an illusion.
    – user20502
    Apr 11, 2016 at 23:55
  • In that case you're just disagreeing with the premises of the thought experiment. Apr 12, 2016 at 12:57
  • well I guess I would be saying that is doesn't make sense to think of something not-an-eye causing visual perception in the brain. Isn't arguing that the premises are nonsensical a legitimate way to solve a problem?
    – user20502
    Apr 12, 2016 at 13:41

A version of your solution to the brain in the vat problem was proposed by Daniel Dennett in the opening chapters of his book "Consciousness Explained" (Chapter 1 - How are hallucinations possible? - Section 1 - The brain in a vat).

His reasoning goes like this:

  • It is easy to simulate reality for a passive person. It would be easy for the scientists setting up the inputs to the brain in the vat to simulate the sensations for someone sitting on a beach doing nothing, sitting on a moving train or boat, watching the scenery move by.
  • People in real life however are not passive, and the brain in the vat would soon realize that something is amiss if it is not capable of doing anything other than observe the surroundings.
  • For the brain in the vat to be truly fooled by the simulation, it should be able to interact with the world around it. It should be able to get up and walk away from the beach, or get off the train at whichever station it chooses. This leads to an infinite number of possibilities, and it would be impossible for the scientists controlling the inputs to the vat to account for all these possibilities.
  • Therefore any pure simulation the scientists can conjure up will have limited scope, and sooner or later the brain in the vat will realize that something is wrong with the world it inhabits, because it will inevitably be stuck with a limited amount of choices in its actions.
  • The only way around this is for the scientists to copy data from reality, instead of creating a simulation from scratch. To fool a brain in a vat that it is a person walking around New York city, they will have to actually use thousands and thousands of hours of footage filmed by real people walking around New York city to cover all the possible scenarios that a person walking around NYC will encounter. The analogy he gives is:

"If you really want to fool someone into thinking he is in a cage with a gorilla, enlisting the help of an actor in a gorilla suit is going to be your best bet for a long time."

To your point in your suggested solution:

Therefore, in order to create an illusion of reality, the machine would have to be identical in causal function to an actual eye, but it that case there would be no illusion, but just actually seeing (with some sort of cyborg eye machine).

The only sense data good enough to fool the brain in a vat would have to be real sense data, so that the brain will be living a sort of reality, it just won't be its own reality.

  • it sounds like dennet actually believes it is possible for a brain to actually experience some sort of phenomenal perceptions via "inputs". what I am saying is that any inputs, since they are in the causal process, would have to be identical in causal function to the eye, and so therefore would not produce anything different. I don't understand how its possible to scientists to stimulate nerves in a way that would produce phenomenal experience.
    – user20502
    Apr 11, 2016 at 23:43
  • 1
    @user20502 I see your point clearer now, but to that I reply that empirical evidence seems to contradict your premise: Hallucinations (as in fake but definitely phenomenological experiences) do happen all the time, due to drugs, dreams, psychiatric conditions. Presumably the scientists performing the brain in a vat experiments know enough about how hallucinations happen from dreams and hallucinations to the point where they can reproduce them in a controlled fashion. Artificially stimulated phenomenological experiences seems entirely possible, its their sustainability that's a problem. Apr 12, 2016 at 4:33
  • Im not sure that hallucination, drugs etc contradicts what I am suggesting. These are abnormal perceptual states that are caused by abnormal brain function. the causal process of perception is still the same, its just that the brain itself is no longer normal, so it produces the "fake" phenomenon. The case of dreaming I think is totally different from visual perception or hallucination.
    – user20502
    Apr 12, 2016 at 13:07
  • @user20502, Then we can just imagine a machine that disrupts normal brain function, and uses that to create a fake world. Really makes no difference in relation to the brain-in-a-vat thought experiment. Apr 12, 2016 at 18:47
  • Ameet, the "machine" you are describing could be psychedelic drugs or a lobotomy or brain injury that changes a persons brain structure and therefore their perception(I know drugs don't exactly work this way). This would not be a fake world, since people really do experience those things or are schizophrenic etc, they are experiencing the reality of their altered brain, not a fake reality. I agree that it makes no difference in the brain in a vat thought experiment
    – user20502
    Apr 12, 2016 at 18:50

But how do you know that there are brains or vats or eyes or any other such things?

I thought the brain in a vat was just a way to illustrate the logical separation between sense-data and a hypothetical outside world.

Once you see that separation, then there's no 'eye' or 'actual function of the eye' to refer to... we are all brains in vats in the sense that we don't have direct access to the outside world.

  • I am trying to answer the skeptics question "how do we know we are not brains in vats attached to a simulation machine". my answer is that if that were the case, then we would not see anything, since the only "machine" that can cause visual perception in the brain is an eye. there isn't a question of whether the brain exists or not. The question is if knowing the actual function of the eye is causing "real perception" would it possible to skip the step of receiving "real images" from the eye and instead just plug in fake images to the brain, and I am saying this is not possible.
    – user20502
    Apr 12, 2016 at 17:33
  • Ok. I think I see your point now. But I don't see why a mechanical eye with artificial stimuli from a computer can't fake the outside world. Why do the causal processes have to be the same... for example, say I'm in a holodeck. The root causes of my sense-data are different (real objects vs holographic light), but both simulate an outside world. Apr 12, 2016 at 17:41

I tend to agree with Zane's answer the most. The OPs solution necessitates an optic device to feed live environmental data to the brain. In order for the illusion to be consistent, the brain should be refused to interact with the illusion. If the brain is allowed to interact with it, then the "simulated reality" is actually real and it is the conscious action of the brain that determines events.

An alternative suggestion, to elaborate on Zane's point, is as follows:

Replace the optic sensor with an analog video feed, like the VGA on the back of your graphics card.

Additional to this, hook your motor cortex up to the wires of a keyboard and mouse.

With some final preparations, like hooking up the nerves to a sensation generator, you are ready to start Earth Simulator 3021.

Then you have a totally immersed brain-in-a-vat happily roaming through a world which for all intents and purposes for the brain is real without ever knowing the real reality.

Is this not kind of what is happening in The Matrix?

Just as a further "real life" illustration, an AI researcher, Tom7, created an AI that plays NES games by feeding a picture of the RAM to it and it controls a virtual controller. This is very similar to what I described above. What if that AI is conscious? Then the game is it's reality and the computer is the vat.

If on the other hand you literally just ask the eye is the only thing replaced, then yes, by inference as you and other answerers pointed out, the reality of the simulation cannot hold if the brain is allowed to interact with it's surroundings, thus it actually just is seeing.

  • in reference to zanes answer, i am not thinking of a video feed that could literally transmit the video directly to the brain. I am saying that this is impossible. the only way to "play" visual data phenomenally in the brain is with an eye or an eye like machine, since they are the only things that can fulfill the causal function of perception. the only illusion possible would be a real illusion, like a person in a gorilla suit appearing to be a gorilla and then transmitted to the brain via eye, and in that case the whole brain in a vat scenario becomes unnecessary.
    – user20502
    Apr 12, 2016 at 18:39

You are thinking of some mechanism that plays video images from outside the eyes and this is incorrect. The brain in the jar has no eyes. The information is fed into the optic nerve as electrical impulses. Have you ever played Call of duty? You can go anywhere in the game and the graphics are incredible. Imagine what virtual places will look like in another thousand years! Images don't have to exist, they can be created by expectations of what you think you should see. Remember that all expectations are built on memories. Eg, pictures of the places you intend to visit. Well even those memories could be implanted.

  • I am saying that the information that is "fed" into the optic nerve, in order to produce the visual phenomenon, would have to cause same information that the eye would to travel to the brain. During visual perception, the eye "feeds" information to the brain and it produces visual phenomenon. If it was not an eye that was doing this, then there would be no visual experience.
    – user20502
    Apr 12, 2016 at 13:00
  • Do you ever dream? Do you see images in your dreams? Aren't those visual experiences created without eyes? Apr 12, 2016 at 13:44
  • No, I think visual phenomenon and dreaming are different. they cannot be confused from the qualitative standpoint, and they are both caused by normal brain activity
    – user20502
    Apr 12, 2016 at 16:39
  • The eye sends electrical impulses along the optic nerve to the brain which interprets these impulses and creates images. My point is, if we could simulate those electrical impulses and feed them directly into the optic nerve, we would believe the images we see originate from our eyes. Science has already linked a digital camera directly to a blind persons brain and it allows them rudimentary vision. So it's not as farfetched as most would believe. Apr 13, 2016 at 22:41
  • what i am contending is that the only thing that can cause those impulses is an eye. the skeptic believes that there is some sort of machine that can cause those impulses. I don't believe this is true. in order to form a phenomenal image, i contend that if some sort of machine could do that, it would have to function very similar to an eye, with photo receptors to receive light. In that case, the only room for illusion would be a real illusion that the fake eye sees, and that cancels the whole point of the machine anyways.
    – user20502
    Apr 14, 2016 at 1:54

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