The ‘Brain in a Vat’ thought experiment

The ‘Brain in a Vat’ thought experiment

7 March 2018

Arguably the most famous thought experiment on knowledge and reality, this exercise asks you to imagine that a mad scientist (or another entity) has taken your brain from your body and placed it in a vat of some life-sustaining fluid. Electrodes have been connected to your brain, and these are also linked to a computer that generates images and sensations. So the brain can still think and act as if it were still in a skull, but all the information it processes comes from the computer, which has the ability to simulate your everyday experience. Assuming this was scientifically possible, how would you ever know whether the world around you was real, or just a simulation generated by a computer? From the point of view of that brain, it would be impossible to tell.

The experiment – also called ‘the brain in a jar’ – has been discussed both amongst philosophers and more widely, in film and literature. The Matrix is a well-known example. In philosophy, the exercise reminds us of Descartes’ Meditations, where he questions whether his sensations were really his own, or just a dream, or an illusion caused by an “evil demon”. (Note that this is only a stage in Descartes’ line of interrogation and not the conclusive one).


  • What view of reality and knowledge can this thought experiment be used to support?
  • If someone told you today that your brain was removed at birth, put in a vat and attached to electrodes, and that nothing you’ve seen, heard, or done was real – it was all simulated, what argument could you invoke against this?
  • What if we don’t even know if our brains are real? What if our minds are stimulated directed by an evil demon? What could we say against that?
  • Now, think about the version of scepticism presented in The Matrix. Is it more similar to –
    • Plato’s cave,
    • Descartes’ dream or demon hypothesis, or
    • The brain in a vat scenario?

8 thoughts on “The ‘Brain in a Vat’ thought experiment”

  1. If someone told me that my brain was removed at birth, put in a vat and attached to electrodes, and that nothing I’ve seen, heard, or done was real, then I would argue that I believed that what I experienced was real. This scenario is similar to The Matrix and to the act of dreaming. The people living in the matrix believed that they were living in reality. Only when Morpheus told Neo that he was living in the simulation did he realize that none of it was real. When Neo is trying to wrap his mind around the fact that he had been living in a simulation his whole life, Morpheus says “What is real? How do you define ‘real’? If you’re talking about what you can feel, what you can smell, what you can taste and see, then ‘real’ is simply electrical signals interpreted by your brain.” Based on this logic, the experiences and memories I had while my brain was in the vat would have in fact been real. Even though nothing I experienced would have actually happened, I would fully believe that it did. I think this blind trust in oneself and the world is a major part of how reality can be interpreted. Similarly, when we are dreaming, very rarely do we recognize that we are experiencing a dream. We assume that whatever is happening is reality because we can see, feel, hear, and otherwise sense the world around us. Only when we wake up, like when Morpheus told Neo the truth, do we realize we were dreaming.

  2. I think it is most like the brain in a vat scenario, but it shares some similarities with Plato’s Cave. The people in the Matrix experience almost the exact same physical conditions, being in a vat of life-sustaining fluid. The machine that they are connected to generates their lives; the images they see, the voices they hear, etc. Because the computer has the permanent understanding of the world, who does what in it and so on, it has the ability to create a world that is undetectably unreal. Neo has absolutely no idea that he is living in the Matrix until he is brought out of it, this proves that the brain was unable to prove whether what he was experiencing was real or not.
    It is also similar to Plato’s cave, in the way that Neo has emerged and he knows, due to the experience of taking the pill, how doubtful people really are of there being another world.

  3. Question: Now, think about the version of scepticism presented in The Matrix. Is it more similar to – a) Plato’s cave b) Descartes’ Dream or demon hypothesis, or c) The brain in a vat scenario

    I believe the topic that best relates to the idea of scepticism presented in The Matrix is the brain in the vat scenario. Like the brain in a vat, the scenario presented in the matrix is that in which there are thousands, potentially millions, of bodies all housed in a room where they are hooked up to feeding tubes that keep them alive in their state of “sleep.” When Neo wakes up in one of the pods, he sees all of the hosts around him and is shocked. Just like the brain in the vat, the bodies are in vats as well.

    Another important point comes in the idea that both subjects have no control of finding the truth of reality on their own. If this brain has been in this vat for its whole life, how will it find a distinction between its two lives. Likewise, Neo is not able to see reality without taking the pill offered my Morpheus. This may suggest that our brains are too weak to recognize what reality truly is. Though Seneca and Plato suggest through deep thought fulfillment can be achieved, I interpret this as finding oneself or what is truly good- not what is actually real. This goes back to the overarching question of how could you ever prove that the world around you was real, and not just a simulation generated by a computer?

    In my opinion, I do not think you could. As Seneca states, the body is merely necessary for the mind. If this vat is able to stimulate our brains in the same way as what we are used to, how would we ever know. Besides, aren’t our thoughts inspired by stimulants from the outside world? How can we ever decipher what we think is real and what is actually real.

    Mitchell Kotheimer

    1. In addition to what I have commented on above, I think it is important to take an alternative view and play devils advocate. In life, there are times when a supernatural (like God) presence is felt. This may occur in a spectacular event or a unique "miracle." I think the brain in the vat theory is quite cynical if one is a religious person and believes in God. Whoever would be controlling these brains would be playing God and that should be left to the maker himself.

  4. 4.) The issue of living in a simulation within The Matrix closely relates to the brain in the vat scenario. Both of these concepts mimic each other greatly. Just as the brain sits in a vat controlled by a computer the people within The Matrix live in a deeply controlled reality. Both of these scenarios force you to wonder if the brain and these people believe they are living a full life or if they are able to realize nothing is in their control. In my opinion both consciousness understand their reality as true. It would take an outside force, like Neo in the Matrix to notify people that their reality is out of their control and there is a life better beyond what they are living. The fulfillment of life starts with a person having full control of their ideas and decisions. Although, there are people who would find comfort in their reality knowing that they aren’t in control of it. Knowing that others are controlling of reality yet you are able to get whatever you want like the brain in the vat, people become attracted to this.

  5. 2) The argument you could use to revoke this is what would the self-conscious and awareness and soul be if you were just being stimulated. If our brain had been put in a vat and everything was being stimulated, our brains would be the main thing being controlled. If this was the case then we would not need self-awareness as we would not need to be aware of what we were doing because we were being controlled. But we know this cannot be true because we as humans have self-awareness. It is a hard feeling to explain but most people will know if they are being looked at. If you stare at someone for long enough they will eventually look at you back and this is because they have self-awareness.

    Humans are also self-conscious. We are fully aware of what we are doing a lot of the time and the only way we wouldn’t be aware would be by the use of alcohol and drugs. People who have been in this situation know what it is like to not feel self-conscious and this is not how we feel on a day to day basis, therefore showing we have this. Our souls are also very important and if we were being stimulated or controlled then we would not have this or have emotions or feelings because there would not be any point for them. But we know this is not the case and all humans have emotions and feelings therefore invoking the argument that we could all be stimulated.

  6. 4. When Neo has to choose between a blue pill and red pill in the Matrix, I think this scenario is closely related to Plato’s story of the cave. My thoughts on this are that Neo had a choice to either stay in the life he was currently living, or he could venture into the Matrix. The "cave" is the life he currently is living while taking the blue pill will bring him into a different world where he really doesn’t know what will happen. The Matrix is just an illusion, but it also could be a better life for some. It’s a risk that Neo is willing to take for a possibly better opportunity. Once Neo decides to go into the Matrix and gets hooked up to a machine, I think this relates to the brain in a vat scenario. Now Neo is being controlled by a machine and learning new skills from this machine. He is able to enter into "reality" by a machine and is no longer the guy that is working a normal desk job. He is now being filled with knowledge that he didn’t have before. This knowledge isn’t actually Neo’s, it’s what has been now programmed into his body. So now he is no longer his true self but computer generated.

    1. I believe Neo was made for the Matrix. He is an intelligent guy that had more to offer in this world then in the world he was living in. I think he is able to see how much he grew as a person when he was in the Matrix, even if he was being programmed. He broke out of his comfort zone by taking the blue pill and learned that he is much more than just a guy doing a normal desk job.

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