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Imagine a brilliant neuroscientist
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
black and white.
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But even though she has never seen color,
Mary is an expert in color vision
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and knows everything ever discovered
about its physics and biology.
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She knows how different
wavelengths of light
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stimulate three types of cone cells
in the retina,
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and she knows how electrical signals
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travel down the optic nerve
into the brain.
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There, they create patterns
of neural activity
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that correspond to the millions
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
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
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
that wasn't captured in all her knowledge?
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Philosopher Frank Jackson proposed
this thought experiment,
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called Mary's room, in 1982.
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He argued that if Mary already knew
all the physical facts about color vision,
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and experiencing color still teaches
her something new,
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then mental states, like color perception,
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can't be completely described
by physical facts.
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The Mary's room thought experiment
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describes what philosophers call
the knowledge argument,
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that there are non-physical properties
and knowledge
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which can only be discovered
through conscious experience.
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The knowledge argument contradicts
the theory of physicalism,
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which says that everything,
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
that actually seeing color
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will be totally different
than learning about it.
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Therefore, there most be some quality
of color vision
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that transcends its physical description.
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The knowledge argument isn't just
about color vision.
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Mary's room uses color vision
to represent conscious experience.
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If physical science can't entirely
explain color vision,
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then maybe it can't entirely explain
other conscious experiences, either.
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For instance, we could know every
physical detail
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about the structure and function
of someone else's brain,
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but still not understand
what it feels like to be that person.
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These ineffable experiences
have properties called qualia,
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subjective qualities that you can't
accurately describe or measure.
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Qualia are unique to the person
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
mental states like this.
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Philosophers interested
in artificial intelligence
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have used the knowledge argument
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to theorize that recreating
a physical state
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won't necessarily recreate
a corresponding mental state.
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In other words,
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building a computer which mimicked
the function of every single neuron
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of the human brain
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won't necessarily create a conscious
computerized brain.
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Not all philosophers agree that
the Mary's room experiment is useful.
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Some argue that her extensive knowledge
of color vision
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would have allowed her to create
the same mental state
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produced by actually seeing the color.
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The screen malfunction wouldn't
show her anything new.
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Others say that her knowledge
was never complete in the first place
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because it was based only
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
stance on his thought experiment.
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He decided that even
Mary's experience of seeing red
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still does correspond to a measurable
physical event in the brain,
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not unknowable qualia beyond
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
learn anything new
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when she sees the apple.
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Could it be that there are fundamental
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
aspects of the Universe
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that lie permanently beyond
our comprehension?
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Or will science and philosophy allow
us to overcome our mind's limitations?