Mary's room is a philosophical thought experiment proposed by Frank Jackson in his article "What Mary didn't know" in 1986.
Imagine that from birth our hypothetical friend Mary is put to live in a room where no colors are shown. Her food, her books, and even the color of her skin are all in black and white. Now imagine that Mary, a very bright girl, has a wealth of information at her disposal, and through concentrated study she comes to learn everything there is to know, which includes that colors are mental processes in the brain and how the brain produces them. But she has never experienced color.
If she is set loose and starts experiencing colors, will she have learned anything new?
She may know everything she can learn, but can she know what "red" is if she has never seen red? Jackson contends that, yes, she has learned something new, via experience, and hence, physicalism is false.
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