We Are All Brains In Vats

The following post is from my new article for Knowing Neurons, an online publication about neuroscience and the mind.  

The Matrix made all of us ask the same disturbing questions: How do I know that the world I see, hear, and touch is real? Can I prove that I’m not actually in a pod created by machines bent on harvesting my bioelectricity?

These unsettling questions are related to a thought experiment called “The Brain In a Vat” problem. The thought experiment was originally proposed by one of my old advisors at Princeton, Gilbert Harman. It’s an updated version of Descartes’ “Evil Demon” thought experiment, which asks whether the world we perceive is an illusion. Given the possibility that the world is an illusion, all we can know for sure is that our thoughts exist: “I think therefore I am.”

The “brain in a vat” scenario makes this frightening possibility biologically plausible. Imagine that a mad scientist takes out your brain and puts it in a vat with life-sustaining liquid. The scientist then connects your brain via wires to a supercomputer, and the supercomputer provides electrical impulses to your brain that are indistinguishable from those that your brain normally receives from your nerves. If the supercomputer could accurately simulate your world, then you would have no idea that nothing you perceive is real.

The “brain in a vat” idea opens up some troubling philosophical questions, but for me it also drives home just how remarkable the brain is. Our brains are, literally, in vats. They float inside our skulls, surrounded by about 0.15 liters of cerebrospinal fluid, and they receive complex patterns of stimulation from the sensory nerves that enter our skull. The brain takes these patterns of stimulation and turns them into a sensory universe so complex and vivid that we forget that it is, at best, only a representation of the outside world.

Think about it this way. Our visual cortex may create the experience of light, but the cortex itself is wrapped in complete darkness, shielded by our skull from any actual light. All it can do is receive information about light outside of our skull. What’s astounding is that the brain can turn this information into experience!

The brain in a vat thought experiment reveals that the information our brain receives could be made up. And if that information is made up, then the brain would turn that made up information into a false sensory world – like The Matrix. I’ll let philosophers puzzle over what this means for our certainty about the outside world. As a neuroscientist, I’m still hung up on the incredible fact that the brain is even capable of turning information – whether accurate or made up – into experience.

So, yes, we are brains in the vats of our skulls – and that is breathtaking.



Harman, G. (1973). Thought. Princeton University Press.


Cover image: Michael Faraday in his laboratory by Harriet Jane Moore

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