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Imagine a brilliant neuroscientist[br]named Mary.

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Mary lives in a black and white room,

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she only reads black and white books,

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and her screens only display [br]black and white.

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But even though she has never seen color,[br]Mary is an expert in color vision

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and knows everything ever discovered[br]about its physics and biology.

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She knows how different [br]wavelengths of light

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stimulate three types of cone cells[br]in the retina,

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and she knows how electrical signals

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travel down the optic nerve [br]into the brain.

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There, they create patterns [br]of neural activity

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that correspond to the millions[br]of colors most humans can distinguish.

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Now image that one day,

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Mary's black and white screen[br]malfunctions

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and an apple appears in color.

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For the first time,

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she can experience something[br]that she's known about for years.

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Does she learn anything new?

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Is there anything about perceiving color[br]that wasn't captured in all her knowledge?

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Philosopher Frank Jackson proposed[br]this thought experiment,

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called Mary's room, in 1982.

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He argued that if Mary already knew[br]all the physical facts about color vision,

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and experiencing color still teaches[br]her something new,

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then mental states, like color perception,

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can't be completely described [br]by physical facts.

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The Mary's room thought experiment

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describes what philosophers call[br]the knowledge argument,

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that there are non-physical properties[br]and knowledge

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which can only be discovered[br]through conscious experience.

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The knowledge argument contradicts[br]the theory of physicalism,

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which says that everything, [br]including mental states,

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has a physical explanation.

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To most people hearing Mary's story,

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it seems intuitively obvious [br]that actually seeing color

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will be totally different [br]than learning about it.

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Therefore, there most be some quality[br]of color vision

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that transcends its physical description.

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The knowledge argument isn't just[br]about color vision.

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Mary's room uses color vision[br]to represent conscious experience.

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If physical science can't entirely[br]explain color vision,

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then maybe it can't entirely explain[br]other conscious experiences, either.

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For instance, we could know every[br]physical detail

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about the structure and function[br]of someone else's brain,

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but still not understand[br]what it feels like to be that person.

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These ineffable experiences [br]have properties called qualia,

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subjective qualities that you can't [br]accurately describe or measure.

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Qualia are unique to the person[br]experiencing them,

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like having an itch,

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being in love,

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or feeling bored.

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Physical facts can't completely explain[br]mental states like this.

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Philosophers interested[br]in artificial intelligence

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have used the knowledge argument

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to theorize that recreating [br]a physical state

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won't necessarily recreate[br]a corresponding mental state.

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In other words,

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building a computer which mimicked [br]the function of every single neuron

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of the human brain

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won't necessarily create a conscious[br]computerized brain.

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Not all philosophers agree that[br]the Mary's room experiment is useful.

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Some argue that her extensive knowledge[br]of color vision

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would have allowed her to create[br]the same mental state

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produced by actually seeing the color.

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The screen malfunction wouldn't[br]show her anything new.

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Others say that her knowledge[br]was never complete in the first place

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because it was based only[br]on those physical facts

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that can be conveyed in words.

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Years after he proposed it,

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Jackson actually reversed his own[br]stance on his thought experiment.

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He decided that even [br]Mary's experience of seeing red

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still does correspond to a measurable[br]physical event in the brain,

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not unknowable qualia beyond[br]physical explanation.

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But there still isn't a definitive answer

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to the question of whether Mary would[br]learn anything new

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when she sees the apple.

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Could it be that there are fundamental[br]limits to what we can know

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about something we can't experience?

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And would this mean there are certain[br]aspects of the Universe

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that lie permanently beyond[br]our comprehension?

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Or will science and philosophy allow[br]us to overcome our mind's limitations?